AU600129B2 - Improvements in chemical processes utilising particulate adsorbents or catalysts - Google Patents

Description

K-

S F Ref: 58876 FORM COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE: Class Int Class oo 0 0 0 lComplete Specification Lodged: ,a Accented: Published: riority: 'Related Art: JjUfl coUsraW' Cl O f. ods su*ctiou cer _C CLII tar Name and Address of Applicant: Sentrachem Limited (NCP Division) Bedford Centre Smith Street 2008 Bedfordview REPUBLIC OF SOUTH AFRICA Address for Service: Spruson Ferguson, Patent Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, New South Wales, 2000, Australia Complete Specification for the invention entitled: Improvements in Chemical Processes Utilising Particulate Adsorbents or Catalysts -he following statement is a full description of this invention, including the ,est method of performing it known to me/us 5845/3

ABSTRACT

Particulate adsorbents such as activated carbon, ion-exchange resins and the like, as well as particulate catalysts, are encapsulated in capsules provided with a plurality of holes in their walls. In the carbon-in-pulp process for gold recovery the use of such encapsulated activated carbon, with the latter occupying only about 25% of the internal space of the capsules, leads to improved recovery, since abrasion losses are essentially eliminated. The rate o, of adsorption of the gold complex on'to the encapsulated carbon is still remarkably fast compared to that on to non-encapsulated carbon particles. In packed columns or beds, the invention provides a lower ao pressure drop without sacrificing too much on rates of diffusion.

ao o a a tt o 4 is tf This invention relates to improvements in chemical processes making use of granular or particulate adsorbents or catalysts, such as activated carbon in carbon-in-pulp and carbon-in-leach processes for instance for the recovery of gold from gold ore pulps, processes using ion-exchange resins or other adsorbents in columns, chromatographic processes, and heterogeneous catalysis processes. The invention further relates to a capsule °o suitable for encapsulating a plurality of solid particles and to a method of encapsulating such particles.

0 00 00 V 0 00 0 Processes making use of activated carbon such as the so-called carbon-ino 0 pulp process and the carbon-in-leach process (the latter involving minor modifications of the former) for the recovery of gold from gold ores 0 have been rapidly replacing the zinc cementation process in the gold-mining 0 00 industry over the past few years.

0 04 In the recovery process the milled ore is agitated with a dilute alkaline cyanide solution in the presence of air, resulting in the dissolution of a4 the gold in the form of the Au(CN) 2 complex. In the cementation process it is necessary to separate the liquid phase from the solid phase before the gold can be precipitated by zinc dust.

In the C-I-P process (see, for example G.J. McDougall and R.D. Hancock, Minerals Sci. Engng, vol. 12, p. 85, 1980) granular activated carbon is stirred directly into the cyanided pulp in adsorption contactors. After equilibrium has been established, the gold-bearing carbon is recovered from the pulp by screening. In order to ensure minimum gold loss and high gold recovery, countercurrent operation is used, and usually three to eight 2 contactors are used in series. The gold is subsequently recovered from the activated carbon by elution with hot NaOH/cyanide solution, optionally after an intermediate acid-washing stage (particularly required at South African mines) to remove adsorbed calcium carbonate, and gold (and silver) is recovered from the eluate, usually by electrowinning. The carbon is thermally reactivated with steam at about 700 to 8000 C before recycling.

I Although the C-I-P process generally works well, it suffers from a number i of disadvantages. The carbon, which must be used in the form of tough, abrasion-resistant granules, preferably of coconut-shell carbon, generally of a size range 1.2 to 2.4mm in diameter, inevitably undergoes some attrition when stirred together with the pulp, the latter consisting of a viscous, abrasive suspension of the ore (of particle size generally smaller than 200um) in the dilute cyanide solution. Because of the aforementioned I attrition, the abraded fine particles of gold-bearing carbon are partially or completely lost together with the pulp in the screening stages, thus leading to significant losses of gold.

