AU4350399A

AU4350399A - Recovery of activated carbon

Info

Publication number: AU4350399A
Application number: AU43503/99A
Authority: AU
Inventors: Steve McDonnell
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-08-13
Filing date: 1999-08-12
Publication date: 2000-03-09

Description

r/UU/U I I 28/5W9 Regulation 3.2(2)

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE

SPECIFICATION

STANDARD

PATENT

*C Application Number: Lodged: p. Invention Title: RECOVERY OF ACTIVATED CARBON The following statement Is a full description of this Invention, Including the best method of performing it known to me RECOVERY OF ACTIVATED CARBON The present invention relates to an apparatus and a process for the recovery of activated carbon particularly, although not exclusively, envisaged for the use in the recovery of activated carbon from a base material, such as, for example gold processing waste, for allowing further stripping of gold from the base material.

The waste from gold processing is characterised by containing activated carbon, gold, clay, quartz, calcine, dust and other fines (including root hairs, wood chips and other plant material). The dust is mostly in the form of silica 10 based sand and may contain iron.

Ever since activated carbon, which includes any charcoal, wood, coconut or like derived product for adsorbency of gold, has been used as a recovery S:stage in the processing of gold from ore, mining companies have sought a costeffective way in which to increase the amount of gold that can be stripped from the activated carbon without success. So presently, millions of dollars worth of gold lie bound up in activated carbon waste dumps around the world waiting for the invention of a way in which to reclaim the activated carbon and strip out the gold. It has not previously been possible to remove the activated carbon, which binds the gold, from other particles which have a similar size and specific gravity to the activated carbon.

Processing of particulate materials usually relies upon either differences in particle size or differences in specific gravity (where the particle sizes are substantially the same size). In conventional gold processing systems the carbon becomes wet with water and its specific gravity increases from about 0.55 to about 1.5 which is similar to that of dirt which is about 2.0. Once the size and the specific gravity of the activated carbon and dirt are substantially the same there is no known way to separate them. Also, typically, where there remains dirt there is silica which interferes with known processes for stripping gold from activated carbon.

Therefore it is an object of the present invention to provide a process for the recovery of activated carbon involving the removal of dust, clay, fines and I M M waste materials therefrom to prepare a feedstock suitable for gold and precious metals recovery.

With this object in view, the present invention provides, in a first aspect, a process for the recovery of activated carbon from gold processing waste comprising the steps of: washing the waste material with a liquid to remove fine particulate material such as dust, clay and plant material; drying the washed material to remove the washing liquid from the activated carbon for lowering its specific gravity so that it will be different to that 10 of the remainder of the washed material; separating particles of differing sizes in a first separator means for removing fine particulate material which remains after the washing step from the dried material to avoid inhibiting the operation of a second separator means; and 15 separating particles of substantially the same size but different specific gravity in a second separator means using a gas, such as air, as a carrier for stratifying the particles so as to separate the activated carbon from the remainder of the particles.

In accordance with a second aspect of the present invention, there is provided an apparatus for recovery of activated carbon from waste material produced in the processing of gold from ore using activated carbon, the apparatus including: a washer means for spraying a liquid such as water through the waste material for removing fine particulate material such as dust, clay and plant material from the waste material; a dryer means for drying the washed material for removing the washing liquid from the activated carbon for lowering its specific gravity so that it will be different to that of the remainder of the washed material; a first separator means for separating particles of differing particle sizes for removing fine particulate material which remains after washing from the dried material to avoid inhibition of operation of a second separator means; and a second separator means for separating particles of substantially the same size but having differing specific gravities using a flow of gas such as air as a carrier for stratifying the particles according to their differing specific gravities so as to separate the activated carbon from the remainder of the particles.

The invention is predicated on the discovery that waste particles, typically of silica, clay, dust and other fines are often wet and adherent to the activated carbon particles interfering with the recovery of activated carbon therefrom and further, that a selective method must be employed in the removal of these waste materials if gold recovery is to be optimised.

