ZA200205033B - Process for treating precious metal ores. - Google Patents

Description

PROCESS FOR TREATING PRECIOUS METAL ORES

This application claims benefits to U.S. provisional application 60/174,080, . filed December 30, 1999.

BACKGROUND OF THE INVENTION

This invention relates to recovering precious metal values from refractory ores, which include carbon- and sulfur-containing components, and to the control of environmental emissions during the treatment of those ores. In particular, this invention relates to a method of roasting those ores.

The purpose of roasting precious metal ores. such as gold ore. 1s to release for extraction the small particles of precious metal that are surrounded by refractory stone or minerals. Refractory refers to non-conventional ores. such as oxide. which implies extreme process measures must be taken to extract the metal. The roasting simply opens up passages for the penetration of a leaching solution nto the mterior of the ore particles. This is accomplished by the removal by volatilization or formation of volatile oxides of certain constitutents such as sulfur, arsenic or antimony.

For example, the gold in refractory sulfide ores 1s angstrom-sized and physically locked in the arsenian pyrite mineral species. Roasting of this ore oxidizes the sulfide mineral and changes the structure. which allows the cvanide leaching solution to come into contact with the gold. Temperature 1s an important parameter. High temperatures tend to form a dense particle rather than a “spongy” calcine. The dense particles trap the smaller precious metal particles. and result in lower metal recoveries. High temperature can cause melting of some components. which also results in metal encapsulation.

In the case of recovering gold from gold ores. roasting of refractory gold ore concentrates has been practiced for decades. Multiple hearth. rotary kiln and muffle ) reactors were first used for roasting. Fluid bed roasting provided a low-capital cost. ] low-maintenance technology with better process control and soon became the favored technology. The first fluid bed concentrate roasters were commissioned in the late 1940’s. Early fluid beds were “bubbling” type. Environmental considerations did not significantly impact on the design. Feedstocks were highly exothermic and reaction rates were relatively rapid.

Roasting today must compete with other technologies for treatment of refractory ores. Ore bodies which are not amenable to concentration must be . 5 handled. Foremost, processing must be done in an environmentally acceptable manner.

Table 1 presents some of the minerals commonly present in refractory gold ores. Many of these minerals include sulfur and other elements that may require costly processing and disposal. In addition, ores may contain organic carbon. This carbon may have “preg robbing” characteristics. which takes up or “robs” the solubilized gold from being recovered during gold leaching operations.

TABLE 1

MINERAL ASSOCIATED WITH GOLD ORES

NON-SULFIDIC SULFIDIC

Name Formula Name Formula

Quartz SiO. Pyrite FeS,

Dolomite CaC0,;.MgCO; Pyrrhotite Fe:Sq to Fe ,Si-

Calcite CaCO, Arsenopyrite FeAsS

Muscovite K,0.2A1,0,.6510:.2H,0 Orpiment AssSa

Albite Na;0.Al0:.6510- Realgar AsS

Talc 3Mg0.4Si0-.H,0 Tetrahedrite 1Cu,S.5bsS:

Clay AlLO;.(x)S10-.(V)H-.O Chalcopyrite CuFeS,

Calaverite AuTe, Sphalente ZnS

Petzite AgsAuTe, Galena PbS

Gold Au Stibnite SbaS;

Scorodite FeAsO,.2H,O Enargite Cu:AsS,

Selenium Se Cinnabar HeS 135 Environmental issues which must be addressed are primarily the fate of the sulfur gases, arsenic, and mercury. Other poliutants such as antimony may be ; important depending on the specific ore mineralogy.

High concentrations of sulfur gases, primarily sulfur dioxide, will be present in the exhaust gases from all concentrate roasters. Generally, the concentration of these sulfur oxide gases should be substantially reduced prior to discharge to atmosphere. One option is the manufacture of sulfuric acid. A second option would be a wet scrubbing system using alkali. Because of the low value of sulfuric acid, very few plants utilize the first option; however, that decision also depends on the availability of a market for the sulfuric acid, and the cost to dispose of the sulfur otherwise.

In the case of concentrates with high arsenic contents efforts have been made to volatize the arsenic as arsenic trioxide. This results in higher gold recoveries.

There are several technologies available for the removal of arsenic trioxide from the exhaust gases.

The roasting of whole or unconcentrated ores has also been commercialized

There are several characteristics of the whole ores that differ {rom concentrates. which significantly affect design. The ore has a low heating value. Dry feeding of the ore is required, whereas most concentrates are fed in a slurry form. Reaction rates are slower with whole ores, thus requiring long solids retention time. Whole ore, as opposed 10 concentrates, can have a higher vanability in the amount of sulfur. and therefore requires blending of different ore lots to the roaster feed. But. blending ores to obtain consistent overall sulfur content can be problematic. and therefore, alternative methods may be required to help contro! SO- content.