In addition, particularly in large plants, the inter stage screens tend to become blocked by the carbon granules.

Both of these problems, it has now been found, may be overcome or significantly ameliorated, by the present invention.

Activated carbon granules, ion-exchange resin beads and other adsorbents such as molecular sieves, silica or alumina may also be used in columns.

3 a ii

I

I

*1 .1

'I

However, processes involving the use of packed columns or beds almost invariably have to contend with the problem of a pressure drop occurring along the length of the column or bed in the direction of flow of the fluid. This results from the fact that the particles or granules in the bed need to be as small as possible, compatible with adequate mass transfer rates. However, the smaller the particle size the higher is the pressure drop experienced. In practice the particle size chosen for a column or bed process therefore represents a compromise between two opposing requirements, namely high mass transfer rates and chemical reaction kinetics on the one hand, and large particles for fast flow rates at low pressure drops along the length of the bed on the other hand.

Problems due to pressure drop in columns are further aggravated in cases where the column becomes fouled during operation, resulting in a build-up in pressure drop. For example, in the use of activated carbon in column processes for water treatment, such columns tend to experience increasing pressure drops as a result of fouling of the granules by particulate material filtered out by the column bed.

These problems may, once again, be eliminated or substantially ameliorated by the present invention.

Similar problems with unacceptably large pressure drops are encountered in packed reactor beds comprising heterogeneous catalysts. For example, in the conversion of sulphur dioxide and oxygen/air mixtures into sulphur trioxide over a catalyst bed of vanadium pentoxide, the pressure drop along the length of the column bed limits throughput, and the catalyst particle 4 size and shape selected represent a compromise between a desired high flow rate of the gas stream through the bed a high rate of throughput) and the smallest compatible particle size of the catalyst material which will ensure adequate mass transfer and hence reaction kinetics.

These problems may also be overcome or significantly ameliorated by the present invention.

According to a first embodiment of the present invention there is provided a process for interacting a fluid with solid particles, including the step of contacting the fluid with a plurality of solid particles encapsulated in a plurality of capsules the walls of which are provided with a plurality of holes, each hole being smaller in its smallest dimension that the smallest dimension of the smallest particle in the range of particle sizes of the said particles, the particles comprising a sorbent or a catalyst.

The fluid may be a clear or unclarified aqueous or non-aqueous liquid, a suspension or a pulp, which may be stirred; or it may comprise a a o"o gas stream, with or without entrained particulate matter.

o° The solid may be a sorbent, for example activated carbon, an ion-exchange resin or other ion-exchanger particles, silica gel, alumina, a molecular sieve or any other micro- or macro-porous medium such as sorbents used for imbibing an organic extractant or extract, for example for use in reversed-phase partition chromatography. The solid may alternatively be a catalyst or catalyst carrier for use in heterogeneous catalysts.

The process may be a batch process, and may include several stages.

Alter- Ii i a C oaisic S0 a a 6 4 C C "I 7?,A KWK:795y L-7 natively, the process may be continuous or semi-continuous. Preferably in this embodiment of the invention, the fluid is contacted with the particulate solid in countercurrent flow, for example in a packed or fluidised bed.

The contacting step may be followed by filtration or screening out of the capsules.

In an advantageous embodiment of the process according to the invention, a gold-bearing pulp is contacted in a number of contactor stages, with granular activated carbon encapsulated in porous capsules, the pores of the capsules being smaller in diameter than the minimum in the range of granule diameters, whilst still being large enough to permit the pulp to freely flow into and out of the interior of the capsules so as to be contacted with the activated carbon.

The shells of the capsules thus prevent the granules entrapped inside the capsules from coming into contact with the walls of the contactor and other equipment such as agitators and the like, thus preventing the abrasive action of the carbon particles thereon during stirring and screening operations, and the larger size of the capsules substantially eliminates problems due to the blocking of the screens during screening operations.

