CA2994578A1 - Hydrometallurgical method for silver recovery - Google Patents

Abstract

A process for recovering silver from silver-bearing gold concentrate or other silver-bearing material. The process comprises: (i) optionally regrinding the input silver-bearing material; (ii) optionally treating the reground gold concentrate with sulfuric or other aqueous acid in an acidulation step; (iii) adding oxygen, water and/or acid to the acidulated concentrate slurry and reacting them together in an autoclave at an elevated pressure and temperature in a pressure oxidation step; (iv) processing the oxidized concentrate slurry in a post pressure oxidation conditioning step; (v) applying a first solid/liquid separation and wash step, comprising at least one of a thickening step, a counter current decantation (CCD) wash step, and a filter and wash step, to form a first washed slurry/solid and first acid-containing solutions; (vi) reacting the first washed slurry/solid with sulfur dioxide (and optionally sulfuric acid and/or water) in a reductive leach step; (vii) applying a second solid/liquid separation and wash step, comprising at least one of a thickening step, a CCD wash step and a filter and wash step, to form a second washed slurry/solid and second acid-containing solutions, wherein the first and second acid-containing solutions can be recycled to the acidulation and/or pressure oxidation steps; and (viii) applying an optional surface cleaning step, to produce a free-milling silver-bearing material, which is amenable to conventional cyanidation to recover the silver therefrom.

Description

HYDROMETALLURGICAL METHOD FOR SILVER RECOVERY
Field of the Invention The present invention relates to methods for recovering silver from silver-bearing material.
More particularly, the present invention relates to a process for recovering silver from gold concentrate and other silver-bearing metal ore concentrates from mining operations.
Background of the Invention In silver-bearing mineral ores, silver is typically found together with other metals (such as gold copper, lead, zinc, etc.). Various silver extraction techniques can be applied to silver-bearing mineral ore/concentrate, depending on what the main metal in the ore/concentrate is (e.g.
whether the major metal is gold, copper, zinc or lead).
Gold and silver are sometimes present together in mined ore - sometimes alloyed together (as electrum). They are also sometimes found in the form of a solid solution within sulfide minerals, such as iron sulfide (pyrite - FeS2), iron arsenic sulfide (arsenopyrite -FeAsS), lead sulfide (galena - PbS) and zinc sulfide (sphalerite - ZnS). These ores typically contain sub-microscopic gold and/or silver that are encapsulated within a crystal matrix of the sulfide mineral. These ores are naturally resistant to recovery/extraction of the gold and silver using conventional cyanide leaching processes (cyanidation), and are often referred to as "refractory ores". These refractory ores conventionally have to undergo pre-treatment in order to break down and oxidise the metal sulfide mineral matrix so that the subsequent cyanidation process will be effective in recovering the silver or gold. In the case of gold concentrate, such pre-treatment conventionally involves a pressure oxidation process, which is used to prepare the concentrate for subsequent conventional metal extraction processes such as cyanidation. (Roasting and bacterial leaching are also possible steps for this pre-treatment process, but, for various reasons, are generally less popular than pressure oxidation).
Although not specifically discussed herein, it is conventional practice in gold mining operations when dealing with refractory ores, that the mined ore will also undergo froth flotation (or other concentration or separation techniques) to first concentrate the target metal content, as one part

- 2 -of the pre-treatment process, before putting the concentrate through the pressure oxidation process. As used herein, "concentrate" can generally refer to the flotation concentrate, the gravity concentrate, or the ore after oxidation, dissolution, leaching or any pre-treatment, or any solid product from combinations of the foregoing pre-treatments.
The above-mentioned pressure oxidation process is typically performed in an autoclave at high pressure and temperature, where high-purity oxygen is mixed with a slurry of the refractory ore or concentrate. In this pressure oxidation reaction, the original sulfidic mineral is oxidised and broken down, releasing the trapped target metal (which in the case of gold concentrate, is gold).
In gold mining/extraction, pressure oxidation produces a high gold recovery -normally 10%
higher than when roasting is used instead.
In the case of gold ore concentrate, in which the gold is associated predominately with pyrite (FeS2) or arsenopyrite (FeAsS), the sulfide minerals are oxidised in the pressure oxidation step to form sulfuric acid, soluble compounds such as ferric sulfate, and solid compounds such as hematite, basic ferric sulfate, jarosite, scorodite or basic iron arsenate sulfate. The iron sulfate-based solid compounds are generally undesirable, since they can potentially release acid and heavy metals into the environment, which presents an environmental challenge.
They can also make subsequent precious metal recovery more difficult due to their acidic nature when being subjected to alkaline conditions. During pressure oxidation, conventionally, arsenic in the ore/concentrate is converted to solid scorodite inside the autoclave, allowing it to be easily disposed of due to its alleged stability in natural environment. This is an advantage over other processes such as roasting where arsenic is released as toxic gases which must be fully captured or scrubbed. During pressure oxidation, basic ferric arsenate sulfate may also form in addition to scorodite; however, basic ferric arsenate sulfate is typically less stable than scorodite.
A disadvantage of conventional pressure oxidation, however, is that any silver in the feed material will often react to form silver jarosite, AgFe3(OH)6(SO4)2 inside the autoclave under pressure oxidation conditions, which makes it difficult and expensive to recover the silver using conventional processes. This is because silver jarosite does not dissolve in cyanide solution, which is conventionally used to leach out the silver and gold content. Thus, in conventional processing of gold concentrate which also happens to contain silver, where a conventional pressure oxidation step is employed as part of that processing (typically in the case of refractory

