AU2019426739B2

AU2019426739B2 - Process for recovering non-ferrous precious metals by pelletisation and calcination of leaching activated carbon powder

Info

Publication number: AU2019426739B2
Application number: AU2019426739A
Authority: AU
Inventors: Yvan Grac
Original assignee: Individual
Current assignee: Individual
Priority date: 2019-01-31
Filing date: 2019-01-31
Publication date: 2021-08-12
Anticipated expiration: 2039-01-31
Also published as: CA3111658A1; AR117919A1; CA3111658C; AU2019426739A1; WO2020157543A1

Abstract

The invention relates to a process for recovering precious metals, in particular gold, from activated carbon powder used in a cyanide leaching process for extracting metal, said process comprising the following steps: - an activated carbon powder containing residues of precious metals is mixed with at least one combustible component and water in order to form a mixture based on activated carbon powder loaded with metal, - the mixture is compressed in a compression machine in order to form granules, - the granules are subsequently calcinated in an oven under controlled combustion conditions, particularly at a temperature between 500 and 800°C and for a time between 60 and 120 minutes so as to produce slag and cinders which are mixed in order to form a concentrate, the carbon ratio of which is less than or equal to 1% by weight.

Description

PROCESS FOR RECOVERING NON-FERROUS PRECIOUS METALS BY PELLETISATION AND CALCINATION OF LEACHING ACTIVATED CARBON POWDER TECHNICAL FELD OF THE INVENTION

[0001] The present invention concerns a process for recovering precious metal and in particular gold from activated carbon waste.

STATE OF THE PRIOR ART

[0002] The process for recovering gold by leaching on activated carbon is well known. In this process, which has numerous possible variants, the gold-bearing ore is first crushed then concentrated in a flotation circuit to remove sulfites and other undesirable residues. The flotation sludge is sent through leaching tanks where it is mixed with a mixture of sodium cyanide and lime slurry in the presence of oxygen. The gold solubilized by the cyanide is adsorbed on activated carbon filters. The activated carbon can be directed in counter current to the sludge. The carbon adsorbs the gold and is then washed with high-pressure water and a hydrochloric acid solution, after which the carbon is desorbed by injecting a solution of cyanide and caustic soda. The auriferous solution obtained is circulated through electrolytic extraction cells arranged in parallel. The gold and other metals are precipitated onto cathodes immersed in the circulating electrolyte. At the anode, oxygen is formed and cyanide degradation occurs. The cathodes loaded with gold cations are washed with a high pressure water jet. The product obtained ("mud") is dried and calcined in the presence of borax, silica and sodium carbonate. The mixture obtained is melted in a furnace at 13000C and the melt is poured into a cascade of moulds where the gold is separated from slag to produce gold ingots.

[0003] At the time of adsorption, the carbon also loses its activity. To restore the active surface and allow reuse thereof in the leaching circuit, it can undergo thermal regeneration. However, after several cycles of leaching and regeneration, the activated carbon becomes exhausted and changes to a powder laden with precious metals (including predominantly gold and silver) and non-precious metals. The carbon powder can have a gold content in the region of 200 grams to 4 kg per tonne. Recovery of gold from the spent carbon powder generally entails burning the powder in a furnace, but the yield is relatively poor since about at least 10 % of the gold is not recovered. In addition, the conventional process is highly energy-consuming.

[0004] The invention starts from the finding that the conventional recovery process generates poor combustion of the carbon; this prevents recovery of a greater quantity of gold. In addition, most of the gold particles of small size (1-10 microns) are contained in the combustion flue gases which volatilize in air and cannot be recovered by the filters on account of their very small size.

[0005] The invention sets out to overcome these problems. In particular, the invention proposes providing a solution with a process having low energy consumption and which significantly increases the recovery yield of precious metal particles in spent activated carbon.

DISCLOSURE OF THE INVENTION

[0006] The invention therefore relates to a process for recovering a non-ferrous metal, gold in particular, from activated carbon powder used in a cyanidation leaching process for extracting precious metals, comprising the following steps:

- The activated carbon powder containing residues of precious metals (gold, silver, platinum, copper, etc.) is mixed with at least one combustible component and water to form a metal-laden activated carbon powder mixture; - The mixture is compressed in a compressing machine to form granules; - The granules are calcined in a furnace under controlled combustion conditions in particular at a temperature of between 500 and 800 °C for a time of between 60 and 120 minutes, to produce slag and ash which are mixed to form a concentrate having a carbon content less than or equal to 1 weight %.

[0007] Therefore, the invention starts from the principle that mechanical agglomerating of spent activated carbon powder, to form wet granules, associated with a fuel allows improved combustion and hence a reduction in the loss of fine particles of precious metals. In addition, by means of this transformation, it is easier to handle activated carbon in granule form rather than a powder, and it allows easy calculation of quantity of material as a function of the volume of the furnace combustion chamber, and adjustment of the amount of oxygen to be supplied to obtain more efficient calcining.

