EP2670707A1 - Production of a crystallized nickel salt from hyperaccumulator plants - Google Patents

Abstract

The present invention relates to the phytoextraction of heavy metals from the soil, and more particularly nickel in the form of a crystallized salt. The present invention includes, in particular, a process of producing a crystallized nickel salt from the ashes of a hyperaccumulator plant that accumulates metal elements nickel-containing, a use of the hyperaccumulator plant for the production of the crystallized nickel salt, and a purified composition of the crystallized nickel salt which is derived from the ashes of the hyperaccumulator plant.

Description

 PRODUCTION OF CRYSTALLIZED NICKEL SALT FROM PLANTS

 hyperaccumulating

FIELD OF THE INVENTION

The present invention relates to the phytoextraction of heavy metals from the soil. More specifically, it consists of a process for producing a crystallized nickel salt from the ashes of a hyperaccumulator plant of metallic elements including nickel.

STATE OF THE ART

Phytoextraction is an agro-ecological process using plants to extract heavy metals from soils. The process can then make it possible to render two types of services, an environmental service for reducing the impacts of metallic pollution and an industrial service for the production of metals. The latter is called phytomining.

Phytomining uses heavy metal mining plants as miners to extract metals of commercial value, these metals being initially contained in mineralized soils that can not be exploited by traditional mining processes (Barbaroux, 2009, Hydrometallurgy, 100, p.10-14 Chaney, 2007, J. Environ., Quai., 36, p.1429-1 43). The application of the principles of phytoextraction by phytoremediation or phytomining emphasizes the use of plants allowing a sufficient annual accumulation in metals such as Ni, Zn, Cd, Se, Co, As or Ti (Baker, 1989, Biorecovery, 1 , pp. 81-126). Some plants are able to accumulate more than 1,000 mg metal per kilogram of dry matter under natural conditions. They are then called hyperaccumulators (Brooks, 1998, CAB International, Wallinford, England, 380). The leaves, flowers and seeds have a ratio of metal concentration to roots greater than one, while that non-hyperaccumulating plants tend to concentrate the metals in their roots (Shallari, 1998, Sci., Total Environment, 209, p.133-142). The hyperaccumulating plants that accumulate the metal in their aerial parts are harvested by conventional agronomic methods. More than 400 plant species, represented by about 45 families, have been identified as hyperaccumulators and two-thirds of these hyperaccumulators accumulate nickel (Brooks 1998, Chaney 2007).

Nickel is present in all compartments of the terrestrial environment in trace amounts, except in serpentine ores and soils, where it is at higher levels. Serpentine soils are derived from ultramafic rock alteration and are widely distributed worldwide (Brooks, 1987, Dioscorides Press, Portland, Oregon, US 454, Ghaderian, 2007, Plant Soil, 293, p. 91-97). These soils are characterized by a pH of 6 to 8, by abnormally high levels of metals such as Ni, Cr and Co, as well as high levels of Fe and Mg and a low Ca content. The major elements, K, P and S are present in trace amounts (Shallari, 1998). The concentration of Ni in these soils ranges from 0.5 to 8 g Ni / kg soil (Bani, 2007, Plant Soil, 293, pp. 79-89).

Nickel is therefore a relatively widespread metal worldwide. However, economically profitable ore mining sites are located in Canada, Russia, Australia, New Caledonia and Brazil. At the industrial level, nickel is mainly produced from ores by pyro- or hydrometallurgical processes. While the metallurgical industry remains by far the largest consumer of nickel (USGC, 2005, US geological survey ores years book, pp.1-25), the chemical industry ranks second.

Nickel compounds are used for electroplating, battery manufacturing (nickel hydroxide is used as a mass in nickel-cadmium batteries), pigment manufacturing for paints and as a catalyst in many chemical reactions ( Kerfoot, 1995, Handbook of Extractive Metallurgy, Wiley-VCH, Weinheim, Germany, p. 714-786). Since nickel is a transition metal, its compounds are numerous and are not all of interest. Some, however, have a higher economic value than nickel for alloy production, such as the nickel and ammonium double sulphate salt used as an electrolytic salt in the plating industry (Kerfoot, 1995). Precipitation of nickel and ammonium double sulphate salt is possible by adding ammonium sulphate to a mineral solution of concentrated nickel sulphate. The physical properties of nickel and ammonium sulfate in aqueous solution have been studied and measured (Linke, 1965, Solubilities of inorganic and metal organic compounds, Volume 2, 4th Edition, American Chemical Society, 763). The flora of metalliferous soils is represented by many phenotypes, some of which are characterized by their ability to accumulate or tolerate high levels of metals (Shallari, 1998). In general, higher plants have implemented two dominant strategies for supporting high metal concentrations (Chaney 2007, Morel 1997 Soil Ecotoxicology, Lewis Publishers, CRC Press, Boca Raton, Florida, USA, Chap 6, pp. 141-176).

- The widespread exclusion limits the absorption of metal ions by the plant. In many situations, the absorbed metals remain localized at the roots and are not transferred to the stems and leaves; and

- Accumulation coupled with tolerance allows the plant that has this adaptation to grow on soil heavily loaded with metals. The metal ions absorbed by the roots and then translocated to the aerial parts are accumulated in the leaves and seeds where they are present in less toxic chemical forms, eg organometallic complexes (Montargès-Pelletier, 2008, Phytochem., 69, pp. 1695-1709). The formation of these organic molecule / metal complexes allows cellular tolerance and cellular accumulation of metal at high levels (Asemaneh, 2006, Planta, 225 (1), pp. 193-202).

The hyperaccumulator Alyssum mural can accumulate up to 20,000 mg Ni / kg DM and a biomass of 10,000 kg / ha can be harvested per year from this plant (Chaney, 2007). The serpentine soils contain nickel at levels between 1000 and 7000 mg / kg. Thus, the ashes of hyperaccumulative plants contain about 15% of nickel whereas the usual rocks from serpentine soils contain only 0.2% of nickel. The recovery of nickel from rocks is therefore faced with a problem of poor profitability. Although nickel concentrations in hyperaccumulative plants are well below the minimum concentration required for traditional mining (30,000 mg / kg), they are sufficient to consider the accumulation and extraction of nickel through a pathway. treatment of hyperaccumulating plants (Baker 1989, Broadhust 2004, Plant Soil 265, 225-242, Shallari 1998). The profitability of this recovery route by plants, however, remains a challenge.

The walled Alyssum (wall-mounted) plant once dried and once harvested, can be pyrometallurgically processed in a foundry (Chaney, 2007) to produce a nickel metal or leach dry mass to produce a concentrated nickel leachate ( Wood, 2006, HortScience, 41, p. 1231-1234). It therefore seems conceivable to produce pure and marketable nickel from the harvest of the mural plant A. on serpentine soils, and then by chemical treatment of this plant.

It has been shown that the leaching of seeds of A. Mural in 0.5 M sulfuric acid at 90 ° C with a solid fraction of 15% for 120 min followed by two residue washing steps allowed extracting 97.0 ± 6.8% of available nickel (Barbaroux, 2009). A three-stage countercurrent process extracts 100% of available nickel. Economic analysis has showed that a profitable industrial application was conceivable. Wood (2006) had previously produced a concentrated nickel solution from the dry mass of a hyperaccumulator plant. However, the process used, a leaching of biomass concentrated in nickel with a slightly acidic solution of pH 5 for 24 hours at 90 ° C., did not allow a profitable industrial application (Barbaroux, 2009).

Moreover, Couillard and Mercier (1992, The Canadian Journal of Chemical Engineering, 70, p.1021-1029) used selective precipitation to selectively extract metals from aqueous solutions. Similarly, the selective electroplating of metals such as nickel has been conducted from synthetic aqueous solutions and not from plant leachate. The selective extraction of metals such as nickel by these methods, selective precipitation and electroplating on the mural A leachate was not possible (Barbaroux, 201 1, submitted to Sep. Purif Technol in 2010). Ghorbani et al. (2002, Russian Journal of Electrochemistry, 38, 11, pp. 1,173-1,177) had shown the influence of organic complexes in the electrolysis of metals. The absence of nickel reduction at the cathode during electroplating from the leachate shows that the nickel is in a complexed form. The removal of nickel by the precipitation of leachate organic matter by ferric chloride treatment validates the hypothesis of a nickel complexed with organic ligands. Only extraction with the organic solvent Cyanex 272 made it possible to extract the nickel to pick it up selectively to an aqueous solution (Barbaroux, 201 1). Despite the selectivity and high efficiency of the selective extraction by extraction solvent, a difficult optimization would be required given the prohibitive price of Cyanex 272 (Barbaroux, 201 1).