-3 -ores or concentrates) and cyanidation is used to extract the gold, much of the silver content cannot be easily recovered using such cyanidation (because it forms silver jarosite, which does not readily dissolve in the cyanide leaching solution). Therefore, in conventional gold extraction operations, much of the silver content is either lost or requires further processing such as using lime boil treatment (discussed below) to break down silver jarosite before cyanidation is applied (thus adding to the processing cost). In ores/concentrates where the silver content is significant, this can mean that a significant amount of the silver value of such ore is not realized.
One known commercial application that is utilised following a pressure oxidation step to break down silver jarosite before recovering the silver from cyanidation, is lime boil treatment. This treatment involves treating the silver jarosite-containing material (slurry) with strongly alkaline lime slurry at elevated temperature near the boiling point of water under atmospheric pressure for several hours. The general reaction for this lime boil treatment is as follows:
2AgFe3(SO4)2(OH)6 + 4Ca(OH)2 + H20 ---> 6Fe0OH + Ag20 + 4CaSO4.2H20 This reaction serves to decompose the silver jarosite, thus releasing the silver so that it is amenable to cyanide leach. (The iron associated with jarosite may be converted to goethite Fe0OH (as shown), or ferric hydroxide Fe(OH)3, or a mixture of these two).
However, this lime boil treatment has a number of downsides: firstly it results in a significant increase in the slurry viscosity; secondly, any arsenic in the solid is made less stable (which is generally not preferred).
Further, the lime boil treatment consumes a large amount of lime, which is very costly. Further, the product slurry after lime boil may have a pH which is too high for conventional cyanidation.
Also, gold grade and silver grade are diluted due to the formation of gypsum (CaSO4.2H20) and goethite (Fe0OH) in the solid. As such, lime boil treatment is not fully satisfactory and usually not justifiable in many circumstances.
Accordingly, it is contemplated that it would be advantageous to provide an alternative process for enhancement of silver recovery from silver-bearing material, such as gold concentrate or gold concentrate material from pressure oxidation operations. Furthermore, there are advantages in being able to provide a process which, in addition to recovering the silver content, can also produce a significant mass reduction in the concentrate material. The solid mass reduction from

- 4 -the concentrate can lead to processing efficiencies, since less material needs to be transported and less material itself has to be further processed downstream.
We shall describe the present invention as a process for recovering silver in the context of processing of gold concentrate from gold mining operations. However, it should be understood that the disclosed process may also be applied to the processing of any silver-bearing metal concentrate (e.g. gold/silver/copper concentrate, gold/silver concentrate) or other silver-bearing materials. The disclosed invention may also be considered as a process for recovering silver from the products of pressure oxidation or from silver jarosite.
Brief Summary of the Invention Disclosed herein is a process for recovering silver from silver-bearing gold concentrate or other silver-bearing material, including from the solid that is derived from pressure oxidation operations.
The disclosed process comprises several basic steps (such as regrinding, acidulation, pressure oxidation, solid/liquid separation and washing and reductive leaching, etc.) that may sometimes be included among some of the processing steps in which mined gold ore may be processed/concentrated into a more concentrated form. The input for the subject process is silver-bearing gold concentrate (which generally speaking is gold/silver concentrate that has been extracted from a mine, and which may or may not have undergone some pre-treatment in the form of froth flotation or gravity concentration, but which generally has not yet been subject to hydrometallurgical, biological or pyrometallurgical processing).
The disclosed process comprises: (i) optionally regrinding the input silver-bearing material; (ii) optionally treating the reground silver-bearing material in an acidulation step; (iii) applying a pressure oxidation step; (iv) applying a post pressure oxidation conditioning step; (v) applying a first solid/liquid separation and wash step; (vi) applying a reductive leach step to the underflow (solid) from the first solid/liquid separation and wash step; (vii) applying a second solid/liquid separation and wash step; and (viii) applying an optional surface cleaning step, to produce a free-milling silver-bearing material, which is amenable to conventional cyanidation to recover the silver therefrom.

- 5 -Brief Description of the Drawings Embodiments of the present invention are described below with reference to the accompanying drawings in which:
Fig. 1 is a simplified flowchart illustrating the process in accordance with one aspect of the present invention.
Detailed Description of the Invention The present invention now will be described more fully hereinafter with reference to the accompanying drawing(s), which form a part hereof, and which show, by way of illustration, exemplary embodiments by which the invention may be practiced. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The following detailed description is, therefore, not to be taken in a limiting sense.
Referring to Fig. 1, this is a simplified flowchart setting out an exemplary method and process 10 for recovering silver from silver-bearing gold concentrate. The disclosed process comprises: (i) optionally regrinding the input silver-bearing gold concentrate; (ii) optionally treating the reground silver-bearing gold concentrate in an acidulation step; (iii) applying a pressure oxidation step; (iv) applying a post pressure oxidation conditioning step; (v) applying a first solid/liquid separation and wash step; (vi) applying a reductive leach step to the underflow (solid) from the first solid/liquid separation and wash step; (vii) applying a second solid/liquid separation and wash step; and (viii) optionally applying a surface cleaning step, to produce a free-milling silver-bearing material.
The initial input for the subject process is silver-bearing gold concentrate or other silver-bearing material 12 (which we shall generally refer to as "silver-bearing gold concentrate"). Preferably, the input silver-bearing gold concentrate has already undergone some pre-processing or concentration before it arrives at this input stage, in which case it is generally in the form of an aqueous slurry or moist filter cake. For example, froth flotation techniques (and/or other applicable gravity concentration techniques) may have been applied to the gold ore to