10 In particular, potential interference effects are: 1. The silica has the effect of glazing over the carbon and forming a barrier to any further processing if heat is applied to the waste material such as in a S furnace.

S2. The dust, of fine particle size, may fill pores and interstices of the particles 15 of waste material which defeats the operation of gravity separators.

3. Water has the effect of increasing the specific gravity of the activated carbon such that it is too close to the specific gravity of the remainder of the waste material to allow separation by stratification.

4. The fines, including root hairs, wood chips, pulp and other organic material, contain gold but inhibit the gravity separation of activated carbon particles by preventing particle migration and separation. The order and nature of the separator steps of the invention are designed to counter these effects.

Most advantageously, the first separator means (size) involves terminal velocity separation. Typical of such separation is suspension of particles in a gas, particularly air, stream. Particle separation occurs in accordance with terminal velocity of the particles which is a function of particle shape, surface texture and weight. Fine dust separation is facilitated by inclusion of this process. Screens and sieves, specific to separation by size, are used advantageously in combination with terminal velocity separation especially in series and especially as a primary step. Reverse screening or scalping may be advantageously employed in the screen series. This allows particles both over- and undersize a desired range to be separated and rejected or used in further processing as appropriate. A scalper screen is advantageously employed for this purpose. Separation into size fractions by screening and terminal velocity separation is found to be desirable because certain size fractions are not as amenable to treatment for recovery of gold as others. Also, gold content may vary between the size fractions.

Most advantageously, the second separator means (specific gravity) is a dry bed gravity separator. A suitable dry bed gravity separator is a table or series of tables having a perforated deck to allow air, or a suitable gas, to be driven through the perforations to entrain the particles and facilitate 10 stratification on the basis of differing specific gravity. A flat deck without ridges is preferred. Liquids having specific gravity similar to water are not used as carriers. The table product of desired specific gravity is subjected to gold So extraction.

In the washing step, a suitable washing liquid is water though it may be 15 advantageous to add a surface active agent, for example being detergent in effect, to ensure a more complete removal of dust and dirt from the activated carbon particles. Effective washing of the activated carbon is important to the invention. Addition should be such as to compensate for water used in the process having a high concentration of total dissolved solids as is the case in 20 the Western Australian goldfields, that is facilitating wetting. Thus addition may be a function of total dissolved solids concentration.

The washing step may occur in multiple stages to further assist in the removal of fine particles, for example of talc. It is important that the washing stage result in a high proportion of fines about 0.5 mm) removal, most preferably more than 99% removal is required to allow satisfactory separation and downstream processing of.activated carbon for recovery of gold. It may be possible to remove some talc, in a balled up form, by screening. After screening, the drying may not proceed to completion but a small amount of moisture may be left to assist in downstream processing.

Gold extraction from the table product having desired specific gravity may be accomplished by leaching, for example by a cyanidation route. Other gold processing techniques that are suitable for this purpose include chloride hydrometallurgy using saline and like solutions. Pyrometallurgy could be used by re-use of activated carbon is not possible. The process for stripping of gold may take into account the salinity of the water used in the separation stages, that is a pyrometallurgical recovery route if used should be selected at chloride levels that will not encourage formation of volatile gold chlorides and losses of gold in fume from a roaster or furnace. A hydrometallurgical recovery route is typically preferred, for example, in the Western Australian environment.

Treated activated carbon may be returned to the gold processing plant for re-use following any desired regeneration process.

10 The process may conveniently be conducted in mobile apparatus that may be moved between sources of activated carbon for treatment.

The invention may be more fully understood from the following description made of preferred embodiments thereof made with reference to the accompanying example and drawings in which: Figure 1 is a process flowsheet relative to one embodiment of the process of the present invention; Figure 2 is a process flowsheet relative to a further embodiment of the process of the present invention; and Figure 3 is a plant diagram showing the layout of a plant for conducting 20 the process in accordance with a most preferred embodiment of the present *e.

invention.

