WO1995031577A1

WO1995031577A1 - Process for recovery of gold and silver from complex pyrite and arsenopyrite ores and concentrates

Info

Publication number: WO1995031577A1
Application number: PCT/US1994/005376
Authority: WO
Inventors: Eduardo Gubbins; Rafael Com
Original assignee: GUCOM Inc
Current assignee: GUCOM Inc
Priority date: 1994-05-12
Filing date: 1994-05-12
Publication date: 1995-11-23
Anticipated expiration: 1996-11-12
Also published as: CO4520149A1; AU7311394A; ZA953738B; PE6895A1

Abstract

A process for recovering gold and silver from complex ores of pyrite and arsenopyrite. The process involves the smelting of pyrite, obtaining molten pyrrhotite, contacting a layer of molten pyrrhotite containing a quantity of precious metals to be recovered with a layer of molten lead in the holding furnace (11) at a temperature where both layers exist as separate liquid phases, and obtaining the migration of the precious metals from the pyrrhotite to the molten lead. The enriched lead bath held in the holding furnace is discharged into a holding pot (18) where zinc is added. After removal of zinc, the molten lead is pumped to heating pot (17).

Description

PROCESS FOR RECOVERY OF GOLD AND SILVER FROM

COMPLEX PYRITE AND ARSENOPYRITE ORES AND CONCENTRATES

BACKGROUND OF THE INVENTION a. Field of the Invention

This invention relates to a process for the recovery of gold and silver from complex pyrite and arsenopyrite ores and concentrates. In particular, this invention provides for a pyrometallurgical process for the recovery of gold and silver which avoids production of oxide gas by-products.

b. Description of the Related Art

Three conventional processes for recovery of gold and silver from complex pyrite or arsenopyrite ores are roasting, pressure leaching (autoclaving) and bioleaching. In each of these processes, iron sulfide in the ore is turned into iron oxide, rendering the material amenable to cyanidation and further recovery of the gold and silver contained in the ore. However, the by-products of these processes, such as sulfur dioxide, sulfuric acid, and arsenic trioxide, require special or costly treatment for disposal.

SUMMARY

In view of the processing required for treatment of the by-products associated with these prior art methods, it is an object of the present invention to provide a process for recovery of gold and silver from complex ores of pyrite or arsenopyrite without the creation of harmful by-products.

It is a further object of the present invention to provide for the recovery of gold and silver from pyrite or arsenopyrite without turning sulfides in the ore into oxides.

It is a further object of the present invention to reduce the cost and complexity of by-product disposal associated with a process for the recovery of gold and silver from complex ores of pyrite or arsenopyrite.

Briefly, the present invention is directed to a process for recovering precious metals from complex pyrite ores comprising the steps of smelting the pyrite, obtaining molten pyrrhotite, contacting a layer of molten pyrrhotite containing a quantity of precious metals to be recovered with a layer of molten lead at a temperature where both layers exist as substantially separate liquid phases, and obtaining the migration of the precious metals from the pyrrhotite to the molten lead. The process eliminates any significant quantity of gaseous oxide by-products.

Other objectives and advantages of the present invention will become apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

Figs. 1A and IB are a schematic representation of a preferred embodiment of the process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION According to the present invention, a charge of pyrites and/or arsenopyrites containing gold and silver, fluxes, and coal and/or fuel oil, is fed to and smelted in a cyclone furnace under reducing conditions. The coal or fuel oil is used to supply heat for smelting. Oxygen is kept below the stoichiometric requirement to burn the fuel for smelting (i.e. 0.5% free oxygen) to obtain molten iron sulfide containing the gold and silver from the charge at the bottom of the cyclone furnace. The product of the smelted charge, which includes molten iron sulphide, gold, silver, and other products from the smelting of complex pyrite and arsenopyrite ores is referred to herein as pyrrhotite.

Gases of this smelting operation consist mainly of elemental sulfur and other sulfides (i.e. arsenic, antimony, lead, bismuth, etc.). Only minimum amounts of sulfur dioxide or arsenic trioxide are generated due to the virtual absence of free oxygen in the smelting environment.

Molten pyrrhotite exits the cyclone furnace and flows directly into a holding furnace, a modified reverb furnace, connected to the bottom of the cyclone furnace. The holding furnace contains a molten metallic lead bath. When the molten pyrrhotite enters the holding furnace and contacts the molten lead, the pyrrhotite remains as a separate and distinct phase residing on the surface of the molten lead due to the lower density of the pyrrhotite. At temperatures ranging from about 1,100 to about 1,200 °C, where both phases coexist in the liquid state, the gold and silver migrate from the pyrrhotite to the molten lead phase. Thus, in the holding furnace, the precious metals contained in the molten pyrrhotite are effectively transferred to the molten lead phase which is acting as a collector of the gold and silver. After the pyrrhotite has been in contact with the molten lead bath for the required amount of time, and the transfer of precious metals to the lead has taken place, the molten pyrrhotite, substantially barren of precious metals, flows continuously off the opposite end of the holding furnace for granulation and storage. The molten lead now enriched with gold and silver remains in the holding furnace, until further processing takes place.