The recovery of metals from a bio-ore (from the dry harvest of hyperaccumulative plants concentrated in metals) can be carried out pyrometallurgically. Koppolu et al. (2004, Biomass Bioenergy, 26, pp. 463-472) have shown that the incineration of a hyperaccumulator plant concentrated in Ni, Cu and Zn at 873 ° K made it possible to recover 99% of the total metals in the form of concentrated ash. The metal concentrations had increased in a ratio of 3.2 to 6 relative to the concentrations in the dry matter. This approach appears to be the most feasible route from an economic and environmental point of view (Harris, 2009, J. Cleaner Product, 17, p.194-200). During incineration, metals such as Ni, Cr or Cu are concentrated in the ashes. The ashes of plants resulting from the incineration of bio-ore would then be processed in the foundry to recover a high purity metal. Li et al. (2003, Plant Soil, 249, p.107-1 15) have shown that the recovery of nickel metal from a bio-ore could be carried out by incineration of the harvest followed by the foundry treatment of the ashes. Most of the ashes of the plant are nutrients that do not interfere with nickel recovery. The ashes of the A. wall biomass are the richest in known nickel ores. Given the low productivity of serpentine soils for agricultural crops and the high value of annually recoverable nickel from phytomining with normal fertilizer inputs, nickel phytomining is expected to be a profitable crop production (Chaney, 2007; Li, 2003). It is also possible to use the energy produced during the incineration of the bio-ore to produce electricity (Li, 2003).

The biomass of A. mural can be treated by incineration to produce ashes. The most commonly suggested route for recovering metals from plants produced by phytomining is the incineration of biomass from hyperaccumulator plants and the treatment of ashes in foundries to recover nickel metal (Li, 2003). Chaney et al. (2007) treat the biomass of the mural plant A. in the foundry to recover nickel in metal form.

The technology for extracting metals from hyperaccumulating plants and their valorization presents difficulties and uncertainties that call into question the profitability of phytoextraction and phytomining. There is therefore a need for an invention that would free itself at least some of the disadvantages mentioned above, which would make the production of metals and metal derivatives extracted by plants reliable and profitable.

SUMMARY OF THE INVENTION

The present invention meets at least some of the above requirements by providing a cost effective recovery of the nickel contained in the ashes of hyperaccumulative plants by the production of a chemical compound of nickel. Crystallized nickel salts have proven to be an effective selective recovery route from aqueous ionic solutions to solubilize nickel contained in hyperaccumulating plants. The crystallized nickel salts preferably include nickel ammonium salt sulfate salt, nickel sulfate salt, and nickel chloride salt.

According to one aspect of the invention, there is provided a method of extracting nickel from ash of a hyperaccumulator plant of metal elements comprising nickel, the method comprising using an ionic solution to recover nickel in the form of a crystallized nickel salt. The process more particularly comprises a step (a) of leaching the ash by the addition of an acid solution, thereby producing a leachate comprising nickel and iron. This results in a step (b) of selective precipitation of leachate iron by addition of a basic solution producing a purified supernatant rich in nickel and an iron-rich precipitate. Then the process comprises a step (c) of crystallization of the crystallized nickel salt.

According to an optional aspect of the process, which may be combined with any of the above aspects, it comprises washing the ashes with an aqueous solution prior to step (a) to reduce the ash content in soluble mineral elements and in chlorine. Soluble mineral elements include potassium and sodium.

According to another optional aspect of the method, which can be combined with any of the above aspects, the method comprises a mechanical separation step, after each steps (a), (b) and (c). The mechanical separation may preferably be a filtration separating a filtrate and a residue.

According to another optional aspect of the process, which can be combined with any of the above aspects, the acid solution is added to the ashes in step (a) with a molar concentration of 0.25 mol / L to 7 mol / L.

According to another optional aspect of the process, which may be combined with any of the above aspects, the leachate has a mass ash concentration of 50 g / L to 300 g / L. Preferably, the mass concentration of ash would be 150 g / L.

According to another optional aspect of the process, which may be combined with any of the above aspects, the leaching step (a) is at a temperature between 0 ° C and 100 ° C.

According to another optional aspect of the process, which can be combined with any of the above aspects, the basic solution used in step (b) is a solution of quicklime, hydrated lime, hydroxide sodium, potassium hydroxide, magnesium hydroxide, calcium carbonate, magnesium carbonate, sodium carbonate or potassium carbonate.

According to another optional aspect of the process, which can be combined with any of the above aspects, the basic solution is added to the leachate with a concentration chosen so that the leachate has a pH between 3.2 and 7, favoring thus the precipitation of iron in order to purify the leachate. Preferably, the solution can be added with a concentration adapted to obtain a pH equal to 5.

According to another optional aspect of the process, which may be combined with any of the above aspects, the method comprises evaporation of the water contained in the purified supernatant of step (b) prior to proceeding with step (c). According to another optional aspect of the process, which can be combined with any of the above aspects, the crystallization (c) is cold.

According to another optional aspect of the process, which may be combined with any of the above aspects, the process comprises purifying the crystallized nickel salt to reduce the magnesium content. The purification may comprise a step (d) of solubilization of the crystallized salt of nickel in water; a step (e) of adding a sufficient amount of an inorganic compound comprising an anion selected to produce an anion-magnesium precipitate; a step (f) of filtering the anion-magnesium precipitate to separate it from a second purified supernatant; and a step (g) of cold recrystallization of a crystallized salt of purified nickel by addition of the ionic solution in the purified second supernatant. The anion would preferably be the fluoride ion. The inorganic compound would be more preferably sodium fluoride to produce a magnesium fluoride precipitate in step (e).

According to another optional aspect of the process, which may be combined with any of the above aspects, the fluoride ion is added with a molar amount of between 2 and 2.5 mol per mol of magnesium present in the solubilized nickel crystallized salt. in step (d).

According to another optional aspect of the process, which may be combined with any of the above aspects, the method comprises mechanical separation, washing and drying of the crystallized salt of purified nickel obtained in step (g). Mechanical separation may preferably be filtration. Drying of the crystallized salt of purified nickel may preferably be carried out at a temperature between 40 and 150 ° C.

According to another optional aspect of the process, which can be combined with any of the above aspects, the nickel is recovered as a nickel ammonium sulphate salt, a nickel sulphate salt or a salt of nickel chloride. According to another optional aspect of the process, which can be combined with any of the above aspects, the ionic solution is an ammonium sulphate solution for recovering nickel in the form of the nickel sulphate salt and the ammonium, the ammonium sulphate solution being added to the supernatant purified during step (c) of crystallization of the nickel sulfate salt of ammonium and ammonium.

According to another optional aspect of the process, which may be combined with any of the above aspects, the ionic solution is a sulfuric acid solution for recovering nickel in the form of a nickel sulphate salt. sulfuric acid being used as an acidic solution in the ash leaching step (a). According to another optional aspect of the process, which may be combined with any of the above aspects, the ionic solution is a hydrochloric acid solution for recovering the nickel in the form of a nickel chloride salt. hydrochloric acid being used as an acidic solution in the ash leaching step (a).

According to another optional aspect of the process, which can be combined with any of the above aspects, the ionic solution is a solution of sulfuric acid with added brine to recover nickel in the form of a chloride salt. nickel, hydrochloric acid being used as an acidic solution in the step (a) of leaching the ashes. Preferably, the brine would be a solution of sodium chloride or potassium chloride. According to another optional aspect of the process, which can be combined with any of the above aspects, the ashes are obtained by incineration of the hyperaccumulator plant of metal elements with the previously dried.

According to another optional aspect of the process, which can be combined with any of the above aspects, the hyperaccumulator plant comprises or is a plant of the family Brassicaceae, Cunoniaceae, Flacortiaceae, Violaceae or Euphorbiaceae or a combination thereof.