- 6 -concentrate the gold (particularly in the case of gold ore where the gold is closely associated with sulfide minerals such as pyrite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite and galena. It should be understood, however, that the subject process could also be applied where freshly mined silver-bearing gold ore is the initial input.
Where appropriate, the input silver-bearing gold concentrate 12 undergoes mechanical regrinding (step 14), to form a reground gold concentrate 15. The extent of such regrinding required, and whether it is necessary at all, will depend on the state of the input silver-bearing gold concentrate (for example, it may already have undergone some degree of grinding during the above mentioned pre-processing, in which case, little or no further regrinding may be necessary). The main purpose of this regrinding step is simply to break down the silver-bearing gold concentrate particles into smaller pieces, so that it has a greater surface area, in order to enhance reaction rate and reaction completeness in the downstream processing steps.
The silver-bearing gold concentrate 12 or the reground gold concentrate 15, as the case may be, (either of which is in the form of an aqueous slurry) is then treated with acid in an acidulation step 16 to form an acidulated concentrate slurry 17. The acid used is preferably either concentrated sulfuric acid or an aqueous sulfuric acid solution. The acid generally serves to react with and break down the carbonates in the gold concentrate to form carbon dioxide (CO2), which can then be removed in the form of CO2 gas. The acidulation step 16 can generally be carried out in one or multiple stirred tanks at ambient pressure for one or a few hours.
One of the objectives, of using acid and dissolved metals (such as dissolved ferric sulfate) during the acidulation step 16, is to make the solid compounds formed during pressure oxidation 18 unstable, so that mass reduction of the solid phase and solubilization of arsenic can be maximized in the ensuing pressure oxidation step 18 and subsequent post pressure oxidation conditioning step 22. This objective is counterintuitive to what is typically desired in the mining industry as a whole when utilizing comparable pressure oxidation steps; in those situations, the general objective is to form the most stable solid compounds such as hematite and scorodite during pressure oxidation (particularly when processing arsenic-bearing materials). The purpose of the acidulation step 16 in the disclosed process, besides adding extra acid for the subsequent pressure oxidation step 18, is to maximise the instability of the solid compounds, and to dissolve and keep as much iron, sulfur and arsenic in solution as possible.

- 7 -For greater operational efficiency and flexibility, it is contemplated that, as shown in Fig. 1, acid-containing solutions from other parts of the process (i.e. the acid-containing solutions from downstream steps such as 30, 31, 32, 46, 47 and/or 48) may be recycled for this acidulation step 16. Where appropriate, cooling may be applied to these warm/hot acid-containing solutions before recycling to minimize heat input when the downstream pressure oxidation is already surplus in heat generation. The extent of recycling of the acid-containing solutions streams 30, 31 and 32 to the acidulation step 16 can be as needed, and can vary from 0% to nearly 100%.
These acid-containing solutions may also contain, besides the acid itself, dissolved iron (e.g. in the form of ferrous and/or ferric), dissolved arsenic (e.g. in the form of arsenite and/or arsenate), and dissolved sulfate salts, etc. For example, the acid-containing solution can either be substantially free of solids (such as the overflow stream 32 from the thickening step (step 24) described below or it can contain solids, (such as the oxidized concentrate slurry 19 from the autoclave discharge or the underflow stream of thickened oxidized concentrate slurry 25, etc.).
Besides free acid (sulfuric acid), acid can also come from the hydrolysis of dissolved metals such as ferric iron. The contained solids in the recycled stream tend to serve as seeding material which is helpful to the reaction rates, to the stability of the solid compounds and to the mitigation of scale formation inside the autoclave.
The acidulated concentrate slurry 17 is then fed into a pressure oxidation vessel or autoclave.
Oxygen and, if required for temperature control, water are added (stream 20) to the autoclave.
In a preferred embodiment, and as shown in Fig. 1, the acid-containing overflow streams from downstream steps (30, 31, 32, 46, 47, 48), as needed, may also be recycled and added to the autoclave. Where appropriate, cooling may be applied to these warm/hot acid-containing solutions before recycling to minimize heat input when the pressure oxidation is already surplus in heat generation. The contents of the autoclave are reacted together to oxidize the sulfide minerals therein under high pressure and temperature conditions (sometimes referred to herein as oxidizing pressure and oxidizing temperature, respectively) to form an oxidized concentrate slurry 19. During this pressure oxidation step 18, as the sulfides are oxidized, the silver in the feed material is initially liberated, but is subsequently precipitated, mostly as silver jarosite [AgFe3(504)2(OH)6] or is incorporated into other jarositic species, all of which are refractory to cyanide leach.

