AU2001261912B2 - Treatment of effluent - Google Patents

Description

WO 01/92162 PCT/AU01/00641 TREATMENT OF EFFLUENT FIELD OF THE INVENTION The present invention relates to a process for the treatment of effluent and relates particularly, though not exclusively, to a process for the treatment of industrial effluent from mining, mineral processing and metal-production plants.

BACKGROUND TO THE INVENTION Gases emitted during various phases of the steel making process are scrubbed to remove particulate material before discharge to atmosphere. Some steel mills suffer from a particularly serious problem in maintaining the scrubber water circuit which may be summarised as follows: The hot gases are scrubbed to recover particulate materials such as dust, coke/carbon and metal fumes, but carbon in the flue gas forms carbonic acid in the scrubber water, which dissolves particulates such as iron, calcium, sulphates, etc.

The scrubber effluent temperature is between 50-65°C and is too hot to adsorb oxygen from the atmosphere.

The scrubber effluent is cooled in cooling towers, which by design are aerators.

The cooling water adsorbs oxygen, which oxidises the iron, which in turn precipitates onto the cooling tower packing causing a reduction in efficiency, and eventually the weight of precipitate collapses the packing.

An alkali, currently caustic soda, is used to control pH, but contaminant salts such as sulphate ions accumulate and a bleed stream must be discarded to avoid pipeline and equipment scaling WO 01/92162 PCT/AU01/00641 -2- A substantial volume of cooling water is discarded to evaporation ponds. This is both costly and wasteful of a scarce resource.

Very significant chemical dosing costs are incurred in maintaining the water quality.

Water treatment consultants are at a loss to find a solution to the problem. Their best solution is to build bigger evaporation ponds.

Scale and suspended solids in cooling water can lead to serious equipment damage.

There is another serious problem with cyanide generation in certain areas within steel making plants. Depending on the process within the steel making plant, the cyanide is produced as a result of high temperature reactions between carbon and nitrogen. This problem is probably common to most steel making processes. The cyanide accumulates in the scrubber circuit water and a bleed stream must be discarded to evaporation ponds to keep the cyanide concentration from attaining dangerous levels. The presence of cyanide in the evaporation ponds presents an environmental hazard, particularly if some of the wastewater leaks into ground water. A typical scrubber water circuit contains between 100 and 150ppm free cyanide. This is a dangerously high concentration, as 50 ppm cyanide in tailing water is fatal to birds and 100-150 ppm cyanide solution would be fatal to an adult if swallowed.

SUMMARY OF THE INVENTION The present invention was developed with a view to providing a process for the treatment of effluent that is effective in substantially reducing the concentration of soluble metal salts and/or cyanide in the effluent. It will be apparent that the process may have application to the treatment of effluent's from sources other than steel mills.

Throughout this specification the term "comprising" is used inclusively, in the sense that there may be other features and/or steps included in the invention not expressly defined or comprehended in the features or steps subsequently defined or described.

WO 01/92162 PCT/AU01/00641 -3- What such other features and/or steps may include will be apparent from the specification read as a whole.

According to one aspect of the present invention there is provided a process for the treatment of effluent, the process comprising the steps of: diverting a stream of effluent containing dissolved metals and/or sulphates through an in-line reactor; reacting the effluent with an oxygen-containing gas in the in-line reactor wherein said dissolved metals and/or sulphates are precipitated out of solution; separating a clarified solution from the precipitated sludge; and, recycling the clarified solution.

Typically the effluent is an aqueous effluent from an industrial processing plant.

Preferably lime or caustic soda is added to the effluent prior to said step of reacting the effluent in order to control the pH. Preferably said oxygen-containing gas is injected 2 0 directly into the in-line reactor adjacent an intake port.

Advantageously the in-line reactor employed in the process of the invention is a FILBLAST (registered trade mark) multiphase staged passive reactor such as that described in US Patent No. 5,741,466. The contents of US 5,741,466 are incorporated 2 5 herein by reference.

According to another aspect of the present invention there is provided a process for the treatment of effluent, the process comprising the steps of: diverting a sidestream of effluent containing cyanide through a first in-line mixer; WO 01/92162 PCT/AU01/00641 -4reacting the effluent with Caro's acid in the first in-line reactor wherein said cyanide is oxidised to harmless cyanate; and, recycling the effluent with cyanate.

Preferably the Caro's acid is produced immediately prior to reacting in said in-line reactor.

Advantageously the Caro's acid is produced by reacting sulphuric acid and hydrogen peroxide in a second in-line reactor which discharges directly into a feedline immediately prior to the first in-line reactor.