Because the sulfur gases may cause some environmental problems, there must be additional processing steps taken with whole ore roasting to meet regulatory compliance. One solution is to scrub the roaster off gases with an alkali. But, whole ore roasting produces more dilute SO, gases, and dilute gases are difficult to scrub and remove. Another solution that has been practiced is to add lime in the roaster to capture the sulfur “in situ,” i.e. by forming solid sulfates. Yet another solution suggested has been to add soda ash (sodium carbonate) in the roaster to control the SO, emissions. In some applications, however, soda ash may cause other problems such as generation of fines, due to its friability.

With low gold prices, the cost of those chemicals becomes more expensive relative to the value of the gold being recovered from the ore. Thus, there is a need for other solutions to the environmental issues that are more cost effective, and offer } potential benefits of enhancing the recovery of the precious metals. ’ 5 SUMMARY OF THE INVENTION

Accordingly, the present invention provides, in one embodiment, a method for treating precious metal ores having sulfur-containing components. The method includes grinding the ore, adding sodium sesquicarbonate to the ore, roasting the ore and sodium sesquicarbonate at an elevated temperature sufficient to oxidize the sulfur-containing components, and recovering the precious metal value from the roasted ore. Preferably, the sodium sesquicarbonate is in the form of mechanically refined trona, which 1s added to the ore before the ore is ground.

In a second embodiment. the invennon provides a method for treating sulfur- containing precious metal ores, ore concentrates or mixtures thereof by adding sodium sesquicarbonate to the ore, roasting the ore in the presence of sodium sesquicarbonate, measuring the sulfur dioxide in the off gas generated by the roasting, and adjusting the amount of sodium sesquicarbonate added to the ore. and recovering the roasted ore.

In a third embodiment. the invention provides a method for controlling ort gas emissions from a mineral ore roaster by introducing a mineral ore into a roaster, introducing sodium sesquicarbonate into the roaster, and roasting the ore and sodium sesquicarbonate at a temperature sufficient to fix any sulfidic material in the ore and fix at least some of the resultant sulfur dioxide.

It has recently been found that trona, or natural sodium sesquicarbonate, when added with ore to a roaster is more effective than lime or soda ash at controlling SO, emissions. Also, in some applications. using trona in the roaster has . improved the recovery of gold from certain gold-containing ores. These and other advantages will be apparent from the detailed description that follows.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Trona is a mineral ore that usually contains 70-95% of a complex salt of - sodium carbonate (Na,CO:) and sodium bicarbonate (NaHCOs) in a hydrated crystal form known as sodium sesquicarbonate (Na,CO; ¢ NaHCO; « 2H,0). Trona also contains between 6-30% insolubles, usually shale oil, and a small amount of NaCl, usually less than 0.3%. A vast deposit of trona is found in southwestern Wyoming, near Green River. The trona in that deposit typically contains between 90-95% sodium sesquicarbonate. But, trona deposits exist elsewhere with a lower sodium sesquicarbonate concentration of between 10-50% by weight.

Trona ore can be mined and mechanically refined to different particle size distributions useful for different applications. Trona can also be chemically refined and processed into sodium sesquicarbonate, soda ash (Na,CO,). sodium bicarbonate (NaHCO:;) and other alkali materials. Unless otherwise specifically noted herein. usage of the term trona refers to raw trona ore, mechanically refined trona or sodium sesquicarbonate.

In a broad aspect of the invention. trona can be added to a roaster to control the SO, emissions derived from oxidation of the sulfur-containing components in the mineral ore. In a narrow aspect of the invention. trona can be added to a roaster feed with gold ores to improve downstream gold extraction.

In a first embodiment of the invention, a method is provided to control the off gas emissions from a mineral ore roaster. The method includes grinding a sulfur-containing mineral ore, adding sodium sesquicarbonate to the mineral ore. and roasting the mineral ore and sodium sesquicarbonate at an elevated temperature sufficient to oxidize the sulfidic material contained in the ore and sufficient to fix at least some of the resultant sulfur dioxide.

Preferably, the sodium sesquicarbonate is in the form of mechanically : refined trona. Preferably, the mechanically refined trona has a particulate size distribution such that about 10 weight percent of the trona is retained on a 30 mesh screen and/or about 86 weight percent of the trona is retained on a 100 mesh screen.

As discussed further below, a trona product having this particulate size distribution 1s commercially available from Solvay Minerals, Inc. of Houston, Texas, and is sold under the trade name Solvay T-50. Preferably, the roasting is conducted at a temperature between about 475°C and about 750°C. More preferably, the temperature is between about 500°C and about 625°C. Even more preferably, the temperature is between about 550°C and 600°C.