- 8 -The reaction conditions for this pressure oxidation step 18 are preferably:
from about 190 to about 240 C; from about 200 to about 600 psig total pressure; from about 15 to about 250 psi oxygen partial pressure; from about 30 to about 120 minutes retention time.
More preferably, the reaction conditions for the pressure oxidation step are: from about 220 to about 230 C; from about 430 to about 530 psig total pressure; from about 25 to about 100 psi oxygen partial pressure; and from about 60 to about 90 minutes retention time.
During pressure oxidation (step 18), sulfide minerals are oxidized to various compounds. For instance, sphalerite (ZnS) is oxidized to zinc sulfate (ZnSO4) in solution.
Chalcopyrite (CuFeS2) is oxidized to copper sulfate (CuSO4) and sulfuric acid (H2SO4) in solution, and iron compounds including hematite (Fe203), jarosite [MFe3(OH)6(SO4)2] and/or basic ferric sulfate (FeOHSO4) in the solid, plus the formation of dissolved ferrous iron sulfate (FeSO4) and dissolved ferric iron sulfate [Fe2(SO4)3] in solution. Galena (PbS) is oxidized to form insoluble lead sulfate (PbSO4) and/or insoluble lead jarosite (plumbojarosite) [Pbo 5Fe3(OH)6(SO4)2].
Depending on pH and temperature conditions, oxidation of pyrite (FeS2) can lead to formation of hematite (Fe203), jarosite [MFe3(OH)6(SO4)2] and/or basic ferric sulfate (FeOHSO4) in the solid, plus the formation of sulfuric acid and dissolved ferrous iron sulfate (FeSO4) and dissolved ferric iron sulfate (Fe2(SO4)3) in solution. For the maximum solid weight loss, the formation of iron sulfates in solution, and of labile basic ferric sulfate (FeOHSO4) as the preferred precipitated form of iron in the solids, should be facilitated during pressure oxidation.
Oxidation of arsenopyrite (FeAsS) during pressure oxidation can result in a series of solid compounds in addition to the formation of sulfuric acid and soluble trivalent arsenic and pentavalent arsenic in solution. The solid arsenic compounds can be the simple scorodite FeAs04.2H20 Or more complicated basic iron arsenate sulfate Fe,(OH)y(SO4),(As04)m.nH20. For the maximum solid weight loss and maximum re-dissolution of iron, sulfur and arsenic solid compounds, the formation of basic iron arsenate sulfate Fex(OH)3.(SO4),(Asa4)11AH20 should be facilitated during pressure oxidation.
As shown in the preferred embodiment of Fig. 1, it is contemplated that the subject process should preferably include a separate acidulation step 16 (for the reasons previously described), before the slurry is subject to the subsequent pressure oxidation step 18.
However, it is to be understood that acidulation may to some extent occur as part of or in combination with the

9 PCT/CA2016/000129 pressure oxidation step (given that the pressure oxidation step generally takes place under acidic condition); as such the separate acidulation step 16 may be regarded as optional.
The oxidized concentrate slurry 19 is then subjected to a post pressure oxidation conditioning step 22, wherein the oxidized concentrate slurry 19 is discharged from the autoclave and maintained for several hours at a temperature in the range of from about 50 to about 100 C (the upper limit of the range being at or near the boiling point of water) and at ambient pressure, to form a conditioned slurry 21; most preferably, the temperature at which the oxidized concentrate slurry 19 is maintained at is about 95 C. As the oxidized concentrate slurry 19 is discharged from the autoclave, there is a reduction in pressure and temperature. In general, most of the unstable solid compounds containing iron, arsenic, sulfate and/or hydroxyl will be dissolved in this step. This step is needed to reduce operating cost in the downstream reductive leach step 36.
The conditioned slurry 21 is then subjected to a first solid/liquid separation and wash step 23.
Where appropriate, cooling may be applied to the hot conditioned slurry 21 before this first solid/liquid separation and wash step 23. This step comprises at least one of several conventional techniques for facilitating the separation of a slurry into solids and a solution, and for washing of the resultant solids. The first solid/liquid separation and wash step 23 can include at least one or a combination of: a thickening step; a counter current decantation (CCD) step; and a filter and wash step. Optionally, re-pulping of the filter cake may be included as well between filtrations to further improve wash efficiency.
In a preferred embodiment, as shown in Fig. 1, the first solid/liquid separation and wash step 23 comprises: a thickening step 24; a CCD wash step 26; and a filter and wash step 28. In the thickening step 24, the conditioned slurry 21 is thickened to form a thickened oxidized concentrate slurry 25 and an overflow stream of acid-containing solution 32 comprising a solution of acid and dissolved metal sulfates/arsenates. The thickening step 24 improves the wash efficiency of the subsequent CCD wash step (step 26) and also recovers a portion of relatively concentrated solution of acid and dissolved metal sulfates/arsenates for recycling to the acidulation step (step 16) and/or the pressure oxidation step (step 18). In the CCD wash step 26, wash water is applied to the thickened oxidized concentrate slurry 25 to form an underflow stream 27 of washed oxidized concentrate slurry and an overflow stream of acid-containing solution 30 comprising a solution of acid and dissolved metal sulfates/arsenates. The CCD wash