The various stages, and apparatus, shown in Figures 1 to 3 are conducted with a view to addressing appropriate removal of the waste materials in a manner to produce an activated carbon fraction amenable to treatment for gold recovery. Figure 2 varies from Figure 1 by incorporating washing and sizing which facilitates further processing and is thus related to the apparatus shown in Figure 3. Any suitable conveyor means may be employed to transfer material between stages.

Referring first to Figure 3, the apparatus 10 for conducting recovery of activated carbon is shown. Prior to washing at rotating sand screen washer (approximate size 0.5 mm),operated in the manner of such washers, removal of oversize particles, trash and so on is achieved using grizzly 14 (approximate 6 size 25 mm)and scalper 16 (approximate size 6 to 10mm). Washed material is then conveyed to kiln dryer 26. Dried material from the washer is sized into specific size fractions particularly using primary separation means including scalper screen 32, having aperture size about 4.0 mm, screen 34, having aperture size about 1.5 to about 2.0 mm and a terminal velocity separation 50 to remove fine dust about 0.5mm) as follows: Fraction A <0.5 mm (rejected by terminal velocity separation) Fraction B +0.5 mm to approximately 1.5 to 2.0 mm (screen 34 undersize) Fraction C about 1.5 to 2 mm to about 4.0 mm (screen 34 oversize) Fraction D about 4.0 mm (scalper 32 oversize) These sizes are typical but may vary in accordance with the feedstock, mine operational requirements and assay results for gold. Fraction A may usually be rejected as its gold assay is low. Similarly, fraction D is typically of little value containing stone, plastic and wood chips though re-processing through apparatus 10 may be preferred where gold is present in appreciable quantity, for example in association with wood chips.

S 20 Fractions B and C contain a large proportion of the gold, typically over and most particularly in the range of about 95% to 99% by weight as meaasured by fire assay. These size fractions are conveyed to table 40 for fractionation in accordance with specific gravity after removal of fines by terminal velocity separation 50. The fractions may be separately fed to the table though they could be fed in combination, and may otherwise be subjected to further processing stages for recovery of gold in accordance with their characteristics of gold content and size. Product material from this table 40 may, optionally following still further size fractionation if necessary, be treated for gold recovery by a method taking into account the considerations described above, particularly total dissolved salts content of local water sources.

Referring additionally to Figures 1 and 2, in the first stage of the process, washing is conducted to substantially remove dust from the waste material.

Such dust is typically rich in silica. Also, there is little dust to fill the interstices and pores of the remainder of the particles. Hence, gravity separators subsequently employed then may function more efficiently to separate particles of similar size but differing specific gravity namely activated carbon from the remainder of the waste material). Washing also removes some of the remainder of the fines.

The washing stage must be conducted in a manner such as to remove most of the silica or mineral constituting the dust so that, especially in the case of 10 silica, there is no substantial glazing when heat is applied to the washed material in later processing by thermal techniques.

Therefore, it is necessary that the material be washed "hard" enough to achieve this result. Multiple stage washing may be conducted. Desirably, it has been found most convenient by the applicant to introduce an additive such as a surface active agent to facilitate washing. A detergent is suitable for this purpose. However, it is important that the dispersancy not be excessive. In this respect, many dispersants and flocculants generally used in mineral processing plants may not be suitable or should be added in trace concentrations only.

Figures 2 and 3 is directed to a desirable embodiment in which sizing accompanies washing as this may facilitate later processing, low size material being rejected at an early stage.

The washing stage is conveniently and advantageously conducted using a rotating screen 20 bringing the material into contact with the water and detergent with sufficient agitation. Undersize at the screen is rejected. Washed material is then transferred to the next drying stage of the process. Washing need not involve screens, other washing apparatus could be used, for example washing tanks. Feed material to the washing stage is advantageously prescreened using a grizzly 14 and scalper 16 to ensure that useless oversize material such as trash, branches and so on are removed.