Preferably, the sodium sesquicarbonate or trona is added to the mineral ore before grinding the mineral ore. This helps to achieve intimate mixing of the materials before roasting, which may reduce the amount of trona additive required to achieve the same results compared to adding trona after the grinding step.

Preferably, the amount of sodium sesquicarbonate or trona added to the mineral ore is less than about 10 kilograms of sodium sesquicarbonate per metric ton (tonne) ot mineral ore. More preferably. the amount of sodium sesauicarbonate or trona 1s more than about 2 kilograms per metric ton of mineral ore. Preferably, the method is used for controlling off gas emissions from a gold ore, where the gold ore is preferably a refractory sulfide gold ore.

In a second embodiment of the invention, a method is provided for treating mineral ore, ore concentrates, or combinations thereof having recoverable precious metal values and including sulfur-containing components. This method includes adding sodium sesquicarbonate to the mineral ore, ore concentrates or combinations thereof, roasting the mineral ore. ore concentrates, or combinations thereof in the presence of sodium sesquicarbonate at elevated temperatures sufficient to oxidize the sulfur-containing components. Preferably the sodium sesquicarbonate 1s present in amounts sufficient to fix at least a portion of the sulfur dioxide created by oxidation of the sulfur-containing components. The method also includes measuring the concentration of sulfur dioxide in the off gas generated by the roasting step, and adjusting the amount of sodium sesquicarbonate added to the mineral ore, ore concentrates or combinations thereof in response to the difference ) between the measured concentration of sulfur dioxide and a predetermined concentration of sulfur dioxide. The method includes recovering the roasted ore as a calcine whereby the precious metal value may be recoverable from the calcine.

Preferably, the sodium sesquicarbonate is in the form of mechanically refined trona, where the trona preferably has a particulate size such that about 86 weight percent of the trona is retained on a 100 mesh screen. Preferably, the method includes grinding the trona and the mineral ore, ore concentrates or combinations thereof together before roasting the mixture. Preferably, the trona and the mineral ore mixture are ground to a particulate size such that about 68 weight percent will pass through a 400 mesh screen. Preferably, the mixture is roasted at a temperature between about 475°C and about 750°C. more preferably at a temperature of between about 500°C and about 625°C, even more preferably at a temperature between about 550°C and about 600°C.

Preferably. this method is operated with a predetermmed concentration of sulfur dioxide between about 4% and about 12%. More preferably, the predetermined concentration of sulfur dioxide is between about 8% and about 10%. and even more preferably at about 9.5%. Preferably, the trona is added in an amount less than about 10 kilograms per metric ton of mineral ore. ore concentrates or combinations thereof. More preferably, the trona is added in an amount of more than about 2 kilograms per metric ton of mineral ore. The method preferably includes conducting the roasting step in an oxygen-enriched atmosphere. Also. 1t 1s preferred that, in contrast to a step-wise batch operation. the method is conducted in a continuous process operation.

In a third embodiment of the invention, a method is provided for controlling the off gas emissions from a mineral ore roaster that includes the steps of introducing a mineral ore containing sulfidic material into a roaster, introducing sodium sesquicarbonate into the roaster, and roasting the mineral ore and sodium sesquicarbonate at an elevated temperature sufficient to oxide the sulfidic material contained in the ore and sufficient to fix at least a portion of the resultant sulfur dioxide. Preferably, the sodium sesquicarbonate is in the form of mechanically refined trona. Preferably, the mineral ore is in the form of an ore concentrate. The preferred temperature for the roasting step is between about 475°C and about 750°C.

Basically, the foremost factors that cause the refractoriness of pyritic- carbonaceous-siliceous gold ores, and thus require oxidative pretreatment in a - 5 roaster, are: (1) the intimate association between gold and sulfides or sulfo-salts (such as of iron, arsenic, antimony, etc.); (2) the association of gold-bearing minerals with carbon or carbonaceous compounds; and, (3) the encapsulation of gold-bearing minerals within host rock (such as silicates, carbonates, etc.).

Extracting the most gold from these ores would therefore require either the destruction of the associated minerals and/or the release of gold and its associations from the physical barrier that prevents them from responding to cvanidation.

The use of trona as a roaster feed additive affords a solution to two of the taree probiems mentioned above. Firsi. ronz “fixes” the sulfides mm the ore. The use of the terms “fix,” “fixing,” or “fixes” herein is intended to refer generally to what is believed to occur in the roaster, that is. that the sodium sesquicarbonate in the trona reacts with the sulfide or sulfur dioxide formed from the sulfide to form a sodium sulfate solid. rather than allowing the sulfide to escape as SO, gas. Second. trona is believed to be a flux that lowers the melting point of solids, and in doing so allows structural changes that could result in coalescence of dissolved gold bv diffusion, the destruction of encapsulating material or conversion of such material to more soluble forms. These are properties that have long been recognized in fire assaying and other methods of chemical analysis, but not obvious as practical for processing large quantities of other material.