According to a further aspect of the present invention there is provided a process for the treatment of scrubber effluent, the process comprising the steps of: diverting a stream of scrubber effluent containing dissolved metals and/or sulphates and cyanide through a first in-line reactor; reacting the scrubber effluent with an oxygen-containing gas in the first in-line reactor wherein said dissolved metals and sulphates are precipitated out of solution; separating a clarified solution from the precipitated sludge; reacting the clarified solution with Caro's acid in a second in-line reactor wherein said cyanide is oxidised to harmless cyanate; and, recycling the clarified solution to the scrubber circuit.

BRIEF DESCRIPTION OF THE DRAWINGS In order to facilitate a better understanding of the nature of the invention one embodiment of the process for the treatment of industrial effluent will now be described in detail, by way of example, with reference to the accompanying drawings, in which: WO 01/92162 PCT/AU01/00641 Figure 1 is a conceptual flow diagram illustrating one embodiment of a preferred process for the treatment of industrial effluent in accordance with the invention; Figure 2 illustrates graphically the reduction in iron concentration achieved using the preferred process for the treatment of scrubber effluent in accordance with the invention; Figure 3 illustrates graphically the reduction in TDS, calcium hardness and sulphate concentration using the preferred process in the treatment of scrubber effluent; Figure 4 illustrates a conceptual flow diagram illustrating a second embodiment of a preferred process for the treatment of industrial effluent in accordance with the invention; Figure 5 illustrates graphically the reduction in CaO consumption achieved using the preferred process for the treatment of acid mine water effluent in accordance with the invention; and, Figure 6 illustrates graphically the reduction in iron concentration, heavy metal, salt contaminants and sulphate concentration using the preferred process in the treatment of acid mine water effluent.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS As noted above, cyanide gas is formed during the steel-making process and is adsorbed in the gas scrubber circuit. As shown in Figure 1, a typical gas scrubber circuit comprises a scrubber 10 through which the scrubber wash water falls and is collected in a scrubber bottoms surge tank 12. The scrubber wash water is recycled via pump 14 to the top of the scrubber 10. It will be seen that as cyanide is adsorbed in the gas scrubber circuit the concentration of cyanide in the scrubber wash water can rise to dangerous levels. Cyanide concentration is controlled by sending a bleed stream 16 to an evaporation pond.

However, as noted above this presents the potential risk of cyanide leakage into the environment. The preferred process of treating this bleed stream, as illustrated in Figure 1, addresses this problem.

WO 01/92162 PCT/AU01/00641 -6- A cyanide monitor 18 continuously analyses the concentration of cyanide in the scrubber bottoms surge tank 12. An upper limit of the cyanide concentration is set for the monitor 18, and whenever the concentration exceeds this upper limit a flow control valve 20 is operated to control the amount of bleed in the bleed stream 16. The large volume of wash water in the scrubber circuit buffers the rate of change in cyanide concentration and therefore a slow rate change is expected. The flow control valve 20 also controls the bleed stream from the scrubber water pump 14 for the removal of dissolved metals and/or sulphates from the scrubber wash water. In order to reduce the concentration of dissolved metals and sulphates in the scrubber water, the bleed stream 16 is passed through an inline reactor 22.

In the illustrated embodiment, in-line reactor 22 is a static reactor in the form of a FILBLAST reactor, however it will be understood that any suitable in-line reactor may be employed. Oxygen or an oxygen-containing gas is injected into the FILBLAST reactor 22 to react with the scrubber effluent so that the dissolved metals and sulphates are precipitated out of solution. Preferably lime or caustic soda is added to the scrubber effluent in the bleed stream 16 prior to reacting in the in-line reactor 22 in order to raise the pH and neutralise the solution. Some precipitation will occur as a result of this 2 0 neutralising, however the reaction is quite slow and inefficient. Reacting the scrubber effluent with an oxygen-containing gas in the in-line reactor 22 is believed to accelerate the process of oxidation/precipitation. Without wishing to be bound by theory, it is believed, for example, that the ferrous (Fe 2 ions are converted to ferric (Fe 3 ions in the in-line reactor. The ferric ions precipitate much more quickly from solution, for example, by reaction with the lime to produce Fe(OH) 3 Precipitation of any sulphates and/or other dissolved metal salts in the scrubber effluent is also accelerated by reacting in the in-line reactor 22.

