CA3111658A1 - Process for recovering non-ferrous precious metals by pelletisation and calcination of leaching activated carbon powder - Google Patents

Abstract

The invention relates to a process for recovering precious metals, in particular gold, from activated carbon powder used in a cyanidation leaching process for the extraction of the metal, comprising the following steps: - a carbon powder active agent containing precious metal residues is mixed with at least one combustible component and water to form a metal-loaded activated carbon powder mixture, the mixture is compressed in a compression machine to form granules , the granules are then calcined in an oven under controlled combustion conditions, in particular at a temperature of between 500 and 800 ° C and for a time of between 60 and 120 minutes so as to produce slag and ash which are mixed to form a concentrate whose carbon content is less than or equal to 1% by weight.

Description

PRECIOUS NON-FERROUS METAL RECOVERY PROCESS BY
PELLETIZATION AND CALCINATION OF ACTIVE CARBON POWDER
LEACHING
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for recovering metal valuable especially gold from waste activated carbon.
STATE OF THE PRIOR ART

The process for recovering gold by coal leaching active is good known. In this process which knows many possible variants, the ore loaded with gold is first crushed and then concentrated in a flotation circuit to remove sulphites and other unwanted residues. Flotation sludge past then in leach tanks where it is mixed with a mixture of sodium cyanide and milk of lime in the presence of oxygen. The gold that is solubilized by cyanide is adsorbed on activated carbon filters. Activated carbon can be advanced against the current of mud. The coal is then charged with gold for to be then washed with high pressure water and acid solution hydrochloric then the carbon is desorbed by injecting a solution of cyanide and welded caustic. The gold solution obtained is then put into circulation at through electrolytic extraction cells arranged in parallel. Gold and others metals are precipitated on cathodes which are immersed in the electrolyte circulating. 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 then calcined in the presence of borax, silica and sodium carbonate. The mixture obtained is melted in an oven at 1300 C
and the melt is poured into cascading molds where the gold is separated from the slag for produce gold bars.

[0003] During adsorption, carbon also loses activity.
In order to restore the active surface and allow its reuse in the circuit of leaching, it can undergo thermal regeneration. However after several cycles of leaching and regeneration, the activated carbon is used up and turns into powder loaded with precious metals (of which gold and silver predominate) and non-precious. The charcoal powder may contain a gold content in the range of 200 grams to 4 kg per tonne. Gold recovery from used coal powder consists in general to burn the powder in an oven but the yield is relatively bad since about at least 10% of the gold is not recovered. In addition, the traditional process is very energy intensive.

[0004]
The invention starts from the observation that the conventional recovery process causes poor combustion of coal; what prevents recovery of a greater quantity of gold. In addition, a good part of the gold particles of small size (1-10 microns) are contained in the combustion fumes which se volatilize in the air and cannot be recovered by the filters of the made them too small size.

[0005]
The invention proposes to remedy these problems. In particular, the invention proposes to provide a solution by an economical process energy and which significantly increases the efficiency of particle recovery of precious metals in used activated carbon.
DISCLOSURE OF THE INVENTION

[0006]
The invention thus relates to a method for recovering a metal non-ferrous, especially gold, from activated carbon powder used in a cyanidation leaching process for metal extraction precious comprising the following steps:
-Activated carbon powder containing precious metal residues (gold, silver, platinum, copper, etc.) is mixed with at least one fuel component and water to form a mixture based on activated carbon powder loaded with metal, - The mixture is compressed in a compression machine so as to form granules, - The granules are then calcined in an oven under conditions controlled combustion, in particular at a temperature between 500 and 800 C and for a time between 60 and 120 minutes so as to produce slag and ash which is mixed to form a concentrate whose carbon content is less than or equal to 1% by weight.

[0007] Thus, the invention assumes that an agglomeration mechanics of the used activated carbon powder in the form of wet granules associated with a fuel improves combustion and therefore reduces the loss of fine particles of precious metals. Moreover, thanks to this transformation, it is more easy to handle activated carbon in granules than in powder and allows easily calculate the quantity of material according to the volume of the chamber of combustion furnace and regulate the quantity of oxygen to be supplied to obtain a more efficient calcination.

[0008] The controlled combustion thus aims to oxidize the granules without reach vitrification of the material; which would not otherwise allow a solubilization of the metal during the subsequent cyanidation.

It has also been identified in the invention that the control of a stocking rate carbon in the concentrate is important to ensure a yield of maximum recovery in the subsequent leaching process. In the case where the rate of carbon would exceed the defined threshold, the calcination operation continues until reaching this threshold. In this case, the calcined products can be combined Has pellets not yet treated for a new combustion operation.