After washing, the washed material is wet, and some of the dust remains attached to the activated carbon particles. This has the effect of bringing the specific gravities of the two materials closer together making processing difficult.

Accordingly, in the next stage, the washed material is dried either slowly in the sun, in industrial dryers burning fossil fuels; or using microwave energy for drying.

The next stage in the process is to separate particles of differing sizes to remove the remainder of the fines. A set of screens or sieves is conveniently employed for this purpose. Terminal velocity separation follows screening for removal of fine dust about 0.5 mm). Screening may occur in two stages, a scalper 32, or reverse screening stage having screen size 4.0mm; and a screen 34 having about 1.5 to 2.0 mm size. Any desired arrangement of screens, 10 usually including a scalper screen 32, may be employed. In accordance with a typical process, a typical product material of sizes between about 1.5mm and about 4.0mm contains most of the gold. Use of such screens allows selective size fractionation of oversize and undersize to achieve the desired particle size *fraction(s) for further processing, particularly for subsequent recovery of gold.

15 Additionally, this stage has the effect of extracting most of the remainder of the fines from the dried material the fines which had been adhered to the activated carbon by the water during washing. The resultant material consists of particles 0 of substantially the same particle size but differing specific gravity. If the fines 00o.

are allowed to remain in the material they may inhibit the efficiency of operation of the second separator means.

In the final stage of the process, the particles of differing specific gravity, but substantially the same size are then separated using a second separator means being most preferably a dry bed gravity separator in the form of a table or series of tables 40. The secondary separator means typically uses an updraft of air or other suitable gas to stratify the particles according to their differing specific gravities. Other carrier gases could be used. It is essential that the separator be of the dry bed kind since a wet bed separator (as conventionally used in gold processing) will wet the activated carbon which increases its specific gravity so that it is too close to that of the remainder of the waste material to allow stratification. A most preferred apparatus, especially for mobile plants, is a Heid Gravity Separator of suitable capacity sourced from Heid Agrartechnik Ges. mbH, Vienna, Austria. A suitable model, for example GA-30 or 9 might have an aluminium deck surface, of approximate area 0.9 to m 2 covered with an overcover of porous material formed from square wire-mesh or textiles and through which pressurised air may flow to achieve stratification by specific gravity. Feed rate table inclination, oscillating intensity and frequency; and pressurised air supply may be adjusted, as necessary, to achieve optimum separation. Typical air flows would be between 150 and 250 m 3 /hour. Dust recovery may be provided for. Product brochure A050/010-1190 (Heid) provides more detailed description of the apparatus and is incorporated herein by reference.

10 Once the stratification occurs, the activated carbon can be recovered, sized as desirable to remove intractable fractions, and the gold stripped from it by an appropriate process in accordance with the above description. Activated carbon may then be regenerated in a conventional manner.

An example of the application of the process of the present invention is as 15 follows:

EXAMPLE

136m 3 of contaminated gold processing waste including activated carbon was *processed with the apparatus of the present invention. The processing waste contained about 20% by weight activated carbon with the balance clay, dust and other fines.

•After processing there was 16m 3 of activated carbon which had a particle size of greater than 2 mm across (referred to as the large sample); 10m 3 of activated carbon which had a particle size between 500 microns and 2mm(referred to as the small sample); and about 1 m 3 of activated carbon with particle size less than 500 micron (referred to as the last sample).

The large sample may be stripped of the gold or, if having a significat fraction having size above 4.0 mm, may be re-introduced into a gold processing plant to collect more gold by adsorption as for example in a Carbon-In-Pulp (CIP) or Carbon-In-Leach (CIL) plant. The small sample is typically no longer suitable for use in gold processing and may be stripped of the gold and discarded or used in a base metal smelter, for example a copper smelter which typically includes a precious metal recovery stage.

The last sample is typically still loaded with contaminants and is not suitable for further processing or use as activated carbon.