A clarified solution is then separated from the precipitated sludge in clarifier or thickener 24 and the clarified solution 26 is recycled to the scrubber circuit. Pump 28 discharges the precipitated sludge to a settling dam. The precipitate is of the jarosite species of iron WO 01/92162 PCT/AU01/00641 -7complexes and is readily settled and filtered, rendering possible the disposal of filter cake as a viable alternative to sludge disposal. This represents a significant improvement in view of the more environmentally acceptable materials handling characteristics of the filter cake compared to sludge. The sludge may contain cyanide in which case it will probably require treatment through a cyanide destruction process, for example, using Caro's acid.

The clarified solution 26 may also contain cyanide and is therefore pumped via pump 32 to a cyanide destruction facility 30. There are two processes commonly used for cyanide destruction. Either process may be employed in the cyanide destruction facility 30. The Inco sulphur dioxide method uses air or oxygen, copper sulphate and sulphur dioxide (either gaseous, or a sodium sulphide salt) to convert the cyanide to cyanate. The second method utilises Caro's acid which also oxidises the cyanide to non-toxic cyanate. A method of mixing ambient temperature hydrogen peroxide and sulphuric acid in a mixing funnel is described in US Patent Nos. 5,439,663 and 5,470,564. A process for the treatment of cyanide in effluent's with Caro's acid is described in US Patent No.

5,397,482.

In the process illustrated in Figure 1, hydrogen peroxide and sulphuric acid are employed but the mixing efficiency and Caro's acid yield are enhanced by utilising a suitably sized FILBLAST reactor 34. The in-line reactor 34 is used to react the acid and hydrogen peroxide and discharge the Caro's acid directly into the feedline of clarified solution from pump 32 immediately prior to a cyanide destruction reactor 36. Because the Caro's acid is discharged into the feedstream immediately prior to entering the cyanide destruction reactor 36, there is no time for the Caro's acid to decompose prior to reaction. A further advantage of the described cyanide destruction facility 30 is that it is totally enclosed, and as it operates under pressure there is no loss of decomposition products to atmosphere, if indeed any decomposition products have time to form during the milliseconds time elapse from mixing to reacting. Cyanide-free water is discharged from the cyanide destruction reactor 36 to the scrubber water surge tank 12.

WO 01/92162 PCT/AU01/00641 -8- Preliminary test results indicate that the described process is able to reduce the iron in solution by 97.6%, total dissolved solids (TDS) by 47.8%, calcium hardness by 53.6% and sulphate by 62%. Figures 2 and 3 illustrate graphically the improvement in the treatment of scrubber effluent from a steel mill using the described process. An interesting result is the lowering of the calcium content even though lime was added to the scrubber water to raise the pH. The steel mill currently uses caustic soda because lime addition results in scale formation. Test work has shown that the product from conventional caustic soda dosing in an agitated vessel leaves a film of scale on the walls of the beaker, whereas the product from the FILBLAST reactor leaves no evidence of scale on the beaker.

In the described process the treatment of the scrubber effluent to remove dissolved metals and/or sulphates is combined with a cyanide destruction facility to address both problems in the steel mill scrubber effluent. However, the removal of dissolved metals and/or sulphates is important in its own right to maintain cooling tower efficiency. Iron precipitation from hot scrubber bottoms is important to prevent iron oxidation in the cooling towers, resulting in sludge build-up and a lowering of cooling tower efficiency with eventual collapse of the packing. Hot scrubber bottoms may or may not contain cyanide, depending on the process within the steel making plant. However, when the process of precipitation of dissolved metals is combined with cyanide destruction, it is preferable to remove the iron before cyanide destruction as the iron will increase the consumption of Caro's acid. By maintaining the scrubber circuit water below say 30 ppm free cyanide, it will be safe to discharge the precipitated sludge to a storage pond. The optimum free cyanide concentration in the scrubber water will be maintained as low as possible, as it can be reliably controlled by the cyanide monitor 18.

Figure 4 illustrates a second embodiment of the process for the treatment of industrial effluent in accordance with the invention, in this case for the treatment of acid mine water (AMW). Acid mine water can be generated within a mine site, for example, from underground operations, from open pits or in the run-off from mine waste dumps.

Bacterial oxidation effluent converts sulphide minerals into sulphuric acid and the associated metals into soluble sulphate such as iron, copper, nickel, cobalt, zinc, etc which WO 01/92162 PCT/AU01/00641 -9report to the wastewater streams (effluent). The resultant acid mine water is therefore also contaminated with heavy metals. Other metallurgical processes, apart from mining operations, also generate wastewater streams that contain heavy metals and anion species such as sulphates, carbonates and chlorides. Manganese, chromium, arsenic, etc are typically found in these process effluent streams or within contaminated ground water.