In addition, larger slits may be used in the screens.

Surprisingly it has been found that, although the kinetics of mass transfer in the adsorption process is slower with granules encapsulated'in the process in accordance with the invention than with conventional C-i-P 6 processes, the kinetics of adsorption of the gold is still acceptably fast, particularly if the capsules are only partially filled with granules, to thus permit free mixing of the granules with the pulp within the capsules during the tumbling motion of the capsules when stirred in the pulp.

Particularly good results have been achieved when about 20 preferably about 25% of the volume of the capsules was occupied by activated carbon granules.

Similar considerations apply to resin-in-pulp processes or any batch-type process using an adsorbent to treat a clarified or unclarified liquor.

If the encapsulated granules or particles cannot be regenerated or reacted directly, for example in the case of encapsulated activated carbon used in gold recovery, where reactivation requires thermal treatment at high temperatures, the capsules may be broken up and the granules removed and treated by the conventional method, before re-encapsulation of the granules in an encapsulation plant.

In another embodiment of the process according to the invention, activated carbon, ion-exchange resin or the like, may be used in the form of encapsulated granules or particles in porous capsules packed into a column, thus providing larger voids and channels in the bed than with unencapsulated granules or particles, but without unduly sacrificing on the efficiency of mass transfer of the species involved in the adsorption or separation process. Again, the pores of the capsules should be small enough to ensure retention of the granules or particles in the capsules during process operation, but large enough for the fluid being processed to be permitted 7 to exchange or mix with the fluid that is at any time within the void space between the particles in the capsules.

The process according to the inventionomay also be applied to packed reactor beds comprising heterogeneous catalysts, for example in the conversion of sulphur dioxide and oxygen/air mixtures into sulphur trioxide, by encapsulating the catalyst particles in porous capsules. Since the capsules contain catalyst particles of small size relative to the size of the capsules themselves, the benefit of a relatively low pressure drop in a o°o bed may be achieved without sacrificing too much on mass transfer rates and o reaction .kinetics.

ao In accordance with another aspect of the invention, there is provided a o "0 capsule suitable for encapsulating a plurality of solid particles, the capsule having a plurality of pores, the diameter of each pore being smaller than the smallest particle to be encapsulated in the capsule.

*The capsule may comprise means adapted to permit the solid particles to be introduced into the capsule, and to be removed therefrom. Alternatively, each capsule may comprise two halves adapted to be joined together after at least partial filling thereof with the particulate solid.

The capsule may be spherically shaped. Alternatively, it may be in the form of a tube, a sachet, a basket or a bag.

Materials of construction may vary according to the particular application of the capsules. Thus, for example, hollow spheres or baskets made of stainless steel mesh may be used to encapsulate catalysts for use in high temperature applications. Alternatively, for example in ambient temperature applications, the capsules may be made of woven or non-woven fabrics, plastics, or the like.

Example 0 0 4t o o a Ping-pong balls (of internal volume about 30ml each) were cut in half, and S a plurality of holes of 1mm in diameter drilled in the shells about 3mm S,,o apart. Into one half of each pair of halves was placed 5g of coconut-shell derived, activated carbon granules, the granules being 8 to 14 mesh ASTM having diameters ranging from 1.4 to 2.36mm). (The 5g of activated o.9 carbon occupied a volume of about 9ml.) The two halves were then sealed together.

The rate of adsorption of gold was compared for non-encapsulated activated carbon and activated carbon encapsulated in the ping-pong balls, for both a clear gold-bearing solution and a gold-bearing pulp. An aqueous solution was prepared containing 10mg/l of gold as KAu(CN) 2 100mg/l of calcium as Cal 2 and 100mg/l of cyanide as KCN.

To 5 litres of gold-bearing solution was added 25g of the granulated activated carbon, and the mixture was stirred by means of a laboratory stirrer for 40 minutes. Samples were removed at regular intervals, and their gold content determined by atomic absorption analysis (Curve A, Fig. 1).