According to another optional aspect of the method, which may be combined with any of the above aspects, the hyperaccumulator plant comprises or is a plant of the mural species A.

According to another aspect of the invention, there is provided a use of a plant hyperaccumulator of metal elements comprising nickel for the production of a crystallized nickel salt from the ash of the hyperaccumulator plant.

According to an optional aspect of the use, which may be combined with any of the above aspects, an ionic solution is used to recover nickel from ashes as the crystallized nickel salt.

According to another optional aspect of the use, which may be combined with any of the above aspects, the crystallized nickel salt is the nickel and ammonium double sulfate salt, the nickel sulfate salt or nickel chloride salt. The crystallized nickel salt would preferably be the nickel sulfate salt of nickel and ammonium.

According to another aspect of the invention, there is provided a purified composition of crystallized nickel salt derived from ash of a hyperaccumulator plant of metal elements including nickel.

According to an optional aspect of the composition, which may be combined with any of the above aspects, the crystallized nickel salt is nickel ammonium salt sulfate salt, nickel sulfate salt or salt. nickel chloride. Preferably, the crystallized nickel salt would preferably be the nickel ammonium sulphate salt.

According to another optional aspect of the composition, which may be combined with any of the above aspects, the crystallized nickel salt has a mass concentration of magnesium less than 5 g / kg. Preferably, the mass concentration of magnesium would be less than 0.7 g / kg.

According to another optional aspect of the composition, which may be combined with any of the above aspects, the crystallized nickel salt has a calcium mass concentration of less than 10 g / kg. Preferably, the mass concentration of calcium would be less than 2.5 g / kg.

According to another optional aspect of the composition, which may be combined with any of the above aspects, the crystallized nickel salt has a potassium mass concentration of less than 10 g / kg. Preferably, the mass concentration of potassium would be less than 2.5 g / kg.

According to another optional aspect of the composition, which may be combined with any of the above aspects, the crystallized nickel salt has a nickel mass concentration equal to or greater than 128 g / kg, preferably 130 g / kg.

The objects, advantages and other features of the present invention will appear more clearly and will be better understood on reading the detailed and non-limiting description of the invention, with reference to the accompanying drawings.

SUMMARY DESCRIPTION OF THE DRAWINGS

The present invention and its embodiments are described and will be better understood in light of the following figures:

FIGS. 1A, 1B, 1C and 1D collectively refer to FIG. 1, which is a diagram of the steps of the nickel extraction and purification process in the form of the nickel and ammonium from A. mural according to an optional aspect of the present invention. Figure 2 is a graph showing a mass balance of the incineration of 1.0 kg dry matter of the mural plant A. according to an optional aspect of the present invention.

Figure 3 is a diffractogram of a sample of nickel and crude ammonium double sulfate salt according to an optional aspect of the present invention.

Figure 4 is a diffractogram of a purified ammonium and purified ammonium sulfate salt sample according to an optional aspect of the present invention.

Although the invention is described in connection with figures illustrating embodiments, it is understood that this is not intended to limit the scope of the invention to these embodiments. On the contrary, it is intended to cover all variants, modifications and equivalents which may be included as defined by the appended claims as well as the present description.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a hydrometallurgical process for treating hyperaccumulator plant ash of metallic elements using a low cost acid as an ash leaching agent. The original way of crystallizing a crystallized nickel salt (Ni) is chosen for nickel recovery from ash leachate.

The process according to the present invention makes it possible to economically and economically produce a crystallized nickel salt such as nickel sulphate salt, nickel chloride salt and nickel and ammonium sulphate salt from ashes. hyperaccumulating plants such as preferably the A. Murale variety. The high added value of the nickel sulphate salt of nickel and ammonium makes it a salt preferably targeted by the present invention. The process steps, as described and claimed, are however adaptable (type of ionic solution used, concentrations of the various compounds, etc.) to the production of other nickel salts such as nickel sulphate or nickel chloride. The present invention relates to the use of hyperaccumulator plants of metal elements comprising nickel to produce a crystallized nickel salt, obtainable by the process described and claimed.

The present invention also relates to a purified composition of a crystallized nickel salt derived from ash of plants hyperaccumulating metal elements comprising nickel, this composition can be obtained by the method described and claimed.

Figure 1 details the various possible steps of a process for producing the nickel sulfate salt of ammonium and ammonium according to an optional aspect of the process.

The hyperaccumulator plant is the mural A. plant (AP) which was previously dried and then incinerated in an oven at 550 ° C. The nickel-containing ash (AA1) was recovered and washed twice with deionized water. The washed ash (AA3) was then leached in a 1 M sulfuric acid solution at 85 ° C. for 4 hours. The supernatant leachate was recovered by filtration, the residue was washed, and then treated as waste (SW1).

It is understood that the acid solution used to leach the ashes may be a sulfuric acid solution as illustrated in FIG. 1, but it may also be chosen from nitric, hydrochloric, acetic, sulfurous acid solutions, a mixture of thereof, a derivative thereof or from the corresponding spent acid solutions.

This crude leachate (L1) was then brought to pH 5 by addition of 5M NaOH and then evaporated in a beaker by heating at 100 ° C using a hot plate. The treated leachate was recovered, the residue washed, then treated as waste (SW2). A specific mass of ammonium sulfate was dissolved in the treated leachate (L2), the temperature of which was then raised to 0 ° C for 6 hours. The supernatant (PE3) was then separated from the salt which crystallized (NS1) by filtration. The salt was then washed and dried at 25 ° C (NS2). The supernatant (PE3) and the washings (PE4), which still contain nickel, will be recycled in the process later. The salts are then solubilized in deionized water (L3). A specific amount of solid sodium fluoride was then added and dissolved in this solubilization solution. The supernatant was filtered off from the residue of MgF ₂ (SW3) that formed during this step. A specific amount of ammonium sulfate was added and dissolved in the purified solubilization solution (L4). The temperature of this solution was then raised to 0 ° C for 6 h. The supernatant (PE5) was then filtered off from the crystallized salts. These salts were washed, dried and stored at 25 ° C (NS3). The supernatant (PE5), which still contains nickel, will be recycled to the process later.

The following is an example of experiments carried out to produce nickel sulphate salt of nickel and ammonium. Similar results are expected to recover the nickel as a simple salt of nickel sulphate or nickel chloride, using ionic solutions comprising the sulphate or chloride anion. The dosage and concentrations of ionic solutions are easily adaptable to produce nickel sulphate or nickel chloride from the ashes of hyperaccumulating plants.

More particularly, the single salt of nickel sulphate is obtained very similarly to the nickel sulphate salt of nickel and ammonium by direct crystallization of the leachate of ash treated with sulfuric acid (L2, FIG. 1), without the addition of sulphate of sulphate. 'ammonium. The crystallized salt of nickel sulphate can then be separated from a remaining supernatant. The crystallized salt of nickel sulphate can then be re solubilized in the presence of sodium fluoride in order to purify it of its magnesium content by forming a residue of magnesium fluoride. In general, the process steps remain the same as those illustrated in FIG. 1, except that the successive cold crystallizations, in order to obtain the nickel sulphate salt with increasing purity, are carried out without the addition of sodium sulfate. 'ammonium. In the case of the nickel chloride salt, the washed ashes (AA3, Figure 1) are leached with a solution of hydrochloric acid (HCI) in place of sulfuric acid (H ₂ SO ₄ ). As for the nickel sulphate salt, the process steps then remain similar to the production of nickel and ammonium double sulphate salt, apart from the fact that the successive cold crystallizations are carried out without the addition of sulphate of sulphate. ammonium.

Moreover, the production of crystallized salt of nickel chloride could alternatively be done by adding an ionic solution comprising sulfuric acid and an excess of brine, during step (a) in order to obtain livixiat ( L1, Figure 1). This alternative avoids the use of hydrochloric acid, the cost is much higher than sulfuric acid. The brine would preferably be potassium chloride or sodium chloride. Chloride ions are thus introduced in excess into the leachate, thus preferably forming a nickel chloride salt during the crystallization step.

It is understood that the crystallization step is preferably a cold crystallization but could also be a crystallization by evaporation of water, or any other crystallization technique known to a person skilled in the art.