Canadian Environmental and Metallurgical Inc. (CEMI) has developed a water treatment technology known as the high density sludge (HDS) process for the treatment of acid mine drainage. The HDS process is essentially an improved lime precipitation system in which lime is added to the waste water to raise the pH of the effluent. This forms metal hydroxides which in turn form a precipitate and settle out. The settling may be done in a tailings pond or a clarifier. A flocculating agent may be added to improve the settling characteristics of the precipitate. In the HDS process sludge is removed from the clarifier and returned to a tank at the head of the circuit where the lime is added prior to the sludge lime mixture being added to the treatment plant influent. Using this process, the density of the settled sludge can be increased to over 20% solids or higher. The main advantage of the HDS process is the increase from 2% solids to 30% solids which reduces the volume of sludge produced by over 95%. The HDS sludge typically drains to a much higher percent solids. Near-complete precipitation of the metals in the effluent as 2 0 hydroxides during the neutralisation step, occurs according to the following reactions:

M+SO

4 =+Ca+2(OH)+2H 2 0- M(OH) 2 +CaSO4.2H 2 0 2M"-+3(SO 4 )+3Ca"+6(OH)'+6H20.2M(OH) 3 +3CaSO4.H 2 0 (2) A typical HDS acid mine water treatment facility requires a number of mixing tanks, flocculant dosing vessels, sludge recycle facilities, etc. Figure 4 illustrates a modified HDS process in which a stream of the effluent is diverted through an in-line reactor in accordance with the process of the invention. A stream of the acid mine water (AMW) is 3 0 diverted to a first stage in-line reactor 40. Lime slurry formed in a lime slurry mix tank 42 is added to the effluent prior to reaction in the in-line reactor 40. As with the first WO 01/92162 PCT/AU01/00641 embodiment, the in-line reactor 40 is preferably a FILBLAST reactor. The FILBLAST reactor is particularly advantageous, as it provides an intense high-shear mixing zone, and the cross-flow characteristic imparted to the gas stream within the reactor creates minute gas particles in the high shear conditions. The FILBLAST reactor also has a very high power to volume ratio compared with other static or mechanically driven high-shear devices. It is believed that it is the combined effect of these reaction-enhancing characteristics that results in the formation of the jarosite species noted above. The effluent then reacts with an oxygen-containing gas in the FILBLAST reactor 40 and any dissolved metals and/or sulphates are precipitated out of solution.

Without wishing to be bound by theory, it is thought that the high-shear conditions created within the FILBLAST reactor accelerate the process of oxidation/precipitation in accordance with the equations and above. However, other reactions may also be occurring within the in-line reactor. Industrial effluent containing other metal and submetal ions may require removal with other suitable precipitants besides lime. Any sulphates and/or other precipitated metal salts can then be removed from the precipitated sludge. Flocculant may be added to the effluent from a flocculent mixing tank 44 prior to reacting with an oxygen-containing gas in a second stage FILBLAST reactor 46. The main purpose of the second stage FILBLAST reactor 46 is to increase the amount of sulphate removal by the addition of specific chemical reagents. Injection of air at reactor 46 is optional. Depending on the nature of the sludge additional chemical reagents may be injected in the second stage reactor 46 to facilitate precipitation of the metal and/or sulphate species in solution.

The precipitated sludge from the second stage FILBLAST reactor 46 flows to a clarifier or thickener 48 for separation of a clarified solution which is discharged to a clear water tank 50. The clarified water may be recycled for use in the lime slurry or flocculent mixing tanks 42 and 44, or may undergo further treatment 52 to produce potable water.

The precipitated sludge is discharged from clarifier 48 to a sludge disposal facility, or 3 0 optionally to a heavy metals recover plant 54. In the modified HDS process described, the precipitated sludge may be sufficiently dense that recycling is not required. However, if WO 01/92162 PCT/AU01/00641 -11recycle is applied, the recycle stream is injected into the feed stream pipeline prior to the first stage FILBLAST reactor 40, and does not require a separate mixing vessel.

Figures 5 and 6 illustrate graphically the improvement in the treatment of acid mine water effluent using the described process.

Now that preferred embodiments of the process for the treatment of industrial effluent have been described in detail, it will be apparent that the process provides a number of significant advantages, including the following: it substantially reduces the concentration of dissolved metals and/or sulphates in the effluent; (ii) it reduces the wastage of water as most of the water can be recycled after treatment; (iii) in the case of the treatment of scrubber effluent, it improves the efficiency of the cooling tower and extends the life of the cooling tower packing and it provides a fully enclosed cyanide destruction facility; and (iv) it reduces the capital costs by eliminating some plant, and provides a more energy efficient process for the treatment of effluent.