The molten lead bath in the holding furnace becomes progressively enriched with precious metals throughout the process. After an appropriate concentration is achieved, enriched lead is tapped from the holding furnace into a steel holding pot for precious metals recovery. Any recovery method known to those in the art may be used to remove the gold and silver. For example, a zinc desilverizing process, which is known to those in the art, is useful for removing gold and silver values from an enriched molten lead bath. The depleted lead from the desilverizing operation is reheated and fed back to the holding furnace in a closed circuit operation to replenish the lead bath.

In the case of a zinc desilverizing process, the zinc crust obtained during the desilverizing operation now contains most of the gold and silver previously dissolved in the enriched molten lead. This crust is skimmed off the top of the molten lead at temperatures near the freezing point of lead (i.e. 320 °C) , and is cast into slabs for further processing. The slabs of zinc crust are smelted in a retort furnace for removal of the zinc. As zinc metal settles in the condenser of the retort furnace, it separates from a lead/silver/gold alloy metal. This alloy metal is cast in bars and fed into a cupel furnace for the removal of lead and any copper or other residual metals, if present, in order to produce an alloy containing 98-99% of gold and silver (Dore metal) . The Dore metal may then be shipped out to refiners. This process can be defined in more detail with reference to Figs. 1A and IB. A blend of ores or concentrates of pyrites and/or arsenopyrites containing gold and silver, along with fluxes and coal make up the charge 1. Charge 1 is placed in a hopper 2, preferably by use of a front end loader. The charge is uniformly fed by means of a first conveyor belt 3 to a rotary dryer 4 where moisture in the charge is reduced to 1-2% by weight. A fraction of the charge is collected as dust produced as a result of the drying operation. Dust may be collected by means of a baghouse system 5 connected to the discharge end of the dryer 4, and then the dust may be fed back to a screw conveyor 6. The bulk of the charge, after the drying operation, flows out of the rotary dryer 4 onto a screw conveyor 6 and onto a second conveyer belt 7 that feeds the dry charge hopper 8. The dry charge stored in the hopper 8 is transported and continuously fed by means of a third belt conveyor 9 to the cyclone furnace 10.

The cyclone furnace 10 is a vertical cylindrical stainless steel water jacketed vessel, fired by four high speed fuel oil burners using air preheated to 250 °C. The charge enters the cyclone furnace 10 centered and is rapidly smelted due to burner heat and the cyclonic effect on the charge. During the smelting operation held under reducing conditions, the charge releases elemental sulfur and other minor sulfides if present in the ore (i.e. arsenic, antimony, lead, bismuth, etc.) in gaseous form.

The smelted charge will contain mainly iron sulfide (pyrrhotite) and precious metals (gold, silver) and will flow down the wall of the cyclone furnace 10 and into the holding furnace 11 that holds a bath of molten metallic lead. The gases from the smelting operation in the cyclone furnace 10 flow into the holding furnace 11 together with the molten charge, then along the holding furnace 11 and out of the opposite end to the cooling tower 12, where water is added, on to a wet venturi scrubber 13, and out into a cyclonic separator 14. At the bottom of the cyclonic separator 14 a sludge is recovered and transferred to settling ponds for storage and disposal. The sludge will contain elemental sulfur as indicated above and other non-ferric sulfides, if present in the ore (i.e. arsenic, antimony, bismuth, etc.), along with pulp conditioning materials such as lime. The clean gas will be drawn up by an exhaust fan 15 positioned next to the separator 14, which will direct it to the stack 16, where it is released to the environment.

The molten pyrrhotite that settles in the holding furnace 11 will form a distinctive liquid phase on top of the molten lead bath and will flow from one end of the holding furnace 11 to the other, releasing its precious metals to the lead and overflowing at the discharge end of the holding furnace 11, for granulation and storage. The enriched lead remains in the holding furnace 11 until further processing takes place.

The enriched lead bath held at the holding furnace 11 is tapped and discharged into a holding pot 18 where zinc is added, stirred at 450 °C, and then allowed to coo! to 320 °C. After removal of the zinc crusts from the holding pot 18, the remaining molten lead (virtually free of gold and silver) is pumped to another heating pot 17 for heating. Heated desilverized lead is pumped back to the holding furnace 11 to replenish the molten lead bath.