The state of non-vitrification of the slag can be checked during of the operation calcination and combustion can be slowed down or stopped before the start of the vitrification. Combustion can be slowed down by reducing air by example or all other way.

The combustion operation is preferably associated with stirring continuous or discontinuous granules and / or slag in the chamber of combustion.
This mixing can be achieved by blades, screws, forced air, a combination of these means or any other appropriate means.

Preferably, the granules are placed in the oven on a wire rack comprising a multitude of openings with a diameter of between 4 and 6 mm.

Such an oven configuration has the advantage of promoting combustion by supplying a quantity of air in the combustion chamber. The air regulation can be controlled manually or automatically. The number number of combustion chambers may vary (1, 2 or more). Several air inlets can be provided in each chamber, for example two air inlets per bedroom. Pulsed air means can be associated in the chamber of combustion; these means possibly being fans and / or turbines can be controlled automatically.

It is also possible to regulate the exhaust of the fumes in the part high in the oven in order to maintain the temperature within the prescribed range and avoid so runaway combustion which may lead to vitrification of the product.

All these control means can be done manually or automatic. Sensors can be arranged in the oven such as sensors temperature and / or pressure in order to control the parameters of combustion and adapt input values such as fan speed or turbines and / or the size of the air openings.

According to one characteristic of the invention, the component fuel is at cellulose base such as bran and / or sawdust, pulp of cellulose, dried vegetable waste, etc.

Preferably, the fuel component represents approximately from 20 to % by volume of the total volume of the mixture and water represents 10 to 25% by volume of the total volume of the mixture.

The mixture based on charged activated carbon powder can advantageously include a chemical reagent which facilitates flammability of granules without explosiveness.

The granules are thus designed to constitute the fuel by so much as such so that it is not necessary to add a heat source as an electric or gas burner. Cellulose thus makes it possible to create a structure permeable in the granules which promote combustion. In addition, the volatility of coal dust is limited without, however, significantly increasing the volume of material to be calcined (about 5% by volume).

The incorporation of the fuel component can be done by compression in an extruding press for example or by any other compression means appropriate.

According to a preferred aspect, the size of the granules is between 10 and 15 mm in length and 6 to 8 mm in diameter. This size of granules allows a optimum combustion quality. The shape of the granules is not decisive.
The granules can form cylinders, pyramids, cubes, spheres or any other three-dimensional shape.

According to an advantageous aspect of the invention, the slag is cooled then crushed in a crusher to a size less than 2mm to produce, in combination with the ashes, said concentrate.

[0023] The grinding of the slag into smaller particle size allows increase the efficiency of the leaching process by increasing the surface area of contact between calcined particles and cyanide.

The metal contained in the concentrate can then be recovered in the known leaching process. The concentrate can have approximately between 1000 grams and 5000 grams per tonne of gold. The recovery rate by leaching can reach 98% by mass relative to the total mass of gold contained in the concentrated.

According to a preferred aspect of the invention, the slag, gas and fumes from combustion are cooled by an air-to-air heat exchanger or air-water.
This cooling takes place after the calcination operation. Slag exits from oven are stored in a cabinet connected to a suction system which collects the puffs of smoke that appear when cooling of slag which sends them to the bag filter. The volutes contain Precious metal microparticles that should be captured. These slags cooled are crushed and mixed with the ash to form the concentrate. Slag are preferably cooled to a temperature less than or equal to 60 C.

Preferably, the dust from the heat exchanger during the cooling are recovered by condensation.

[0027] Thus, the colder air from the heat exchanger loaded with gas escaping slag condenses; thus producing a condensate charged with precious metal particles. For example, the sumps of the heat exchanger heat can be emptied to collect calcination dust.

Preferably, the dust from the exhaust gases of the .. combustion are recovered by filters.

Thus, the recovered dust is compressed into briquettes then calcined in the furnace because the carbon content is too high. Calcination in the oven is operated under the same temperature and time conditions as those carried out for the granules. The briquettes can be calcined independently of or in combination with the granules.

Dust recovery filters can be filters handle of textile material, for example 20 to 200, preferably 80 to 150. Dust can be collected every 12 to 48 hours by example and mixed to then form the briquettes. The extraction of dust from the filters can be done by complete cleaning with a vacuum powerful, with double turbine for example.

Filters can be replaced after a period of activity included between 15 days and 60 days, preferably between 25 and 45 days.