Numerous variations and modifications will suggest themselves to persons skilled in the chemical engineering and process arts, in addition to those already described, without departing from the basic inventive concepts. All such variations and modifications are to be considered within the scope of the present invention, the nature of which is to be determined from the foregoing description and the appended claims.

Claims (23)

1. A process for the treatment of effluent, the process comprising the steps of: Sdiverting a stream of effluent containing a soluble contaminant species through a O 5 first in-line reactor; reacting the effluent with an oxidizing agent in the first in-line reactor and substantially depleting the effluent of the soluble contaminant species; and, recycling the treated effluent. 10

2. The process according to claim 1, wherein the step of reacting the effluent with the oxidizing agent converts the soluble contaminant species to an insoluble species, and the insoluble species is separated from the treated effluent prior to the step of recycling the treated effluent.

3. The process according to claim 2, wherein the soluble contaminant species are dissolved metals and/or sulphates.

4. The process according to claim 2 or claim 3, wherein the oxidizing agent is an oxygen-containing gas.

The process according to any one of claims 2 to 4, wherein the effluent is an effluent from an industrial processing plant.

6. The process according to any one of claims 2 to 5, wherein the pH of the effluent is adjusted prior to said step of reacting the effluent.

7. The process according to claim 6, wherein lime or caustic soda is added to the effluent to adjust the pH of the effluent.

8. The process according to any one of claims 2 to 7, wherein a further precipitant is added to the effluent prior to said step of reacting the effluent.

9. The process according to any one of claims 2 to 8, wherein a flocculent is added to the effluent prior to said step of reacting the effluent. Kirstie/keep/retypeiP48226 claims -13- The process according to any one of claims 4 to 9, wherein the oxygen-containing N gas is injected directly into the in-line reactor adjacent an intake port.

IN 5

11. The process according to claim 1, wherein the step of reacting the effluent with the oxidizing agent converts the soluble contaminant species to a non-toxic species, and the non-toxic species is recycled with the treated effluent.

12. The process according to claim 11, wherein the soluble contaminant species is S 10 cyanide.

13. The process according to claim 11 of claim 12, wherein the oxidizing agent is Caro's acid.

14. The process according to claim 13, wherein the Caro's acid is produced immediately prior to reacting in said first in-line reactor.

The process according to claim 13 or claim 14, wherein the Caro's acid is produced by reacting sulphuric acid and hydrogen peroxide in a second in-line reactor which discharges directly into a feed line immediately prior to the first in-line reactor.

16. A process for the treatment of effluent, wherein the effluent contains a mixture of a first soluble contaminant species and a second soluble contaminant species, the process comprising the steps of: diverting a stream of effluent containing the mixture of the first soluble contaminant species and the second soluble contaminant species through a first in-line reactor; reacting the effluent with a first oxidizing agent in the first in-line reactor and substantially depleting the effluent of the first soluble contaminant species; diverting the depleted effluent to a second in-line reactor; reacting the depleted effluent with a second oxidizing agent in the second in-line reactor and substantially depleting the effluent of the second soluble contaminant species; and recycling the treated effluent. Kirstie/keep/retype/P48226 claims -14-

17. The process according to claim 16, wherein the step of reacting the effluent with the first oxidizing agent converts the first soluble contaminant species to an insoluble species, and the insoluble species is separated from the depleted effluent prior to the step of Sdiverting the depleted effluent. IO

18. The process according to claim 16 or 17, wherein the step of reacting the depleted effluent with the second oxidizing agent converts the second soluble contaminant species to a non-toxic species, and the non-toxic species is recycled with the treated effluent. IO S 10

19. The process according to any one of claims 16 to 18, wherein the effluent is a scrubber effluent containing dissolved metals and/or sulphates and cyanide.

The process according to any one of claims 16 to 19, wherein the first soluble contaminant species are dissolved metals and/or sulphates.

21. The process according to any one of claims 16 to 20, wherein the second soluble contaminant species is cyanide.

22. The process according to any one of claims 16 to 21, wherein the first oxidizing agent is an oxygen-containing gas.

23. The process according to any one of claims 16 to 22, wherein the second oxidizing agent is Caro's acid. Dated this 16th day of December 2005 ATOMAER PTY LTD By Its Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and Trade Mark Attorneys of Australia. Kirstie/keep/retype/P48226 claims