[0008] Controlled combustion is intended to oxidize the granules without reaching vitrification of the material; otherwise, solubilisation of the metal at the time of subsequent cyanidation would not be possible.

[0009] It was also identified in the invention that control over a low carbon content in the concentrate is of importance to ensure a maximum recovery yield at the subsequent leaching process. In the event that the carbon content should exceed the defined threshold, the calcining operation is continued until this threshold is reached. In this case, the calcined products can be associated with granules that have not yet been treated to conduct a new combustion operation.

[0010] The non-vitrification state of the slag can be controlled during the calcining operation, and combustion can be slowed or stopped before the onset of vitrification. Combustion can be slowed by reducing the amount of air for example, or any other means.

[0011] The combustion operation is preferably associated with continuous or discontinuous stirring of the granules and/or slag in the combustion chamber. This stirring can be obtained by blades, screws, pulsed air, a combination of these means or any other suitable means.

[0012] Preferably, the granules are arranged in the furnace on a grate comprising a multitude of openings having a diameter of between 4 and 6 mm.

[0013] Said furnace configuration has the advantage of promoting combustion by supplying a quantity of air to the combustion chamber. Air adjustment can be controlled manually or automatically. The number of combustion chambers may vary (1, 2, or more). Several air intakes can be provided in each chamber, for example two air intakes per chamber. Pulsed air means can be associated in the combustion chamber; these means possibly being fans and/or turbines able to be controlled automatically.

[0014] It is also possible to adjust flue gas exhaust in the top part of the furnace to maintain the temperature within the intended range and thereby prevent combustion runaway which could lead to vitrification of the product.

[0015] All these control means can be operated manually or automatically. Sensors can be arranged in the furnace such as temperature and/or pressure sensors to control the combustion parameters and to adapt input values such as speed of fans or turbines and/or size of air openings.

[0016] In one characteristic of the invention, the combustion component is cellulose based such as bran and/or sawdust, cellulose pulp, dried plant waste, etc.

[0017] Preferably the combustion component represents about 20 to 35 volume% of the total volume of the mixture, and water represents 10 to 25 volume % of the total volume of the mixture.

[0018] The metal-laden activated carbon powder mixture can advantageously comprise a chemical reactant which facilitates flammability of the granules without explosibility.

[0019] The granules are therefore designed to form the fuel as such, so that it is not necessary to add a heat source such as an electric or gas burner. Cellulose allows a permeable structure to be set up within the granules which promotes combustion. In addition, the volatility of carbon dust is limited without however considerably increasing the volume of material to be calcined (about 5 volume %).

[0020] Incorporation of the combustible component can be obtained by compression in an extrusion press for example, or any other suitable compression means.

[0021] In one preferred aspect, the size of the granules is between 10 and 15 mm in length and 6 to 8 mm in diameter. This granule size allows optimal combustion quality. The shape of the granules is not determinant. The granules can form cylinders, pyramids, cubes, spheres or any other three-dimensional shape.

[0022] In one advantageous aspect of the invention, the slag is cooled and milled in a grinder to a size smaller than 2 mm to produce said concentrate in combination with the ash.

[0023] The milling of slag to a smaller particle size allows an increase in the efficacy of the leaching process by increasing the contact surface between the calcined particles and the cyanide.

[0024] The metal contained in the concentrate can then be recovered in the known leaching process. The concentrate may contain between 1000 grams and 5000 grams of gold per tonne. The recovery rate by leaching can reach 98 weight % relative to the total weight of gold contained in the concentrate.

[0025] In one preferred aspect of the invention the slag, exhaust and flue gases derived from combustion are cooled by an air-air or air-water heat exchanger. This cooling takes place after the calcining operation. The slag removed from the furnace is stored in a slag box connected to an aspiration system to collect the fumes which are emanated on cooling of the slag, and directs these towards a bag filter. The fumes contain microparticles of precious metals which are to be captured. This cooled slag is ground and mixed with the ash to form the concentrate. The slag is preferably cooled to a temperature lower than or equal to 60 °C.

[0026] Preferably, the dust derived from the heat exchanger on cooling is recovered by condensation.

[0027] Therefore, the colder air of the heat exchanger laden with gas escaping from the slag is condensed producing a condensate containing particles of precious metals. For example, the sumps of the heat exchanger can be drained to recover calcination dust.

[0028] Preferably, the dust derived from the combustion flue gases is recovered by filters.

[0029] The recovered dust is compressed into briquettes and calcined in the furnace since the carbon content is too high. Calcining in the furnace is carried out under the same conditions of temperature and time as for the granules. Calcining of the briquettes can be carried out independently of the granules or in combination therewith.