- 10 -step 26 involves the removal of acid and dissolved metal sulfates/arsenates in multiple thickeners by applying clean water wash. The amount of dissolved metal sulfates/arsenates in the underflow stream 27 of washed oxidized concentrate slurry (as well as in the first washed slurry/solid 29) should be preferably kept to a minimum, because any dissolved ferric iron (Fe3+) and dissolved pentavalent arsenate (Ass) will consume sulfur dioxide during the subsequent reductive leach step (step 36). The filter and wash step 28 can be one or more of a number of conventional filtration and washing techniques for filtering and washing slurry/solids. In the filter and wash step (step 28), the underflow stream 27 of the washed oxidized concentrate slurry undergoes filtration and/or wash to further reduce the amount of sulfuric acid and dissolved metal sulfates/arsenates. Optionally, re-pulping of the filter cake may be included as well between filtrations to further improve wash efficiency. The filtrate stream 31 from the filter and wash step 28 is acid-containing solution.
In the above embodiment, the first solid/liquid separation and wash step 23 forms an underflow (solid) stream 29 of first washed slurry/solid and overflow/filtrate streams (30, 31 and 32) of an acid-containing solution (which also contains dissolved metal sulfates/arsenates). In the preferred embodiment shown in Fig. 1, the first washed slurry/solid 29 stream corresponds to the underflow (or filter cake) stream from the filter and wash step 28.
There are a number of other variations for the solid/liquid separation and wash step 23. These can include, for example, the following options:
Option #1 - "CCD wash" only;
Option #2 - "Filter and wash" only;
Option #3 - "CCD wash" + "Filter and wash";
Option #4 - "Thickening" + "CCD wash"; or Option #5 - "Thickening" + "Filter and wash".
The different options and the appropriateness of using such given the circumstances will generally be understood by a person skilled in the art. For instance, when the solid content in the conditioned slurry 21 is high, the thickening step 24 may be unnecessary and so can be omitted.

- 11 -Multiple stages of filter and wash may be considered. Also, re-pulping of the filter cake may be included between filtrations to further improve wash efficiency.
The overflow stream 32 from the thickening step 24; the overflow stream 30 from the CCD wash step 26; and the filtrate stream 31 from the filter and wash step 28; all of which consist mostly of process water with acid and dissolved metal sulfates/arsenates in solution, can be recycled, as needed, to the acidulation step 16 and/or to the pressure oxidation step 18, to provide the increased acidity to make the solid iron, sulfur and arsenic compounds less stable and to serve as a water source therefor for temperature control. Where appropriate, cooling may be applied to these warm/hot acid-containing solutions before recycling to minimize heat input when the pressure oxidation is already surplus in heat generation. The surplus of the overflow/filtrate streams (32, 30 and 31) may undergo a further water treatment step 34, where acid is neutralized and any dissolved metal sulfates/arsenates may be precipitated or recovered, if desired, using any commercially available conventional processes. For example, it may be desirable that dissolved copper be recovered from these overflow/filtrate streams. Again, the treated water from the water treatment step 34 may also be recycled to the acidulation step 16, the pressure oxidation step 18, the CCD wash step 26, the filter and wash step 28, and other steps related to the process including flotation and grinding, and/or as wash water or dilution water in various steps in order to serve as a source of water therefor. Again, where appropriate, cooling may be applied to the treated process water before recycling to allow proper heat balance.
The first washed slurry/solid 29 from the first solid/liquid separation and wash step 23 is then subjected to a reductive leach step 36. Sulfuric acid, sulfur dioxide and water (and optionally, a catalyst of copper, such as copper sulfate) are added (stream 35) to the first washed slurry/solid 29 in a sealed reactor, and reacted with the first washed slurry/solid 29 under moderately elevated temperatures and total pressure (see below for suitable reaction conditions) to reduce ferric iron (Fe3 ) and pentavalent arsenic (As5+) in the solid to form soluble ferrous iron (Fe2+) (such as ferrous sulfate FeSO4) and soluble trivalent arsenic (As3 )(such as arsenous acid, H3As03) in solution, respectively. Any sulfates, which are associated with ferric iron (Fe3+) and pentavalent arsenic (As5+) in the solid, will be dissolved into solution as well after ferric iron (Fe3 ) and pentavalent arsenic (Ass) in the solid are reduced to ferrous iron (Fe2+) and trivalent arsenic (As3+). In general, hematite Fe203, jarosite MFe3(OH)6(SO4)2 (including silver jarosite),