Briquettes from the collected combustion dust can contain chemical agents capable of promoting combustion. These briquettes can be manufactured in a compaction device separate from the device at produce the granules.

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

BRIEF DESCRIPTION OF THE FIGURES

[0034]
Other characteristics and advantages of the invention will emerge from reading of the following description, with reference to the appended figures, to which:
- Figure 1 is a schematic representation of the process recovery of the invention;
-Figure 2 is a schematic representation of the process integrated into the leaching process for the recovery of precious metal.
DETAILED DESCRIPTION OF AN EMBODIMENT

A block diagram of the process for recovering gold from Activated carbon waste from leaching is shown in Figure 1.

[0036]
The process begins with the collection of a volume of powder powder and / or pieces of activated carbon loaded with metal and its mixture with a quantity of combustible material based on cellulose such as bran, the cellulose pulp, sawdust, dried vegetable waste., etc. The matter to cellulose-based is a natural fuel that helps in combustion and creates of differential pathways that allow air to enter the pellet and escape boost combustion. The mixture preferably also comprises a volume of water between 10 and 25%. Water provides an inexpensive supply of oxygen and ecological in the mixture which is used as oxidizer in the operation of combustion later.

[0037]
This charcoal-based mixture is made into a granule in a pelletizing or compression apparatus 1. The apparatus produces a agglomeration mechanics of the material of the mixture and forms small granules, of preferably between 5 and 20 mm maximum.

[0038]
Compression of carbon fines associated with the creation of tracks differential by the addition of cellulose promotes the slow combustion of granules in the next operation and solves the problem of volatility of dust of charcoal that affects the quality of combustion and reduces the rate of recovery of precious metals. In addition, thanks to the granules, it is easier to handle and of calculate the optimal quantity of products to be calcined according to the volume of the oven chamber and adjust the amount of oxygen to be supplied to the combustion in order to reduce the loss of fine particles in the exhaust fumes.

In the next step, the granules are burned in an oven during of a operation known as calcination which makes it possible to produce slag, ash as well as fumes containing vapor, gases and matter solid in the form of dust. The volume distribution of solid matter in general represents: 50% slag; 35% ash and 15% dust.
The calcination brings about a weight reduction of about 55% of the weight of granules.
Slag forms the part richest in precious metals (about 50% in weight) while ash and dust contain about two time less (25% by weight).

Calcination must be controlled in temperature so as to avoid the vitrification of slag. Vitrification is detrimental to the dissolution of gold contained in the slag during leaching. Thus, it is important that the calcination is carried out at temperatures between 500 and 800 C, preferably between 550 and 750 C, even more preferably between 600 and 700 C.
The calcination time is also controlled and can last as long as the rate in carbon is less than 1% (weight). The purpose of calcination is to eliminate the matter organic and oxidize mineral matter without vitrification.

It is important to consider that there is a correlation between the rate of final concentrate obtained and the weight at the inlet of the fines. Thus a rate of concentrated between 45 and 50% allows a precious metal recovery rate of around 98% on average. If the rate is greater than 50%, the rate of recovery drops below 90%. A concentrate rate greater than 50% is explained by a poor control of calcination conditions (oxygen level, mixing insufficient, faulty production of the granules, non-compliance with the proportions in mixing before compaction, etc.).

The calcination furnace 2 preferably comprises at least one combustion chamber which includes a perforated grate on which are WO 2020/1575

43 PCT / IB2019 / 050787 deposited the granules. The grille separates an air circulation compartment of a combustion compartment. The grid is perforated to the size of the granules in order to ensure their retention in the combustion chamber. It can be made of cast iron or any material resistant to high temperatures.
The calcination process requires regular stirring of the product.
Also, it is preferable to equip the combustion compartment with means of mixing such as blade (s), screw, propeller (s), forced air, etc. These means can to be activated automatically or manually and / or continuously or intermittent.

The calcination furnace 2 can include several chambers of combustion thus increasing production capacity. Bedroom preferably comprises several air inlets; at least one or two per chamber, which can be arranged in association with a means of circulation air like a fan or turbine. The chamber includes an exhaust fumes, preferably placed on the top of the oven; the means that can understand a means of adjusting the opening of the exhaust as a valve, trap, etc., thus allowing the combustion speed to be adjusted.

After calcination, the slag is taken out of the furnace and cooled. The cooling is done by storing the slag in a connected cabinet 3a at ventilation system to suck in the volutes of smoke and send them to the filter sleeves. The dust contained in the heated air is preferably collected by filters. These dusts can be used to form of briquettes compacted in a compaction device 4.