The case of nickel sulphate salt of nickel and ammonium is here described in more detail in the light of the following example.

EXAMPLE

1. Experiments carried out 1 .1. Sampling and plant preparation

A mural samples were collected at Pojske in the Pogradec region of Albania (latitude: 40 ° 59'55.28 "N and longitude: 20 ° 38'0.92" E). The soils of this region are ultramafic with Ni levels of the order of 3.0 g Ni / kg soil. The Ni content of the top layer of Pojke soil (0 to 25 cm deep) was measured at 3.44 mg Ni / kg. The Plants were harvested by hand, dried in the sun, and stored at room temperature (20 ± 2 ° C) pending experimentation.

The sampling campaign was conducted in July 2009, during the flowering period, at the Pojske site in the Pogradec region of Albania. The leaves were no longer attached to the stems and the flowers were widely present. There were also seeds from this period. A high Ni content in the upper parts of the plant was revealed during the previous sampling. This time it was shown that the stems, which represent 65.4% of the total biomass, were also rich in Ni. The stems had a Ni content of 6,100 mg Ni / kg DM which represented 51.8% of the total Ni. The entire aerial parts of the plant were therefore used as bio-ore. The roots have not been used because they remain difficult to harvest and weakly concentrated in Ni. The plants were crushed finely using a Kika-Werke mill, model M20.S3. The particle size was determined using successive sieves and it was shown that 39% of the particles had a size between 250 and 425 μηι. Metal concentrations in the wall A. dry mass were determined.

1 .2. Incineration of the plant A. mural and ash production

The incineration tests of A. mural were performed by depositing 25 g of finely crushed A. wall plant in a 150 mL porcelain crucible. The crucible was then introduced into an oven (1400 Furnace, Barnstead Thermolyne, Duduque, Lana, USA) whose temperature was raised to 550 ° C for 2 hours. During the incineration, the plants were regularly mixed using a stainless steel rod to avoid the formation of coal. After 2 h, the end of the burning of the plants was noted, the crucible was removed from the oven and the ashes were recovered, weighed and stored in dry at 25 ° C. A total mass of 1000 g of finely ground plant A. was thus treated in this way. The concentration of metals in the ash has been determined. 1 .3. A wall ash wash

The ash concentrated the major elements Ca, K, P, Mg and Ni which were found mainly in the form of carbonates for K and in oxide form for Mg and Ni. Ca was found either in Ca ₃ (PO ₄ ) 2 or in carbonate form. The high solubility of K ₂ CO ₃ , of the order of 1 12 g / 100 mL water (20 ° C), made it possible to eliminate a large part of the K contained in the ash by a simple washing with water. Ash was suspended in demineralized water (20% solids) in a 100 mL beaker. The suspension was stirred for 15 minutes. The supernatant was then separated from the solid residue (AA2) using a vacuum filtration system. The solid residue was placed in an oven at 100 ° C for 2 hours. The contents of major and minor elements of the wash water were measured. This wash water was then treated as waste (PE1). In order to optimize the removal of K from ash, the washing was carried out twice (PE2). The metal concentration in the mass of the washed ash (AA3) was determined. The mass balances for Ca, K, Mg and Ni of these ash washing steps were performed.

1 .4. Solubilization of Ni from washed ash from a mural

Nickel solubilization tests from the washed ash were carried out in order to optimize the leaching parameters, the molarity of the acid solution, the percentage of mass, the period and the leaching temperature. All the tests were carried out by adding a specific mass of ash in a 100 ml beaker containing 50 ml of a H ₂ SO _{4 solution} of specific molarity. The beaker was then placed in a bain-marie system so that the reaction proceeded at a temperature of 100 ° C. A first series of leaching was carried out using an acid solution at 0.25, 0.5 and 1.0 M in the presence of 10 g / l of ash. Samples (1 mL) were collected after 120 and 240 min, filtered and analyzed. In a second step, two series of leaching were carried out with solid concentrations of 100 and 150 g / l of ash. The leaching period was set at 240 min. A The first leaching run was carried out using an acid solution at 0.5, 1, 0 and 1 .125 M in the presence of 100 g / l of ash. The second leaching run was carried out using a 0.5, 1 .0 and 1 .125 M acid solution in the presence of 150 g / l of ash. The Ni contents of the leachates obtained were measured and the extraction yields were determined.

After determining the optimum conditions for washed ash leaching, recovery of Ni from this washed ash leachate was tested using a method of selectively crystallizing a double sulphate salt of Ni and ammonium. The crystallization tests were carried out using concentrated aqueous solutions of Ni obtained by the solubilization of the Ni contained in the ashes. The optimum parameters used during the leaching of Ni from the ashes previously determined consist of an ash concentration of 150 g / l, a molarity in H ₂ S0 ₄ of 1 .9 M and a reaction period of 240 min.

1 .5. Neutralization of A Wall Ash Leachate

The neutralization of the leachate was carried out by introducing 60 ml into a 100 ml beaker with magnetic stirring. The leachate was neutralized to pH 5 by the dropwise addition of a 5M NaOH solution.

1 .6. Evaporation of neutralized leachate

The neutralization step was followed by evaporation. The 100 mL beaker was placed on a hot plate equipped with a magnetic stirring system. The temperature control was ensured by the use of a heating plate equipped with a thermostat. The volume of the solution is controlled by the use of a graduated beaker. The leachate temperature was raised to 100 ° C. Evaporation continued until the leachate volume was reduced by a factor of 3. The supernatant was then separated from the residue by filtration using a vacuum pump placed on a filtration system. magnetic. Filtration was followed by washing the residue (4 mL) produced by this step of neutralization and evaporation of the leachate. The The leachate thus treated and the washing solution were then mixed. The major element and metal contents of this treated leachate (L2) were determined. The mass balance for Ca, K, Mg, Fe and Ni of this step of neutralization and evaporation of the leachate was carried out. The major element and metal contents of the filtered residue (SW2) were measured.

1 .7. Crystallization of ammonium sulphate salt

The principle of selective recovery of Ni from the treated leachate of ash from the mural A. plant was based on the crystallization of the double sulphate salt of Ni and ammonium from the wall ash ash leachate. The physical characteristic exploited during this crystallization was the low solubility of the double salt of Ni at 0 ° C. which is 1 .6 g / 100 ml. The formation of the double sulphate salt of Ni and ammonium from Ni sulphate and ammonium sulphate is presented in Equation 1.

Equation 1 NiSO ₄ + (NH ₄ ) ₂ SO ₄ + 6H ₂ O → Ni (NH ₄ ) ₂ (SO ₄ ) ₂ .6H ₂ O

Referring to Figure 1, the treated leachate (L2) was introduced into a 100 mL beaker. An amount of (NH ₄ ) ₂ SO ₄ equal to the stoichiometric amount of Ni contained in the leachate with an excess of 20% was added to the 100 mL beaker. Dissolution of the ammonium sulfate was ensured by the use of a hot plate provided with a magnetic stirring system. The leachate temperature was then raised to 60 ° C. After dissolving the ammonium sulfate in the leachate, the beaker was brought to room temperature (25 ° C). It was placed in a polystyrene box containing ice for a period of 6 hours. The leachate temperature was monitored and measured at 2 ° C. After 6 h, the leachate was removed from the ice. The supernatant was then separated from the crystallized salts (NS1) by filtration using a vacuum pump placed on a magnetic filtration system. Filtration was followed by washing the residue (2 mL) with deionized water at a temperature of 0 ° C. Filtration waters (PE4) were added to the supernatant (PE3) and the resulting solution (PE4) was analyzed. The Ni salts were dried at 25 ° C and then stored dry at room temperature. The metal concentration of these Ni salts was measured. The mass balance for Ca, K, Mg and Ni of this crystallization step of the crude Ni salts was carried out. XRD analysis of the salts produced by this crystallization phase from the leachate was carried out. The aim of this analysis was to identify the crystalline form (s) present in these salts so that the salts produced as double sulphates of Ni and ammonium can be identified.

1 .8. Purification of double sulfate salts of Ni and ammonium

In order to produce a high purity Ni salt, a step of purifying and removing magnesium from the Ni salts produced was carried out. This step was based on the very low solubility of magnesium fluoride which is 0.076 g / 100 mL. The solubility of NiF ₂ is much higher, ie 40 g / l.