- 12 -residual basic ferric sulfate FeOHSO4, scorodite FeAs04.2H20, and any basic iron arsenate sulfate Fe,(OH)y(SO4),(As04)..nH20, etc, will all be broken down. The added sulfur dioxide is oxidized to sulfate or sulfuric acid during this reductive leach step 36.
Assuming arsenic in the solid is present as basic iron arsenate sulfate (which for example may be expressed as Fe6(SO4)3(As04)2(0F)6.11H20), the reductive reaction may be expressed as:
Fe6(OH)6(SO4)3(As04)2.nH20 + 5S02 + 2H20 6FeSO4 + 2H3As03 + 2H2SO4 + nH20 (1) The reductive reactions for scorodite FeAs04.2H20 may be expressed in the following:
2FeAs04.2H20 + 3S02 ¨> 2FeSO4 + 2H3As03 + H2SO4 (2) The reductive reactions for hematite Fe203, silver jarosite AgFe3(OH)6(SO4)2 and basic ferric sulfate FeOHSO4 may be expressed in the following:
Fe2O3 + S02 + H2SO4 2FeSO4 + H20 (3) 2AgFe3(OH)6(SO4)2 + 3S02 6FeSO4 + H2SO4 + Ag20 + 5H20 (4) 2FeOHSO4 + S02 2FeSO4 + H2SO4 (5) (Note: Besides the formation of silver oxide Ag20 in (4), other insoluble silver compounds may form as well during reductive leach).
Suitable reaction conditions for this reductive leach step 36 are: a temperature of from about 50 to about 150 C; total pressure of from about 5 to about 150 psig, a sulfur dioxide partial pressure of from about 1 to about 100 psi; and reaction time of from about 1 to about 10 hours.
Preferably, the reaction conditions for this reductive leach step 36 are: a temperature of from about 50 to about 100 C; total pressure of from about 5 to about 50 psig, a sulfur dioxide partial pressure of from about 1 to about 30 psi; and reaction time of about 6 hours.
In this reductive leach step 36, over about 80% of solid arsenic compounds in the first washed slurry/solid 29 can be dissolved into solution. In addition to the dissolution of solid arsenic compounds, the majority of other arsenic-free compounds such as hematite (Fe203) and silver jarosite (AgFe3(OH)6(SO4)2), etc., in the solid, are also dissolved during this reductive leach step 36.

- 13 -As mentioned previously, it is generally preferable to keep the amount of dissolved metal sulfates/arsenates in the underflow stream 27 of washed oxidized concentrate slurry, and in the first washed slurry/solid 29 to a minimum, since any dissolved ferric iron (Fe3+) and dissolved pentavalent arsenate (As5) will consume sulfur dioxide (thus adding to the cost and potentially adding to the processing time/requirements). Preferably, the pulp density for reductive leach is chosen such that the dissolved iron and arsenic in the solution is always below the solubility limits of ferrous sulfate (FeSO4) and arsenic trioxide (As203) in solution at the operating temperature or even at ambient temperature.
This reductive leach step 36 may form a vent gas 38 (which may comprise sulfur dioxide that is vented to a sulfur dioxide scrubber) and a leached concentrate slurry 37.
Venting of the off-gas from the reductive leach step 36 will be necessary when less than 100% pure sulfur dioxide is added to the reactor(s). The sulfur dioxide bearing vent gas 38 will also occur when the slurry is discharged out of the reactor(s), and the amount of vent gas may be reduced when cooling or purging with an inert gas such as nitrogen is provided to the slurry before discharge. To minimize or eliminate venting when either 100% pure sulfur dioxide or less than 100% pure sulfur dioxide is added to the reactor(s), the added sulfur dioxide must be adequately sheared and dispersed into the solution/slurry, and the sulfur dioxide in the gas phase must be sufficiently re-entrained into the solution/slurry through appropriately designed reactor(s), baffles, spargers and agitators. The solid in the leached concentrate slurry 37 contains primarily silicates, other inert gangues, and a very low level of arsenic. If lead is present, lead sulfate may be expected in the solid. Small amounts of other compounds, such as gypsum, may be present as well. Silver (any gold and any platinum group elements like platinum and palladium) remains in the solid without loss during the reductive leach step 36.
The leached concentrate slurry 37, which generally contains ferrous sulfate (FeSO4), trivalent arsenic (H3As03 and other arsenite compounds) and acid (H2SO4), etc., in the solution, is then treated in a second solid/liquid separation and wash step 39. Where appropriate, cooling may be applied to the hot leached concentrate slurry 37 before this second solid/liquid separation and wash step 39. This second solid/liquid separation and wash step 39 also comprises at least one of several conventional techniques for facilitating the separation of a slurry into solids and a solution and for washing of the resultant solids (as described above in relation to the first

- 14 -liquid/solid separation and wash step 23). As shown in the preferred embodiment of Fig. 1, the second solid/liquid separation and wash step 39 can comprise: a thickening step 40; a counter current decantation (CCD) wash step 42; and a filter and wash step 44.
In the thickening step 40, the leached concentrate slurry 37 is thickened to form a thickened leached concentrate slurry 41 and an overflow stream of acid-containing solution 46. The thickening step 40 improves the wash efficiency of the subsequent CCD wash step (step 42) and also recovers a portion of the acid in solution, dissolved arsenic and dissolved metal sulfates for recycling to the acidulation step (step 16) and/or the pressure oxidation step (step 18). In the CCD wash step 42, wash water is applied to the thickened leached concentrate slurry 41 to form an underflow stream 43 of washed thickened slurry/solid and an overflow stream of acid-containing solution 47. The CCD wash step 42 involves the removal of acid, dissolved arsenic and dissolved metal sulfates in multiple thickeners by applying clean water wash. In the filter and wash step (step 44), the underflow stream 43 of washed thickened slurry/solid can undergo filtration and/or wash to further reduce the amount of acid, dissolved arsenic and dissolved metal sulfates; the filtrate stream 48 from the filter and wash step 44 is acid-containing solution.
The overflow streams 46 and 47 contain primarily ferrous sulfate FeSO4, trivalent arsenic (H3As03 and other arsenite compounds), sulfuric acid H2SO4, and a small amount of dissolved sulfur dioxide S02 (or sulfurous acid H2S03).
The washed thickened slurry/solid 43 is then subject to a filter and wash step 44, which comprises at least one of a number of conventional filtering and washing techniques for washing the washed thickened slurry/solid 43, to form an acid-containing solution 48 and a second washed slurry/solid 45. Multiple stages of filter and wash may be considered.
Also, re-pulping of the filter cake may be included between filtrations to further improve wash efficiency.
As mentioned above, and as shown in Fig. 1, some or all of the acid-containing solutions 46, 47 and 48 from the second solid/liquid separation and wash step 39 following the reductive leach step 36 are preferably recycled to the acidulation step 16 and/or to the pressure oxidation step 18, generally to help oxidize trivalent arsenic (As3+) to pentavalent arsenic (As5+) and oxidize ferrous iron (Fe2+) to ferric iron (Fe3'), because it is desirable to have pentavalent arsenic and ferric iron when the process water is treated for better stability of the resultant solid precipitate. Where