[0030] The dust recovery filters can be bag filters in fabric material, for example in a number of 20 to 200, preferably 80 to 150. The dust can be collected every 12 to 48 hours for example and mixed to form briquettes. Extraction of dust from the filters can be obtained by complete cleaning with a powerful vacuum system having a double turbine for example.

[0031] The filters can be replaced after a period of activity of between 15 days and 60 das, preferably between 25 and 45 days.

[0032] The briquettes derived from collected combustion dust can contain chemical agents able to promote combustion. These briquettes can be formed in compacting equipment separate from the equipment producing granules.

[0033] The concentrate obtained by mixing slag and ash is then sent into the leaching process.

BRIEF DESCRIPTION OF THE FIGURES

[0034] Other characteristics and advantages of the invention will become apparent on reading the following description with reference to the appended Figures in which:

- Figure 1 schematically illustrates the recovery process of the invention;

- Figure 2 schematically illustrates the process integrated in the leaching process for recovering precious metals.

DETAILED DESCRIPTION OF ONE EMBODIMENT

[0035] A flow diagram of the gold recovery process from activated carbon waste derived from leaching is shown in Figure 1.

[0036] The process starts by collecting a volume of powder and/or pieces of metal-laden activated carbon which is mixed with a quantity of cellulose-based combustible material such as bran, cellulose pulp, sawdust, dried plant waste, etc. The cellulose-based material is a natural fuel which helps combustion and creates differential channels allowing air to enter into the granule and dynamise combustion. The mixture also preferably comprises a volume of water of between 10 and 25 %. Water provides the mixture with a low cost, environmentally-friendly source of oxygen and acts as oxidizer for the subsequent combustion operation.

[0037] This carbon-based mixture is formed into granules in pelletising or compression equipment 1. The equipment produces mechanical agglomeration of the mixture material and forms granules of small size, preferably of between 5 mm and a maximum of 20 mm.

[0038] Compression of the carbon fines associated with the forming of differential channels via the addition of cellulose promotes slow combustion of the granules during the subsequent operation and solves the problem of the volatility of carbon dust which affects the quality of combustion and reduces the recovery rate of precious metals. In addition, with granules it is easier to handle and calculate the optimal quantity of products to be calcined as a function of the volume of the furnace chamber, and to adjust the amount of oxygen to be supplied for combustion to reduce loss of fine particles in the exhaust gases.

[0039] At the following step, the granules are burnt in a furnace during a so-called "calcining" operation which produces slag, ash and flue gases containing vapour, gases and solid matter in dust form. The volume distribution of solid matter generally represents:

% slag; 35 % ash and 15 % dust. Calcining affords a weight reduction of about 55 % of total granule weight. The slag forms the portion with the highest precious metal content (about 50 weight %) whereas the ash and dust contain about twice less (25 weight %).

[0040] Calcining must be temperature-controlled to prevent vitrification of the slag. Vitrification is detrimental to dissolving of the gold contained in the slag at the time of leaching. It is therefore important that calcining should be conducted at temperatures of between 500 and 800 °C, preferably between 550° and 750 °C, more preferably between 600 and 700 °C. Calcining time is also controlled and can last until the carbon content is lower than 1 weight %. The purpose of calcining is to remove organic matter and to oxidize the mineral material without vitrification.

[0041] It is important to consider that there is a correlation between the level of the final concentrate obtained and the input weight of the fines. A concentrate of between 45 and 50 % allows precious metal recovery of approximately 98 % on average. If the level is higher than 50 % the recovery level drops to below 90 %. A concentrate level higher than 50

% can be accounted for by poor controlling of calcining conditions (percent oxygen, insufficient mixing, production of faulty granules, non-heed of proportions in the mixture before compacting, etc.).

[0042] The calcining furnace 2 preferably comprises at least one combustion chamber which has an openwork grate on which the granules are arranged. The grate separates the air circulation compartment from a combustion compartment. The openwork of the grate matches the size of the granules to guarantee retention thereof in the combustion chamber. It can be in cast-iron or any material able to withstand high temperatures.

[0043] The calcining process requires regular stirring of the product. It is therefore preferable to equip the combustion compartment with stirring means such as blades, screws, helical impellers, pulsed air etc. These means can be actuated automatically or manually and/or continuously or intermittently.

[0044] The calcining furnace 2 may comprise several combustion chambers thereby allowing increased production capacity. A chamber preferably comprises several air intakes: at least one or two per chamber which can be arranged in association with air circulation means such as a fan or turbine. A chamber also comprises an exhaust for flue gases preferably arranged at the top of the furnace; these means possibly comprising means to adjust opening of the exhaust such as a valve, hatch etc. allowing adjustment of the rate of combustion.