Still with reference to FIG. 1, the double sulphate salts of Ni and ammonium (NS2) which have been produced are introduced into a 100 ml beaker and solubilized in a volume of 40 ml of demineralized water. This solution (L3) was analyzed. This solubilization solution of the Ni (L3) salts was then neutralized to pH 7 by the dropwise addition of a 5M NaOH solution. A default amount of NaF equal to 95% of the stoichiometric amount in Mg contained in the solubilization solution was added to the 100 mL beaker. The dissolution of NaF is described by Equation 2:

Equation 2 NaF ^ → Na ⁺ + F ^"

Dissolution of NaF was followed by evaporation. The 100 mL beaker was placed on a hot plate equipped with a magnetic stirring system. The volume of the solution is controlled by the use of a graduated beaker. The temperature of the solubilization solution was raised to 100 ° C (boiling). Evaporation continued until the leachate volume was decreased by a factor of 2. The formation of the MgF ₂ residue is described by Equation 3: Equation 3 Mg ²⁺ + 2 F MgF _{2 (S)}

The supernatant was then separated from the MgF ₂ residue (SW3) by filtration using a vacuum pump placed on a magnetic filtration system. The leachate thus treated (L4) was introduced into a 100 ml beaker. Samples (1 mL) were taken from the leachate thus treated (L4) and the metal concentration was measured. The metal concentrations in this purified solution of solubilization of salts were measured. The filtration residue obtained was recovered, dried at 100 ° C. and then stored at room temperature. The metal contents and major elements of this MgF ₂ residue were determined. The mass balance for Ca, K, Mg and Ni of this purification step of this solution (L4) was carried out. The beaker containing the purified Mg solubilization solution (L4) by addition of NaF was placed in a polystyrene box containing ice for a period of 6 h. The leachate temperature that was regularly monitored was measured at 2 ° C. The temperature control was ensured by the use of a thermometer. After the 6-hour period, the leachate was removed from the ice. The supernatant was then separated from the product salts (NS3) by filtration using a vacuum pump placed on a magnetic filtration system. Filtration was followed by washing the Ni salts with deionized water (2 mL) at a temperature of 0 ° C. The wash water (PE6) was mixed in the solution of the supernatant (PE5) whose contents of metals and major elements were then determined by ICP-AES analysis. The Ni salts (NS5) were then dried at room temperature and then stored in a dry state at a temperature of 25 ° C. The metal concentrations in these salts (NS5) were measured by ICP-AES analysis. XRD analysis of the salts produced by this purification phase was carried out. The aim of this analysis was to identify the crystalline species present in these salts in order to identify the salts produced as double sulphates of Ni and ammonium. 1 .9. TCLP test for metals

The danger as a toxic waste of the filtration residue resulting from the purification phase of the solubilization solution of the crude salts was evaluated by the TCLP test. The TCLP test developed by the USEPA assesses the hazardous or non-hazardous nature of solid waste for landfill (EPA Method 131 1) (US EPA, 1992, EPA, method 131 1, United States Environment Protection Agency, Washington , DC, United States, p.35). This test was carried out on the magnesium fluoride precipitate obtained during the purification phase of the double salts of Ni. The concentrations of toxic metals and total fluorides measured in the extraction liquid of the TCLP test were then compared with the maximum concentrations authorized in Quebec.

2. Experimental results

2.1. Preparation of the plant A. mural

The metal distribution within the tissues of the mural A. plant is given in Table 1. The results show a high Ni concentration of 9.7 g / kg of dry matter (DM). This result is consistent with the literature where Ni levels between 8.4 and 9.1 g Ni / kg DM in the mural A. plant were observed (Bani 2007, Shallari 1998). The high and harvestable parts (neglecting stems and roots) of A. mural have high levels of Ni. The stems have Ni contents, of the order of 6 g / kg DM, which is lower than the Ni contents observed in the upper parts. But the stems represent a high percentage of the total biomass of the plant, of the order of 65%. In the experiments mentioned above, the whole plant A. wall, rods and high parts, was used as a raw material to test the solubilization and recovery of Ni.

The results in Table 1 show high concentrations of Mg (4 g / kg DM), Ca (8.4 g / kg DM) and K (7.6 g / kg DM). These results are typical for plants growing on serpentine-based soils. The high Mg content is characteristic of hyperaccumulating plants such as A. mural that are adapted to the serpentine-based soils unique to the Pogradec region of Albania.

2.2. Incineration of the plant A. mural and ash production

A 1000 g mass of finely crushed A. wall plant (AH) was incinerated and 76.3 g of crude ash (AA1) was recovered. The results of the analysis of the metals in these ashes given in Table 1. These results show a high Ni content of 126 g / kg as well as high concentrations of primary primary elements K and P with contents of 121 g / kg and 16.8 g / kg and secondary major element, such as Ca with a content of 131 g / kg. Fe is found with significant levels of 10.9 g / kg. The presence of traces of As, Co, Cr, Cu, Mm, Mo, Pb and Se metals is noted. These results are explained by the behavior of metals during incineration.

 Table 1 Detailed composition of dry matter, ashes and ash washed from the plant A. mural

 Elements Concentration (g / kg MS)

 A. Wall Ash A. Wall Ash Washed

 (AP) (AA1) (AA3)

 Al 0.17 ± 0.03 2.62 ± 0.16 3.82 ± 0.08

 As 0.00 ± 0.01 0.01 ± 0.01 0.01 ± 0.01

 Ca 8.42 ± 1.10 131 ± 2.00 168 ± 3.14

 Cd 0.00 ± 0.00 0.00 ± 0.00 0.00 ± 0.00

 Co 0.01 ± 0.00 0.09 ± 0.01 0.08 ± 0.01

 Cr 0.00 ± 0.00 0.06 ± 0.00 0.10 ± 0.00

 Cu 0.03 ± 0.03 0.11 ± 0.07 0.10 ± 0.03

 Fe 0.41 ± 0.03 10.91 ± 0.43 15.5 ± 0.31

 K 7.63 ± 1.00 121 ± 1.28 17.2 ± 0.70

 Mg 3.93 ± 0.49 71.1 ± 2.54 90.6 ± 1.63

 Mn 0.01 ± 0.00 0.33 ± 0.01 0.48 ± 0.07

 Mo 0.00 ± 0.01 0.03 ± 0.02 0.01 ± 0.00

 Neither 9.73 ± 1.22 126 ± 0.09 158 ± 1.78

 P 0.80 ± 0.07 16.8 ± 0.11 21.1 ± 0.27

 Pb 0.02 ± 0.00 0.04 ± 0.01 0.04 ± 0.01

 Se 0.00 ± 0.05 0.00 ± 0.00 0.04 ± 0.01

Zn 0.03 ± 0.01 0.35 ± 0.05 0.35 ± 0.02 Figure 2 shows a mass balance performed on the metals and major elements contained in the dry matter of the mural A. plant and those contained in the ash. This review shows that the major primary elements K and P and secondary Mg tend to remain in the ashes, recovery rates are close to 100%. The recovery rate of Ni after incineration of the mural A. plant is 98.9 ± 0.1%. The results for As, Cd, Co, Cr, Cu, Mn, Pb, Se and Zn show the high volatility of As, Cd and Se during incineration with recovery rates close to 2.7%, 0% and 3.5. %. The volatility of these metals shows that care must be taken to treat the flue gases. Finally, the results show that Ca, Co, Cr and Zn remain in the ashes during incineration with recovery rates of 1 18%, 96.6%, 1 19% and 92.8%.

The washing of the ashes of A. two-stage mural causes a 23.6% decrease in the mass of ash. The analysis of the metals and major elements of the two wash waters PE1 and PE2 shows that these solutions are highly concentrated in K with contents of 9090 mg / L for PE1 and 346 mg / L for PE2. Other metals and major elements such as Ni or Mg are present in these wash waters only in trace amounts.