Trona, however, 1s not an oxidizer of carbon. Fundamentally, such an imagined system where carbon is oxidized by trona is chemically impossible, since there is nothing else in the system that is available for the corresponding reduction reaction (redox couple). Therefore, for roasting ores, it is considered theoretically impossible for trona to have an effect on the carbon “preg robbing” problem. . Nevertheless, other effects caused by trona addition in the roaster feed are not apparent until the downstream stages of processing. One example. if the

Claims (28)

1. ) 1. A method for controlling off gas emissions from a mineral ore roaster comprising: grinding a sulfur-containing mineral ore; adding sodium sesquicarbonate to the mineral ore; and roasting the mineral ore and sodium sesquicarbonate at an elevated temperature sufficient to oxidize the sulfidic material contained in the ore. and sufficient to fix at least some of the resultant sulfur dioxide.
2. 2. The method of claim 1 wherein the sodium sesquicarbonate is in the form of mechanically refined trona.
3. 3. The method of claim I wherein the trona has a particuiate size such that about 86 weight percent of the trona is retained on a 100 mesh screen.
4. 4. The method of claim 2 wherein the temperature is between about 475°C and about 750°C.
5. 5. The method of claim 2 wherein the amount of trona added to the mineral ore 1s less than 10 kg/tonne ore.
6. 6. The method of claim 5 wherein in the amount of trona is more than 2 kg/tonne ore.
7. 7. The method of claim 1 wherein the mineral ore is gold ore.
8. 8. The method of claim 7 wherein the gold ore is a refractory sulfide gold ore.
9. 9. A method for treating mineral ore, ore concentrates, or combinations thereof having recoverable precious metal values and including sulfur-containing } components, the method comprising: adding sodium sesquicarbonate to said ore. ore concentrates, or combinations thereof; roasting said ore. ore concentrates, or combinations thereof in the presence of sodium sesquicarbonate at elevated temperatures sufficient to oxidize the sulfur-containing components. wherein the sodium sesquicarbonate is present in amounts sufficient to fix at least a portion of the sulfur dioxide created by oxidization of the sulfur-containing components; measuring the concentration of sulfur dioxide in the off gas generated by the roasting; adjusting the amount of sodium sesquicarbonate added to the mineral ore, ore concentrates, or combinations thereof in response to the difference between the measured concentration of sulfur dioxide and a predetermined concentration of sulfur dioxide; and recovering the roasted ore as a calcine whereby the precious metal value may be recoverable from the calcine.
10. 10. The method of claim 9 wherein the sodium sesquicarbonate is in the form of mechanically refined trona.
11. 11. The method of claim 10 wherein the trona has a particulate size such that about 86 weight percent of the trona is retained on a 100 mesh screen.
12. 12. The method of claim 10 further comprising grinding the trona and the ore, ore concentrates or combinations thereof together before roasting the mixture. © 25
13. 13. The method of claim 12 wherein the trona and ore mixture are ground to a particulate size such that about 68 weight percent will pass through a 400 mesh screen.
14. 14. The method of claim 10 wherein the temperature is between about 475°C and about 750°C.
15. 15. The method of claim 14 where the temperature is between about 500°C and about 625°C.
16. 16. The method of claim 9 wherein the predetermined concentration of sulfur dioxide is between about 4% and about 12%.
17. 17. The method of claim 16 wherein the predetermuned concentration of sulfur dioxide is about 9.5%.
18. 18. The method of claim 10 wherein the trona is added in an amount less than 10 kg/tonne ore.
19. 19. The method of claim 17 wherein the trona is added in an amount of more than 2 kg/tonne ore.
20. 20. The method of claim 9 wherein the roasting is performed in an oxygen-enriched atmosphere.
21. 21. The method of claim 9 wherein the method 1s conducted in a continuous process operation.
22. 22. A method for controlling off gas emissions from a mineral ore roaster comprising: introducing a mineral ore containing sulfidic material into a roaster; introducing sodium sesquicarbonate into the roaster; and roasting the mineral ore and sodium sesquicarbonate at an elevated temperature sufficient to oxidize the sulfidic material contained in the ore, and sufficient to fix at least a portion of the resultant sulfur dioxide.
23. 23. The method of claim 14 wherein the sodium sesquicarbonate is in the form of mechanically refined trona.

    PCT/US00/35460
24. 24. The method of claim 14 wherein the temperature is between about 475°C and about 750°C.
25. 25. The method of claim 22 wherein the mineral ore is in the form of an ore concentrate.
26. 26. A method according to claim 1. or claim 22. substantiallv as herein described and illustrated.
27. 27. A method according to claim 9, substantially as herein described and illustrated.
28. 28. A new method for controlling off gas emissions, or a new method for treating mineral ore, substantially as herein described. AMENDED SHEET