- 15 -appropriate, cooling may be applied to these acid-containing solutions before recycling to allow proper heat balance in the downstream. Alternatively, despite being less desirable in terms of capital and operating costs, these trivalent arsenic and ferrous iron bearing acid-containing solutions 46, 47 and 48 can be oxidized and treated separately. Another alternative is to partially or nearly completely oxidize these trivalent arsenic and ferrous iron bearing acid-containing solution streams 46, 47 and 48 in a separate circuit and then recycle them to the acidulation step

16 and/or to the pressure oxidation step 18, to avoid any undesirable effects which may occur during acidulation step 16 and/or pressure oxidation step 18 when a large amount of trivalent arsenic (As'+) and ferrous iron (Fe2+) is present in the solution/slurry The possible variations as described above for the first solid/liquid separation and wash step 23, are also applicable for the second solid/liquid separation and wash step 39, including the following options:
Option #1 - "CCD wash" only;
Option #2 - "Filter and wash" only;
Option #3 - "CCD wash" + "Filter and wash";
Option #4 - "Thickening" + "CCD wash"; or Option #5 - "Thickening" + "Filter and wash".
The different options and the appropriateness of using such given the circumstances will generally be understood by a person skilled in the art. Multiple stages of filter and wash may be considered. Also, re-pulping of the filter cake may be included between filtrations to further improve wash efficiency.
The second washed slurry/solid 45 may then be subjected to an optional surface cleaning step 50.
This surface cleaning step 50 may not be required, for example, where the product second washed slurry/solid 45 is to be sold to a smelter for further processing.
However, where the product second washed slurry/solid is to be hydrometallurgically processed further (such as leaching using sodium cyanide solution), this surface cleaning step 50 will be necessary to enhance the silver leach rate and increase silver recovery.

One suitable possibility for the surface cleaning step 50 involves treating the second washed slurry/solid 45 with hydrogen peroxide (or another oxidizing agent like ozone or hypochlorite), at ambient temperature or slightly elevated temperature, after the second washed slurry/solid 45 is re-pulped or diluted. When hydrogen peroxide is used, acidic conditions are generally preferred in order to ensure the stability of hydrogen peroxide. Another effective surface cleaning method may be achieved through regrinding of the second washed slurry/solid 45 prior to silver leaching.
The product of the foregoing is a free-milling silver-bearing material 52. In comparison with the input silver-bearing material, this free-milling silver-bearing material 52 will have shown a substantial reduction in mass in terms of the amount of solid material involved. The free-milling silver-bearing material 52 can undergo conventional silver extraction (step 54) including, for example, leach treatment with cyanide or other lixiviants (such as thiosulfate, thiocyanate, chloride, bromide, hypochlorite, thiourea, glycine, etc. (including, more specifically, sodium cyanide, calcium thiosulfate, ammonium thiosulfate)).
Example Silver-bearing gold concentrate was processed in accordance with the disclosed process.
Samples were collected from certain of the individual steps in the process, and assays were conducted. When silver-bearing gold concentrate was processed through the conventional process of cyanide leaching following pressure oxidation, the silver recovery was less than 5%.
However, when silver-bearing gold concentrate of similar quality was processed in accordance with the disclosed process (with the additional reductive leach step), the silver recovery from cyanide leach following the reductive leach step (step 52) was increased to over 88%.
Furthermore, there was a substantial reduction in mass of the solid material, which also led to a significant increase in the silver grade. Silver grade in the solid was increased from 20 grams per tonne in the initial input stream 12 to about 175 grams per tonne in the final product stream 52 after reductive leach.

Claims (20)

Claims

1. A process for recovering silver from a silver-bearing material, comprising:
(i) subjecting an aqueous slurry of the silver-bearing material to a pressure oxidation step, wherein the aqueous slurry of the silver-bearing material is reacted with oxygen and an acid in an autoclave at an oxidizing pressure and at an oxidizing temperature, to form an oxidized concentrate slurry;
(ii) subjecting the oxidized concentrate slurry to a post pressure oxidation conditioning step, wherein the oxidized concentrate slurry is discharged from the autoclave and maintained for several hours at a temperature in the range of from about 50 to about 100°C, to form a conditioned slurry;
(iii) subjecting the conditioned slurry to a first solid/liquid separation and wash step, wherein the first solid/liquid separation and wash step comprises at least one conventional technique for facilitating the separation of a slurry into solids and a solution, and washing the resultant solids, to form a first washed slurry/solid and at least one first acid-containing solution;
(iv) subjecting the first washed slurry/solid to a reductive leach step, wherein the first washed slurry/solid is reacted with sulfur dioxide to form a leached concentrate slurry;
(vi) subjecting the leached concentrate slurry to a second solid/liquid separation and wash step, wherein the second solid/liquid separation and wash step is one of a number of conventional techniques for facilitating the separation of a slurry into solids and a solution, and washing the resultant solids, to form a second washed slurry/solid and at least one second acid-containing solution; wherein the second washed slurry/solid is a free-milling silver-bearing material that is amenable to leach treatment to extract the silver.