The mass balance presented in Table 2 gives the recovery rates for K, Ca, Mg and Ni: 10.9% of K, 98.3% for Ca, 97.3% of Mg and 96.1% of Ni. These results show that the two washes of the crude ash removed 89.1% of the K present as carbonates in the ash while concentrating the content of other metals such as Ni. Metal analysis in washed ash (AA3) was performed (Table 1). These results show a high Ni content of 159 g / kg, as well as high concentrations of major elements Ca, Mg and P with contents of 168 g / kg, 90.6 g / kg and 21 g / kg. The secondary major element K has only a content of 17.2 g / kg. Fe is found in ashes with significant levels of 15.5 g / kg. Table 2 Mass balance and recovery percentages for Ca, K, Mg and Ni during the treatment of 100 g of A. mural ash by two successive washing steps

Mass (mg) Recovery (%)

 Ca K Mg Ni Ca K Mg Ni

Crude ashes 13 051 12 107 7 1 13 12 612 100 100 100 100 (AA1)

 PE1 38.1 10 454 2.8 0.1 0.3 86.3 0.0 0.0

PE2 22.3 467 1.2 0.1 0.2 3.9 0.0 0.0

Washed ashes 12 833 1 316 6 921 12 1 17 98.3 10.9 97.3 96.1 (AA3)

2.3. Solubilization of Ni from washed ash from a mural

Table 3 shows the effect of the H ₂ SO ₄ concentration on the solubilization of Ni from the wall ash. The results show that for a total solids concentration of 10 g / L, the recovery rates of Ni for H ₂ SO ₄ acid concentrations of 0.25, 0.5 and 1 M were respectively measured at 91.2, 94.7 and 96.6. % after a reaction time of 120 min and 98.9, 100 and 100% for a reaction time of 240 min.

The reaction time seems to be an important parameter for the dissolution of the Ni. A duration of 2 h results in a variable and increasing recovery rate as a function of the acid concentration. A duration of 4 h allows a low variability of the solubilization of Ni depending on the acid concentration whose recovery rate remains close to 100%. A duration of 4 ha was therefore chosen for the following experiments. But, the solid concentration of 10 g / L does not seem suitable for industrial recovery of the ash of mural A. Higher solids concentrations were therefore tested. Table 3 Yield of Ni Extraction from Wall Ash As a Function of Acid Concentration, Reaction Time and Solid Concentration

Molarity Test Time ST Yield

 (M) (h) (%) (% Ni)

 1 0.25 2 1 91.2

 2 0.25 4 1 98.9

 3 0.5 2 1 94.7

 4 0.5 4 1 100

 5 1 .0 2 1 96.6

 6 1 .0 4 1 100

 7 1 .0 4 10 93.2

 8 0.5 4 10 0.00

 9 1 .13 4 10 96.2

 10 1 .5 4 10 100

 1 1 2.0 4 15 100

 12 1 .7 4 15 75.0

 13 1 .8 4 15 93.3

 14 1 .9 4 15 96.2

Solid concentrations of 100 g / L and 150 g / L were tested for the solubilization of Ni from the ash of A. mural. An assay of the total basicity of the ashes of the mural A. plant was carried out (results not shown) and made it possible to better estimate the amount of H ₂ SO ₄ acid to be used for dissolving the Ni contained in these ashes. .

Table 3 shows the effect of the H ₂ SO ₄ concentration on the solubilization of Ni from mural ash with an ST concentration of 100 g / L. The recovery rates of Ni for H ₂ SO ₄ concentrations of 0.5, 1 .0, 1 .125 and 1 .5 M were measured at 0, 93.2, 96.2 and 100%, respectively, after a reaction period of 4 h.

Table 3 also shows the effect of H ₂ SO ₄ concentration on Ni solubilization from mural ash with an ST concentration of 150 g / L. Ni recovery rates for H ₂ SO ₄ concentrations of 1 .7, 1 .8, 1 .9 and 2.0 M were measured at 75, 93.3, 96.2 and 100%, respectively, after 240 min. These experiments were conducted in order to solubilize a minimum of 95% of the Ni contained in the dry mass of A. mural with a minimum amount of sulfuric acid. The cost of using sulfuric acid is not prohibitive (with an average price of $ 100 / tonne H ₂ S0 ₄ ). But it is important to limit the quantity used because the process currently used for the recovery requires the neutralization of the leachate of A. mural ash and therefore the use of concentrated NaOH whose price is much higher than that of the acid. sulfuric acid (with an average cost of $ 500 / tonne of 100% NaOH in Quebec).

The economic study which followed the series of experiments showed that the ash of the A. wall plant was leached with a 15% ST percentage with a 1 .9 M acid solution for 240 min at 90 ° C. allowed optimal Ni solubilization for the purpose of commercial development.

According to the previous experiments and Table 3, ash leaching with a 1 .9 M H ₂ SO _{4 solution} with a 15% ST percentage at 95 ° C. for 4 hours allowed the dissolution of 96.2% of the available Ni. .

The analysis of the major elements and metals contained in the ash leachate (L1, with reference to Figure 1) is presented in Table 4. The results show that L1 is highly concentrated in Ni with a concentration of 10 160 mg Ni / L as well as in primary major elements such as K and P with concentrations of 1162 mg K / L and 1418 mg P / L, secondary major elements such as Mg and Ca with respective concentrations of 5 984 mg mg / L and 583 mg Ca / L. Fe is present in the leachate (L1) at a non-negligible concentration of 61 1 mg / L. Concentrations of trace metals such as Al, Co and Fe were measured with respective values of 195 mg Al / L, 7.51 mg Co / L, 61 1 mg Fe / L. 2.4. Neutralization of A. wall ash leachate at pH 5 and evaporation

The contents of metals and major elements of the treated leachate (L2, with reference to Figure 1) obtained by neutralization and evaporation of the raw leachate are presented in Table 4. The results show high levels of Ni, Ca, Mg and K with concentrations of 21,280 mg / L, 927 mg / L, 13,680 mg / L and 3,228 mg / L and a low Fe content of 13.7 mg / L. The mass balance and the recovery yields (Table 5) show that the neutralization and evaporation stages make it possible to recover 91.7% of the Ni and 95.8% of the Mg contained in the raw leachate, whereas only 4.9% of the Fe and 83.4% of the Ca are recovered during this step. The high Ni content in the treated leachate (L2) will be optimal for the crystallization phase of the double sulphate salt of Ni and ammonium (Mullin, 1967, J. Chem., And Eng., Data, 12, 4, p. 516-517, Tavare et al., 1985).

 Table 4 Detailed composition of the ash leachate of the mural A. plant, ash leachate treated by neutralization at pH 5 and dehydration of the crystallization water

Elements Leachate (L1) Leachate treated (L2) Water crystallization (PE3)

 (mg / L) (mg / L) (mg / L)

 Al 195 8.07 0.03

 As 0.65 1.67 1.03

 Ca 583 1 086 927

 Cd 0.06 0.23 0.00

 Co 7.51 14.6 1 .27

 Cr 3.91 1.92 0.09

 Cu 3.09 1 .97 0.36

 Fe 61 1 71.5 13.7

 K 1 162 3 228 1 301

 Mg 5 984 13 680 3 486

 Mn 26.8 45.2 17.2

 Mo 0.29 1.21 0.18

 Neither 10 160 21 280 828

 P 1 418 1 558 106

 Pb 2.63 2.88 0.12

 Se 12.4 0.33 0.00

Zn 24.4 10.1 0.40 Table 5 Mass balance and recovery percentages for Ca, K, Mg, Ni and Fe during the treatment of the leachate of 5 g of mural ash by two stages of neutralization at pH 5 and evaporation

Mass (mg) Recovery (° ^")

 Ca K Mg Ni Fe Ca K Mg Ni Fe

Leachate Brut (L1 41 .7 85.1 441 743 44.4 100 100 100 100 100

Leachate treated (L2)) 34.7 104 422 682 2.19 83.4 121 95.8 91.7 4.93

Residue Fe 27.8 8.91 8.66 70.8 52.8 27.8 10.5 1 .96 9.52 1 18

A high concentration of K was also measured with a value of 3228 mg K / L, as well as high levels of secondary major elements, Mg and Ca respectively of 13 680 mg Mg / L and 1086 mg Ca / L. Other metals such as Co, Mn or Zn are found in lower concentrations in the range of 14.6 mg Co / L, 45.2 mg Mn / L and 10.1 mg Zn / L. Other metals such as As, Cd or Cu are present but remain in concentration ranges close to 1 mg / L. The analysis of the metals and major elements of the Fe residue (SW2) shows high Fe contents of the order of 97 g / kg and Ni of the order of 108 g / kg (Table 6). The high Fe content is explained by the precipitation of Fe in the form of Fe hydroxide during the neutralization of the leachate at pH 5 according to the reaction described by Equation 4.