Dust from combustion fumes are also .. preferably collected by a filtration apparatus 5. This apparatus can include a series of bag filters 6 to trap the dust.
The air is drawn into the filter apparatus 5 and extracted by a device extraction air 7. The collected dust is used for the production of briquettes in the compaction device 4.

The slag can be crushed to reduce its size in one crushing apparatus 8. Preferably, the slag is crushed or crushed to a particle size of less than 5mm, preferably less than 2mm.
After mixture of the concentrate, the carbon content can be checked and / or a measurement of the precious metal content carried out.

The products resulting from the calcination, namely the slag (50%
about), ash (approximately 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.

Figure 2 shows a known leaching process in which the recovery process of the invention is integrated as a recycling process of the used activated carbon.

The leaching process comprises a first step 10 of collection of gold ore then a grinding step 11 by balls to release the minerals and flatten native precious metals. Gravity treatment can to be performed to roughly concentrate precious metals, like a sluice, of jigs, spiral, Wifley table, centrifugal concentrator or any other way.

In the next step 12 known per se, the ore undergoes a flotation which releases the metal from sulphides whose cyanicidal properties prevent good subsequent cyanidation.

The ore concentrated in precious metal and freed from sulfites is then collected and introduced into a leaching plant 13. The known method in itself consists in establishing an oxidation-reduction reaction with the gold contained in the ore, cyanide ions and oxygen. Gold as well as other metals as silver are dissolved in the form of gold cyanide in ionic form (ions [AU (CN) 2] -). Gold is extracted by adsorption on coal columns active under granule form. Activated carbon is generally obtained by carbonization of plants such as coconut bark or peach stones.

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

In the method according to the invention, the spent coal and whose yield adsorption is no longer sufficient is collected in a tank 17 to be treaty according to the method 20 for recovering gold as described in detail in FIG.
1. The concentrate of calcination products 8 obtained by this process is returned in the leaching cycle, in particular in the installation 13.

This recovery process allows an increase in the recovery precious metals of the order of 10% by weight relative to the process traditional.

Claims (10)

AMENDED CLAIMS
received by the International Bureau on June 15, 2020 (06.15.20) 1. Process for recovering precious metals, in particular gold, at go activated carbon powder used in a leaching process by cyanidation for metal extraction comprising the following steps:
- The powder activated carbon containing precious metal residues is mixed with at least one combustible component and water to form a mixture based on activated carbon powder loaded with metal, said fuel component being based on cellulose such as bran and / or sawdust, cellulose pulp, waste dried plants, - The mixture is compressed in a compression machine so as to form granules, - The granules are then calcined in an oven under conditions controlled combustion, in particular at a temperature between 500 and 800 C and for a time between 60 and 120 minutes so as to produce slag, ash and smoke containing vapor, gases and solid matter in the form of dusts which are mixed to form a concentrate whose rate of carbon is less than or equal to 1% by weight, said method being characterized in that it comprises the steps additional following aimed at:
- recover said dust from the calcination step of granules, - compress said dust into briquettes, - calcining said briquettes of dust in said furnace. 2. Method according to claim 1, characterized in that the component fuel represents approximately 20 to 35% by volume of the total volume of the mixture and the water represents 10 to 25% by volume of the total volume of the mixed.
AMENDED SHEET (ARTICLE 19) 3. Method according to one of claims 1 or 2, characterized in that the charged activated carbon powder mixture comprising a reagent chemical designed to facilitate the flammability of the granules without explosiveness. 4. Method according to any one 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. Method according to any one of the preceding claims, characterized in that the combustion is brought to a temperature between 600 and 700 C for a time between 60 and 120 minutes. 6. Method according to any one of the preceding claims, characterized in that the slags, gases and fumes resulting from the combustion are cooled by an air-to-air or air-to-water heat exchanger. 7. Method according to claim 6, characterized in that the dust from the heat exchanger during cooling are recovered by condensation. 8. Method according to claim 6 or 7, characterized in that the dust from the combustion exhaust gases are recovered by filters. 9. Method according to any one of the preceding claims characterized in that the granules are placed in the oven on a grid comprising a multitude of openings with a diameter of between 4 and 6 mm. 10. Method according to any one of the preceding claims characterized in that the slag is cooled and then crushed in a crusher to a size less than 2mm to produce in combination, with ash, said concentrate.

AMENDED SHEET (ARTICLE 19)