2. The process of claim 1, wherein prior to the pressure oxidation step, the silver-bearing material is subjected to an acidulation step, wherein an acid is added to the silver-bearing material, to form an acidulated concentrate slurry;

3. The process of claim 2, wherein in the acidulation step, the acid is concentrated sulfuric acid or an aqueous solution of sulfuric acid.

4. The process of claim 1, wherein in the pressure oxidation step, the aqueous slurry of the silver-bearing material is reacted with the oxygen and acid for from about 30 to about 120 minutes in the autoclave, and wherein the oxidizing pressure is in the range of from about 200 to about 600 psig total pressure, the partial pressure of oxygen is in the range of from about 15 to about 250 psi, and the oxidizing temperature is in the range of from about 190 to about 240°C.

5. The process of claim 4, wherein in the pressure oxidation step, the aqueous slurry of the silver-bearing material is reacted with the oxygen and acid for from about 60 to about 90 minutes in the autoclave, and wherein the oxidizing pressure is in the range of from about 430 to about 530 psig total pressure, the partial pressure of oxygen is in the range of from about 25 to about 100 psi, and the oxidizing temperature is in the range of from about 220 to about 230°C.

6. The process of claim 1, wherein the first solid/liquid separation and wash step, comprises one or more of: (i) applying a thickening step, to form an underflow stream of thickened oxidized concentrate slurry and an overflow stream of a first acid-containing solution;
(ii) applying a countercurrent decantation (CCD) wash step, to form an underflow stream of washed oxidized concentrate slurry and an overflow stream of a first acid-containing solution;
and (iii) applying a filter and wash step, to form the first washed slurry/solid and a filtrate stream of a first acid-containing solution.

7. The process of claim 6, wherein the first solid/liquid separation and wash step comprises: (i) applying a thickening step; (ii) applying a countercurrent decantation (CCD) wash step; and (iii) applying a filter and wash step.

8. The process of claims 1, 6 or 7, wherein at least one of the acid-containing solutions from the first solid/liquid separation and wash step is recycled to the acidulation step and/or to the pressure oxidation step.

9. The process of claim 1, wherein in the reductive leach step, the first washed slurry/solid is reacted with sulfur dioxide and at least one of sulfuric acid and water.

10. The process of claim 1, wherein in the reductive leach step, the first washed slurry/solid and sulfur dioxide are reacted together for from about 1 to about 10 hours, at a temperature of from about 50 to about 150°C, at a total pressure of from about 1 to about 150 psig, and at a partial pressure of sulfur dioxide from about 1 to about 50 psi.

11. The process of claim 10, wherein in the reductive leach step, the first washed slurry/solid and sulfur dioxide are reacted together for from about 3 to about 6 hours, at a temperature of from about 70 to about 100°C, at a total pressure of from about 1 to about 30 psig, and a partial pressure of sulfur dioxide of from about 1 to about 30 psi.

12. The process of claim 10, wherein in the reductive leach step, a catalyst of copper sulfate is added to enhance the reaction rate.

13. The process of claim 1, wherein the second solid/liquid separation and wash step, comprises one or more of: (i) a thickening step, to form an underflow stream of thickened leached concentrate slurry and an overflow stream of a second acid-containing solution; (ii) a countercurrent decantation (CCD) wash step, to form an underflow stream of washed thickened slurry/solid and an overflow stream of a second acid-containing solution; and (iii) a filter and wash step, to form the second washed slurry/solid and an filtrate stream of a second acid-containing solution.

14. The process of claim 13, wherein the second solid/liquid separation and wash step comprises:
(i) a thickening step; (ii) a countercurrent decantation (CCD) wash step; and (iii) a filter and wash step.

15. The process of claims 1, 13 or 14, wherein the at least one second acid-containing solutions from the second solid/liquid separation and wash step is recycled to the acidulation step and/or to the pressure oxidation step.

16. The process of claim 1, wherein before the pressure oxidation step, the silver-bearing material undergoes mechanical regrinding in a regrinding step.

17. The process of claim I, wherein following the second solid/liquid separation and wash step, the second washed slurry/solid is subjected to a surface cleaning step to form the free-milling silver-bearing material, wherein the surface cleaning step is selected from treatment with an oxidizing agent, treatment by extended aeration, and treatment by regrinding.

18. The process of claim 17, wherein the oxidizing agent is selected from the group consisting of hydrogen peroxide, ozone, pure oxygen and hypochlorite.

19. The process of claims 1 or 17, further comprising a silver extraction step, wherein the free-milling silver-bearing material is leach treated with a lixiviant to extract the silver.

20. The process of claim 17, wherein the lixiviant is selected from the group consisting of sodium cyanide, calcium thiosulfate, ammonium thiosulfate, thiourea and glycine.