Equation 4 Fe ³⁺ + 3 OH ^" <→ Fe (OH) _{3 (s)}

The high Ni content of the Fe residue can not be explained by the precipitation of Ni in the hydroxide form. Indeed, at pH 5, the solubility of the Ni hydroxide is very high during the evaporation phase. In addition, despite the high sulphate content of the leachate, Ni sulphate has a high solubility, 630 g / L at 80 ° C (Linke, 1965) and would not precipitate according to the reaction described by Equation 5. Equation 5 Ni ²⁺ + S0 ₄ ^{2 "} <→ NiS0 _{4 (s)} Table 6 Detailed composition of the iron residue, the crude Ni and ammonium sulphate salt and the MgF ₂ residue

Elements Iron Residue (SW2) Raw Salt (NS2) Residue of mgF2 (SW3)

 (g / kg) (g / kg) (g / kg)

 Al 30.9 ± 14.1 0.11 ± 0.16 2.19 ± 0.24

 As 0.02 ± 0.01 0.00 ± 0.00 0.01 ± 0.01

 Ca 14.3 ± 6.93 0.56 ± 0.23 11.9 ± 12.1

 Cd 0.01 ± 0.00 0.00 ± 0.00 0.00 ± 0.00

 Co 0.20 ± 0.07 0.07 ± 0.05 0.17 ± 0.02

 Cr 0.53 ± 0.25 0.00 ± 0.00 0.01 ± 0.01

 Cu 0.25 ± 0.10 0.00 ± 0.00 0.03 ± 0.02

 Fe 97.8 ± 44.4 0.80 ± 0.13 1.61 ± 1.52

 K 9.14 ± 4.02 8.23 ± 0.91 2.80 ± 0.66

 Mg 14.6 ± 8.5 25.1 ± 4.9 189 ± 49

 Mn 1.19 ± 0.42 0.16 ± 0.08 0.64 ± 0.51

 Mo 0.01 ± 0.00 0.00 ± 0.00 0.01 ± 0.00

 Ni 108 ± 61 60.2 ± 7.7 78.0 ± 68.4

 P 124 ± 23 0.38 ± 0.03 6.59 ± 4.45

 Pb 0.2 ± 0.2 0.01 ± 0.00 0.01 ± 0.01

 Se 0.00 ± 0.02 0.00 ± 0.01 0.00 ± 0.00

 Zn 2.48 ± 0.97 0.02 ± 0.01 0.12 ± 0.02

 However, there may be adsorption of Ni on the ferric hydroxide during its precipitation, which could explain the loss of Ni in this residue (Julien, 1994, Wat Res., 28, 12, 2567). -2574). Finally, it is possible that a low efficiency washing is at the origin of the high concentration of Ni of the residue.

2.5. Crystallization of ammonium sulphate salt

The concentrations of primary and secondary metals and major elements in the salts (NS2, with reference to Figure 1) collected after crystallization are presented in Table 6. The results showed that these salts had a high concentration of Ni and Mg with respective values of 60.2 g / kg and 25.1 g / kg. Potassium (K) is still present in the salt but at a concentration of 8.23 g / kg. Other metals are present only in trace amounts. Table 7 shows the mass balance and the recovery rate of the crystallization phase from the treated leachate. These results show a recovery rate during crystallization of 64.7% for Ni, 41.3% for Mg and 72% for K. The results also revealed the very low recovery of Ca (16.9%).

These results showed that crystallization did not specifically recover Ni and that Mg and K were included in the crystal structure of salts formed at significant levels. The high percentage of recovery of Mg during crystallization will not allow a cost-effective purification of the salt of double sulfate of Ni and ammonium. A specific Mg removal step is therefore necessary to obtain a high purity Ni salt having acceptable commercial value. The presence of a single crystalline form, that of the double sulfate salt of Ni and ammonium in the crude salt samples filtered during this step was demonstrated by X-ray diffraction (FIG. 3).

The contents of metals and major elements of the solution of the crystallization solution (PE3) are not negligible. The results show significant K, Mg and Ni contents with measured values of 1 301 mg / L, 3 486 mg / L and 828 mg / L. This crystallization solution can be recycled in the process.

Table 7 Mass Balance and Recovery Percentages for Ca, K, Mg and Ni in Crystallization of the Ni-ammonium and crude ammonium sulphate salt from treated leachate

Mass (mg) Recovery ('%)

 Ca K Mg Ni Ca K Mg Ni

Leachate treated (L2) 34.8 104 423 682 100 100 100 100

Crude salt (NS2) 5.9 74.5 174 442 41 .3 72 41 .3 64.7

Process Water 29.2 42 1 10 184 84.1 40.6 26.0 26.9 (TP4) 2.6. Purification of the double sulphate salt of Ni and ammonium

The removal of the Mg contained in the double sulphate salts of Ni and ammonium was achieved by a purification step. This step involves the solubilization of the salts produced in an aqueous solution of demineralised water.

The metal concentrations for the elements Ni, K and Mg in this solubilization solution (L3, with reference to FIG. 1) are presented in Table 8. These results showed that the solubilization solution of the crude salts of Ni had a content of high in Ni with a measured concentration of 9604 mg Ni / L. In contrast, the concentration measured in Mg in the solution to be purified was 4,187 mg Mg / L. This high Mg content poses a problem of purity during the crystallization of the double salt. Neutralization of this solution to pH 7 and hot addition to this solution of a specific amount of solid NaF were carried out. The aqueous solution was evaporated and the MgF ₂ residue was removed from the supernatant by filtration. The concentrations of metals and major elements contained in the purification solution (L4) are presented in Table 8. These results showed that the purification solution (L4) had a high content of Ni with a measured concentration of 9 1 16 mg Ni / L. In contrast, the concentration measured in Mg in the purification solution was only 38.9 mg Mg / L.

The removal percentages for Ni, K and Mg during the purification step are presented in Table 9. These results showed percentages of 0.75% for K, 88.8% for Mg and 0.47% for the Ni. Thus, the purification has only slightly changed the Ni and K concentrations. The Mg content has greatly decreased, with a removal percentage of nearly 90%.

These results tend to explain that the Mg contained in the solubilization solution of the double sulphate salts of Ni and ammonium was precipitated in the form of Mg fluoride poorly soluble and therefore solid and removed by filtration. Table 8 Detailed composition of the solubilization solution of the salt of double sulphate of Ni and ammonium, of the solubilization solution of this purified salt by adding NaF and the crystallization solution

Elements Solubilized salt (L3) Solubilized and purified salt (L4) Sol. of crystallization (PE5)

 (mg / L) (mg / L) (mg / L)

 Al 0.00 0.00 0.00

 As 0.00 0.00 0.00

 Ca 58.2 139 1 1 .1

 Cd 0.00 0.00 0.00

 Co 15.5 12.6 2.40

 Cr 0.81 0.70 0.14

 Cu 0.65 1.58 0.51

 Fe 44.6 1.23 0.00

 K 1 1 10 1 365 1 126

 Mg 4 187 38.9 31 .1

 Mn 32.2 15.8 6.80

 Mo 0.23 0.20 0.19

 Ni 9 604 9 1 16 1 498

 P 43.6 6.64 8.77

 Pb 0.69 0.93 0.05

 From 0.00 0.00 0.00

 Zn 0.00 0.00 0.00

 Table 9 Mass balance and recovery percentages for Ca, K, Mg and Ni during the crystallization of the double sulphate salt of Ni and ammonium purified from the solubilization solution

Mass (mg) Recovery (%)

 Ca K Mg Ni Ca K Mg Ni

Ground. solub. (L3) 5.9 74.5 174 442 100 100 100 100

MgF ₂ (SW3) 0.51 0.56 155 2.07 8.71 0.75 88.8 0.47

Process water 1 .42 38.5 7.07 49.7 24.2 51 .7 4.05 1 1 .3 (TP4)

 Purified salt (NS3) 7.12 8.52 2.42 427 121 1 1 .4 1.39 97.0

The analysis of the metals and major elements of the residue of magnesium fluoride (SW3) filtered during the purification of the solubilization solution of the salts was conducted and the results are shown in Table 6. They show high levels of Mg and Neither with respective concentrations of 189 g / kg and 78.0 g / kg. The high Mg content is explained by the precipitation of 90% of the Mg present in solution in the form of MgF ₂ , a highly insoluble compound as predicted by the solubility of MgF ₂ (Table 10).