[0045] After calcining, the slag is removed from the furnace and cooled. Cooling is obtained by placing the slag in a slag box 3a connected to the ventilation system to aspirate fumes and direct these towards the bag filter. The dust contained in the heated air is preferably collected by the filters. This dust can be used to form briquettes compacted in compacting equipment 4.

[0046] The dust derived from combustion gases is also preferably collected by filtering equipment 5. This equipment may comprise a series of bag filters 6 to trap the dust. The air is aspirated into the filtering equipment 5 and extracted by an air extraction device 7. The collected dust is used to produce briquettes in the compacting equipment 4.

[0047] The slag can be crushed for size reduction in crushing equipment 8. Preferably, the slag is crushed or milled to a particle size of less than 5 mm, preferably less than 2 mm. After mixing the concentrate, the carbon level can be checked and/or the precious metal content measured.

[0048] The products derived from calcining, namely slag (about 50 %), ash (about 35 %) and preferably recovered dust (15 %) are mixed to form a concentrate which can be stored in a container 9 until use in the leaching process.

[0049] Figure 2 illustrates a known leaching process in which the recovery process of the invention is integrated as method for recycling spent activated carbon.

[0050] The leaching process comprises a first step 10 to collect auriferous ore, followed by a ball milling step 11 to release the minerals and flatten native precious metals. Gravity treatment can be carried out for rough concentration of the precious metals such as a sluice, jig, spiral, Wifley table, centrifugal concentrator or any other means.

[0051] At the following step 12 known per se the ore undergoes flotation to release the metal from sulfides which have cyanicide properties preventing good subsequent cyanidation.

[0052] The ore with precious metal concentration and free of sulfites is collected and fed into a leaching installation 13. The method known per se consists of setting up a redox reaction with the gold contained in the ore, cyanide ions and oxygen. Gold and other metals such as silver are placed in solution in the form of gold cyanide in ionic form ([AU(CN) 2 ions). The gold is extracted by adsorption on columns of activated carbon in granule form. The activated carbon is generally obtained by carbonizing plants such as coconut husks or peach stones.

[0053] After the carbon adsorption step, an elution step in an eluting tank 14 takes place to desorb the precious metals in the presence of a concentrated solution of cyanide at atmospheric pressure or under pressure. Finally, the gold is recovered from this eluting solution by electrolysis in an electrolytic installation 15. The exhausted carbon is returned to the adsorption circuit via circuit 16.

[0054] In the process of the invention, the exhausted carbon no longer having a sufficient adsorption yield is collected in a tank 17 to be treated in accordance with the gold recovery process 20 as described in detail in Figure 1. The concentrate of calcining products 8 obtained with this process is sent to the leaching cycle, in particular to installation 13.

[0055] This recovery process allows an increase in the recovery of precious metals in the region of 10 % by weight compared with the conventional process.

Claims

1. A process for recovering precious metals, gold in particular, from activated carbon powder used in a cyanidation leaching process for extracting metal, comprising the following steps: - The activated carbon powder containing residues of precious metals is mixed with at least one combustible component and water to form a metal-laden activated carbon powder mixture; said combustible component being cellulose based such as bran and/or sawdust, cellulose pulp, dried plant waste - The mixture is compressed in a compressing machine to form granules; - The granules are calcined in a furnace under controlled combustion conditions in particular at a temperature of between 500 and 800 °C for a time of between 60 and 120 minutes, to produce slag, ash and flue gases containing vapour, exhaust and solid matter in dust form which are mixed to form a concentrate having a carbon content less than or equal to 1 weight %, said process being characterized in that it comprises the following additional steps for the purpose of: - cooling the slag, exhaust and flue gases derived from combustion by an air-air or air-water heat exchanger, - recovering by condensation the dust derived from the heat exchanger on cooling and derived from the calcining step of the granules, - compressing said dust into briquettes, - calcining said dust briquettes in said furnace.

2. The process according to claim 1, characterized in that the combustible component represents about 20 to 35 volume % of the total volume of the mixture, and water represents 10 to 25 volume % of the total volume of the mixture.

3. The process according to one of claims 1 or 2, characterized in that the metal laden activated carbon powder mixture comprises a chemical reactant intended to facilitate the flammability of the granules without explosibility.

4. The process according to any of the preceding claims, characterized in that the size of the granules is between 10 and 15 mm in length and 6 to 8 mm in diameter.

5. The process according to any of the preceding claims, characterized in that combustion is brought to a temperature of between 600 and 700 °C for a time of between 60 and 120 minutes.

6. The process according to any of claims 1 to 5, characterized in that the dust derived from combustion exhaust gases is recovered by filters.

7. The process according to any of the preceding claims, characterized in that the granules are arranged in the furnace on a grate comprising a multitude of openings of diameter between 4 and 6 mm.

8. The process according to any of the preceding claims, characterized in that the slag is cooled and milled in a grinder to a size of less than 2 mm to produce said concentrate in combination with the ash.