The rate of recovery of activated carbon was about 90% to 95% of that in the waste sample. Gold was stripped from both accepted samples, that is particle size above 0.5 mm, at the rate of about 1 kg (1000 ppm) gold per tonne of carbon recovered by the process of the invention, a grade much higher than typically encountered in regions such as the Western Australian goldfields, to 20 ppm. Hence, there is a very significant amount of gold which can be reclaimed by the process. Also, coconut carbon and extruded carbon cost about 10 $3000/tonne and $5000/tonne respectively and so there are further savings from the process since the larger carbon is available for re-use in a manner not heretofore possible. A typical waste dump treated in accordance with the S: process may yield about 5 years worth of useful carbon. That is, enough useful carbon to operate the gold processing plant for 5 years without having to purchase any further activated carbon.

Modifications and variations may be made to the invention by those skilled in the art on reading this disclosure. For example, other forms of heating could be used in the drying stage and the present invention may be applied to the various types of activated carbon that may be found in gold processing waste dumps. Such modifications and variations are within the scope of the present invention.

Claims

1. A process for the recovery of activated carbon from gold processing waste comprising the steps of: washing the waste material with a liquid to remove fine particulate material; drying the washed material to remove the washing liquid from the activated carbon for lowering its specific gravity so that it will be different to that of the remainder of the washed material; separating particles of differing sizes in a first separator means for removing fine particulate material which remains after the washing step from the said dried material to avoid inhibiting the operation of a second separator means; and separating particles of substantially the same size but different specific gravity in a second separator means using a gas such as air as a carrier for stratifying the particles so as to separate the activated carbon from the remainder of the particles.

2. The process of claim 1 wherein said washing liquid contains a surface active agent. S.

3. The process of claim 1 or 2 wherein said first separator means includes at least one screen.

4. The process of claim 3 wherein said first separator means comprises a reverse screening stage. The process of any one of the preceding claims wherein the primary separation means includes terminal velocity separation.

6. The process of any one of the preceding claims wherein the second separator means is a dry bed gravity separator, preferably a table.

7. The process of any one of claims 2 to 6 wherein addition of surface active agent is a function of dissolved solids concentration of a washing liquid being water.

8. The process of any one of the preceding claims wherein the product from the second separator means is treated for gold extraction.

9. The process of claim 8 wherein activated carbon is regenerated for re- i' use following gold extraction. 9999 An apparatus for recovery of activated carbon from waste material produced in the processing of gold from ore using activated carbon the apparatus including: *a washer means for spraying a liquid such as water through the waste material for removing fine particulate material such as dust, clay and plant material from the waste material; 9 a dryer means for drying the washed material and removing the washing oo liquid from the activated carbon for lowering its specific gravity so that it will be different to that of the remainder of the washed material; I.9 a first separator means for separating particles of differing particle sizes for removing fine particulate material which remains after washing from the dried material to avoid inhibition of operation of a second separator means; and a second separator means for separating particles of substantially the same size but having differing specific gravities using a flow of gas such as air as a carrier for stratifying the particles according to their differing specific gravities so as to separate the activated carbon from the remainder of the particles.

11. The apparatus of claim 10 wherein said first separator means includes at least one screen. 13

12. The apparatus of claim 11 wherein said first separator means comprises a reverse screening stage.

13. The apparatus of claim 11 or 12 wherein said first separator means includes terminal velocity separation.

14. The apparatus of any one of claims 10 to 14 wherein said second separator means includes a dry bed gravity separator, preferably a table.

15. The apparatus of any one of the preceding claims further comprising a gold extraction stage.

16. The apparatus of any one of the preceding claims further comprising a regeneration stage. DATED this 12th day of August 1999. STEVE McDONNELL *0 S S. S S 555 S S @0 0 *5 S 5555 WATERMARK PATENT TRADEMARK ATTORNEYS 290 BURWOOD ROAD HAWTHORN VICTORIA 3122 AUSTRALIA