The presence of Ni can not be explained by the precipitation in the form of soluble Ni hydroxide at neutral pH. In addition, a precipitation in the form of soluble NiF ₂ at neutral pH at 90 ° C is also not possible. The formation of NiF ₂ is described by Equation 6.

Equation 6 NiF ₂ ^ Ni ²⁺ + 2F ^"

The amount of F ^" necessary to precipitate the Ni present in the solubilization solution of the crude salts (L3) as NiF ₂ should be 40 times greater than the amount actually introduced in the protocol. adsorption of Ni during the precipitation of MgF ₂ at pH 7 may be the explanation for the presence of Ni in this residue.This co-precipitation may be due to the too sudden introduction of the specific amount of NaF in solid form in solution L 3. The introduction of the same quantity of NaF in liquid form drop by drop would undoubtedly allow a better specificity of the precipitation of MgF ₂ without trapping Ni.

Table 10 Solubility of MgF ₂ and NiF ₂ at 20 ° C and 90 ° C (Linke, 1965)

Solubility Chemical Compounds (g / L)

 20 ° C 90 ° C

MgF ₂ 0.075 Insoluble

NiF ₂ 25.6 25.9

 In order to produce a Mg and K purified ammonium sulfate and Ni sulfate double salt, recrystallization was performed from the purification solution. Salts (NS5) which were crystallized during this phase were recovered by filtration. The concentrations of metals and major elements within the crystal structure of these salts (NS5, with reference to

Figure 1) were measured and the results are presented in Table 1 1. These results showed Ni, K and Mg contents measured at 132 ± 3.13 g Ni / kg, 2.35 ± 0.10 g K / kg and 0.70 ± 0.02 g Mg / kg. Other metals or major elements, such as Ca, Fe or Cu, are only present in trace amounts at concentrations below 1 g / kg of salts.

Crystallization from the solubilization solution allowed the increase of the Ni content in the purified salt. It was caused by recrystallization of this double salt at 0 ° C where 97% of Ni, 1 1 .4% of K and only 1 .4% of Mg were included in the crystal structure of the double salt. Table 8 gives the contents of metals and major elements of the crystallization solution (PE5, with reference to FIG. 1). These results show significant K and Ni contents with measured values of 1,126 mg / L and 1,498 mg / L. The Ni of this water of crystallization can be recycled in the production process of Ni from the mural plant A.

The presence of the double sulphate salt of Ni and ammonium in the purified salts (NS5) which have been filtered during this step has been demonstrated by XRD, as shown in FIG. 4. These results are almost identical to those shown in FIG. those observed in Figure 3.

Table 11 Detailed composition of purified ammonium and purified ammonium sulphate salt (NS5)

Elements Concentration (g / kg)

 Al 0.00 ± 0.00

 As 0.00 ± 0.00

 Ca 2.24 ± 0.31

 Cd 0.00 ± 0.00

 Co 0.10 ± 0.1 1

 Cr 0.00 ± 0.00

 Cu 0.04 ± 0.01

 Fe 0.00 ± 0.01

 K 2.35 ± 0.10

 Mg 0.70 ± 0.02

 Mn 0.09 ± 0.10

 Mo 0.01 ± 0.00

 Neither 132 ± 3.13

 P 0.17 ± 0.16

 Pb 0.02 ± 0.00

 Se 0.00 ± 0.00

Zn 0.00 ± 0.00 2.7. TCLP test for metals

The measurements of concentrations of contaminants, toxic metals and total fluorides in the TCLP test extraction liquid from the magnesium precipitation residue, resulting from the purification of the double salts of Ni and determination of the fluorides, were carried out and compared with the maximum authorized concentrations in Quebec (Table 12). The results show that the concentrations measured for toxic metals such as As, Cd or Pb are below the standards in force in Quebec with measured values of 0.2 ± 0.02 mg As / L, 0.03 ± 0.001 mg Cd / L and 0.42 mg Pb / L. In addition, the total fluoride concentration is 72.3 ± 10.9 mg F / L, which is lower than the Quebec standard of 150 mg F / L.

Claims

1. A method of extracting nickel from ash of a hyperaccumulator plant of metal elements comprising nickel, the process comprising using an ionic solution to recover nickel as a crystallized nickel salt.

The method of claim 1, comprising the steps of:

(a) leaching the ashes by adding an acidic solution, thereby producing a leachate comprising nickel and iron;

(b) selectively precipitating iron from the leachate by adding a basic solution thereby producing a purified nickel-rich supernatant and an iron-rich precipitate; and (c) crystallizing the crystallized nickel salt.

The process of claim 2 comprising washing the ash with an aqueous solution prior to step (a) to reduce the ash content of soluble inorganic elements and chlorine.

4. The process according to claim 2 or 3, wherein the acid solution is added to the ashes in step (a) with a molar concentration of 0.25 mol / L to 7 mol / L.

5. The process according to any one of claims 2 to 4, wherein the basic solution is added to the leachate with a concentration chosen so that the leachate has a pH between 3.2 and 7, to precipitate the iron contained in leachate.

6. The process according to any one of claims 2 to 5, wherein the crystallization (c) is cold.

The process of any one of claims 2 to 6 comprising purifying the crystallized nickel salt to reduce the magnesium content.

The process of claim 7, wherein the purification comprises the following steps:

(d) solubilizing crystallized nickel salt in water;

(e) adding a sufficient amount of an inorganic compound comprising an anion selected to produce an anion-magnesium precipitate;

(f) filtering the anion-magnesium precipitate to separate it from a purified second supernatant; and

(g) cold recrystallization of a crystallized salt of purified nickel by addition of the ionic solution in the purified second supernatant.

The process of claim 8, wherein the anion is the fluoride ion.

The process of claim 8 or 9, wherein the inorganic compound is sodium fluoride to produce a magnesium fluoride precipitate in step (e).

1 1. The process according to any one of claims 2 to 10, wherein the ionic solution is an ammonium sulphate solution for recovering nickel in the form of a nickel ammonium sulfate sulphate salt, the ammonium sulphate being added to the supernatant purified during step (c) of crystallization of the nickel sulfate salt of nickel and ammonium.

The process according to any one of claims 2 to 10, wherein the ionic solution is a sulfuric acid solution for recovering the nickel in the form of a nickel sulphate salt, the sulfuric acid being used as the the acid solution during the step (a) of leaching the ashes.

13. The process according to any one of claims 2 to 10, wherein the ionic solution is a hydrochloric acid solution for recovering nickel in form. a nickel chloride salt, the hydrochloric acid being used as the acid solution during the step (a) of leaching the ashes.

14. The process according to any one of claims 1 to 13, wherein the ash is obtained by incineration of the plant hyperaccumulator of metal elements previously dried.

15. The method according to any one of claims 1 to 14, wherein the hyperaccumulator plant comprises or is a plant of the mural species A.

16. Use of a hyperaccumulator plant of metal elements comprising nickel for the production of a crystallized nickel salt from the ashes of the hyperaccumulator plant.

17. The use according to claim 16, wherein the crystallized nickel salt is nickel ammonium sulphate salt, nickel sulphate salt or nickel chloride salt.

18. A purified composition of crystallized nickel salt derived from ash of a hyperaccumulator plant of metallic elements including nickel.

19. The purified crystallized nickel salt composition of claim 18, wherein the crystallized nickel salt is the nickel ammonium sulphate salt.

20. The purified crystallized nickel salt composition according to claim 18 or 19, having a nickel mass concentration equal to or greater than 128 g / kg.