The zinc crust formed in the desilverizing operation becomes enriched with gold and silver and is cast for further processing in the zinc retort furnaces 19, where zinc is recovered as zinc metal to return to the desilverizing operation and a lead silver gold bullion is produced. The bullion is melted in the cupel furnace 20 and oxidized with air to remove the contained lead and copper and most of the other trace metals. This leaves behind an approximately 98- 99% pure Dore metal, that is cast into slabs for resale. In the foregoing specification, the present invention has been described with respect to a specific embodiment. This serves as an example to illustrate the invention rather than limit its scope. Modifications may be made without departing from the broader teachings and scope of the invention.

Claims

WHAT IS CLAIMED IS

1. A process for the removal of precious metals from pyrrhotite, comprising the steps of: contacting a layer of molten pyrrhotite containing a quantity of precious metals to be removed with a layer of molten lead at a temperature where both layers exist as separate liquid phases; and obtaining the migration of the precious metals from the molten pyrrhotite to the molten lead thus forming an enriched molten lead and a molten pyrrhotite substantially barren of the quantity of precious metals.

2. The process of claim 1 wherein the molten lead and the molten pyrrhotite are in contact at a temperature in the range of about 1,100 to about 1,200 °C.

3. The process of claim 1 wherein the precious metals are selected from the group consisting of gold, silver, and copper.

4. The process of claim 1 further comprising a step of recovering the precious metals from the enriched molten lead.

5. The process of claim 4 wherein the step of recovering the precious metals from the enriched molten lead comprises a zinc desilverizing process.

6. A process for the recovery of precious metals from complex pyrite ores, comprising the steps of: charging a cyclone furnace with pyrites containing a quantity of precious metals to be recovered, fluxes, and coals and/or fuel oil; smelting the charge under reducing conditions in the cyclone furnace; obtaining molten pyrrhotite containing the precious metals to be recovered, and feeding it to a holding furnace containing a bath of molten lead; maintaining the holding furnace at a temperature sufficient to allow each of the molten pyrrhotite and the molten lead to coexist in contact as substantially separate liquid phases; allowing the molten pyrrhotite and the molten lead to remain in contact for a time sufficient to allow the precious metals from the molten pyrrhotite to migrate to the molten lead thus forming an enriched molten lead and a molten pyrrhotite substantially barren of the precious metals; separating the substantially barren molten pyrrhotite from the enriched molten lead; and recovering the precious metals from the enriched molten lead.

7. The process of claim 6 wherein the holding furnace is maintained at a temperature in the range of about 1,100 to about 1,200 °C.

8. The process of claim 6 wherein the precious metals are selected from the group consisting of gold and silver.

9. The process of claim 6 wherein the step of recovering the precious metals from the enriched molten lead comprises a zinc desilverizing process.

10. A process for the recovery of gold and silver from complex ores of pyrite, comprising the steps of: creating a charge containing pyrites containing gold and silver, fluxes, and coal and/or fuel oil; feeding the charge to a cyclone furnace; smelting the charge under reducing conditions in the cyclone furnace; obtaining molten pyrrhotite containing gold and silver; feeding the molten pyrrhotite to a holding furnace containing a bath of molten lead; maintaining the holding furnace at a temperature sufficient to allow each of the molten pyrrhotite and the molten lead to coexist in contact as separate liquid phases; allowing the molten pyrrhotite and the molten lead to remain in contact for a time sufficient to allow gold and silver from the molten pyrrhotite to migrate to the molten lead thus forming an enriched molten lead and a molten pyrrhotite substantially barren of gold and silver; separating the substantially barren molten pyrrhotite from the enriched molten lead; and recovering the gold and silver from the enriched molten lead.

11. The process of claim 10 wherein the holding furnace is maintained at a temperature in the range of about 1,100 to about 1,200 °C.

12. A process for smelting pyrite to obtain molten pyrrhotite comprising the steps of: creating a charge containing pyrite; feeding the charge to a cyclone furnace smelting the charge in the cyclone furnace under reducing conditions; and obtaining molten pyrrhotite.

13. The process of claim 12 wherein the process is substantially free of generated oxide gases.

14. The process of claim 13 wherein the pyrite comprises a complex ore including gold or silver or gold and silver and the molten pyrrhotite obtained includes gold or silver or gold and silver.

15. The process of claim 14 wherein the charge further includes a fuel to supply heat for smelting.

16. The process of claim 15 wherein the fuel is selected from the group consisting of coal and fuel oil.

17. The process of claim 16 wherein the reducing conditions for smelting include the maintenance of a free oxygen level below the stoichiometric requirement to burn fuel for smelting.

18. The process of claim 17 wherein the free oxygen level is maintained at less than 0.5%.