To 5 litres of gold-bearing solution were added five porous ping-pong 9 balls each containing 5g of activated carbon a total of 25g of activated carbon), and the mixture stirred under the same conditions as under Samples were similarly removed at regular intervals and analysed for gold (Curve B, Fig. i).

To 5 litres of gold-bearing solution was added dry, finely ground j barren gold ore to produce a pulp of 45 per cent solids content.

To the pulp was added 25g of activated carbon granules, and the mixture stirred for 40 minutes. Samples were removed at regular intervals, filtered, and analysed for gold content (Curve C, Fig. 1).

S(d) To a pulp, prepared exactly as under were added five ping-pong balls each containing 5g of encapsulated activated carbon, the mixture stirred, and the progress of gold removal followed exactly as under i (Curve D, Fig. i).

The results plotted in Fig. 1 show that with a clear solution, the rate of adsorption of Au is only slightly slower for the activated carbon encapsulated in the ping-pong balls than for the "free" activated carbon.

For a pulp similar to that used in the C-I-P process, the difference in rate of gold removal between the case where the carbon is "free" and where it is encapsulated in ping-pong balls is appreciably larger. However, given the small number and the size of the ping-pong balls and the high viscosity of the pulp, the rate of removal of gold by the encapsulated activated carbon is still remarkably fast.

Claims (13)

1. A process for interacting a fluid with solid particles, including the step of contacting the fluid with a plurality of solid particles encapsulated in, a plurality of capsules the walls of which are provided with a plurality of holes, each hole being smaller in its smallest dimension than the smallest dimension of the smallest particle in the range of particle sizes of the said S° c particles., the particles comprising a sorbent or a catalyst. o 0

2. A process as claimed in claim 1, wherein the solid particles oB occupy, apart from the inter-particle interstitial voids, substantially the entire internal volume of each capsule. 0

3. A process as claimed in claim 1, wherein the solid particles So occupy only part of the internal volume of each capsule. 0.A

4. A process as claimed in/claim5, wherein the solid is a o 4 sorbent and the fluid contains a material sorbable by the solid.

A process as claimed in claim 4, wherein the solid is activated carbon, and the fluid is selected from the group consisting of an aqueous solution, a suspension or a pulp, the fluid containing one or more metal complexes adsorbable by the activated carbon.

6. A process as claimed in claim 5 which includes the further steps, after the sorption of the metal complex or complexes during the contacting step, of separating the capsules from the fluid, recovering the metal complex or complexes from the sorbent, and if necessary reactivating the activated carbon particles for recycling to the process.

7. A process as claimed in claim 6, including the step of removing the solid particles from the capsules before l- l 11 12 reactivation thereof.

8. A process as claimed in any one of claims 1 to 7, wherein the sorbent is selected from the group consisting of activated carbon, aluminia, silica, molecular sieve and an ion-exchange resin, and is contacted in counter-current flow with the fluid.

9. A process as claimed in any one of claims 1 to 7, wherein the process is heterogeneous catalysts and the solid comprises a catalyst contacted in counter-current flow with a gas or vapour stream.

A capsule for encapsulating a plurality of solid particles therein, the walls of the said capsule being provided with a plurality of holes, each hole being smaller in its smallest dimension than the smallest dimension of the smallest particle in the range of particle sizes of said particles, the particles comprising a sorbent or a catalyst.

11. A process for interacting a fluid with solid particles, substantially as herein described with reference to Example or

12. The product of the process of any one of claims 1 to 9 or 11.

13. A capsule for encapsulating a plurality of solid particles therein, substantially as herein described with reference to Example or DATED this SEVENTH day of MAY 1990 Sentrachem Limited (NCP Division) Patent Attorneys for the Applicant SPRUSON FERGUSON !I 7 U JT KNK:795y