FI125099B - Method and apparatus for recovering platinum group metals and ferrochrome from chromite ore containing platinum group metals - Google Patents

Description

METHOD AND APPARATUS FOR RECOVERING PGM AND FERRO-CHROME FROM PGM BEARING CHROMITE AIR

FIELD OF THE INVENTION

The invention relates to a method and apparatus for recovering platinum group metals and ferrochrome from PGM bearing chromite in air.

BACKGROUND OF THE INVENTION

Most of the world's known Platinum reserves are located in South Africa, which produces most of the world's Platinum. South Africa is also the world's largest single producer of Ferrochrome. Platinum group metals, or PGMs, include Platinum, rhodium, palladium, ruthenium, iridium, osmium. PGMs Frequently occur together with chromites. The Platinum industry in South Africa is rapidly moving from the traditional Meren-sky Reef to UG2 Reef as a raw material. The UG2 Reef contains most of the world's known PGM reserves, and it also has high chromite content.

There are some challenges in using UG2 based raw material with current PGM recovery processes. One of the challenges is that traditional Smelting Furnaces cannot use a Concentrate that contains over 2.5% Cr. If the chromium content is too high, Cr tends to create a crust in the smelting furnace and the risk of explosion is high. Traditional six-in-line Smelting Furnaces are susceptible to build-ups of high-melting chromite spinels if the Cr203 content of the feed is too high. Also the furnace control is very challenging. Furthermore, the concentration process is rather Complicated when the target is to separate the chromite from the PGMs. Traditionally, UG2 ore has been concentrated by removing chromite from the ore as far as possible to reach low chromite content in the PGM Smelting furnace feed. It is very difficult to completely remove chromite from the Concentrate by flotation. Chrome melts at temperatures above 1600 ° C, with PGM Smelting Furnaces operated at 1400-1500 ° C. The presence of chromium in the feed leads to lower furnace reduction efficiency and chromite can also damage the smelting furnace.

The UG2 concentration process used by the Platinum industry in South Africa produces lots of chromite-containing tailings. Ferrochrome producers can use these tailings as raw material. South Africa suffers from a shortage of electricity which is why local producers cannot use all the chromite-containing tails of the Platinum industry but tails are exported to China. The Chinese are building a lot of Ferrochrome capacity now, which is worrying for South African producers. One of the targets of the present invention is to provide a process that allows South Africans to use their UG2 reserves more completely in their own country.

Attempts have been made to develop pyrometallurgical processes that tolerate higher chromite contents in the PGM Concentrate. US 6,699,302 B1 discloses a method for processing metal sulfide Concentrate that contains at least one metal selected from the group consisting of PGMs, nickel, cobalt and zinc. The method comprises dead-roasting the metal sulfide Concentrate, smelting the dead-roasting Concentrate under reducing conditions in an electrically stabilized open-arc furnace, and collecting the metals from the Smelting step in the form of an alloy or vapor. Chrome is an unwanted element and it is removed from the metal alloy in a Converter.

Although the process of US 6, 699,302 B1 can use raw materials with high chromium content, chrome is finally discarded from the process. Furthermore, the process is designed only for use with sulfide raw materials.

The industry lacks a process that effectively combines the recovery of PGMs and Ferrochrome from a PGM bearing in chromite air, such as UG2.

PURPOSE OF THE INVENTION

The purpose of the present invention is to eliminate or at least reduce the problems of prior art. A further purpose is to provide a new process for the effective utilization of PGM bearing in chromite air.

SUMMARY

The method according to the present invention is characterized by what is presented in the claim 1.

The apparatus according to the present invention is characterized by what is claimed in the claim 10.

The new method consists of preparing the Concentrate that contains most of the PGMs and chromite of the air, subjecting the Concentrate to a heating step to dry and / or preheat the Concentrate, and smelting the Concentrate under reducing conditions in a DC Smelting furnace to produce molten metal alloy that contains the PGMs of the feed and molten slag that contains the chromium of the feed. The molten slag is tapped from the smelting furnace into an AC slag furnace, where the reduction of the oxides of iron and chromium contained in the slag takes place so that Ferrochrome is produced.

According to one embodiment of the invention, the heating step additionally comprises roasting the Concentrate to remove the sulfur and / or volatiles contained in the Concentrate.

According to one embodiment of the invention, slag properties are controlled with flux.

Advantageously the method consists of adding flux and / or reductant into the smelting furnace and / or into the slag furnace.

According to one embodiment of the invention, the reducing conditions in the Smelting furnace and / or in the slag furnace are controlled with the addition of a reductant.

According to one embodiment of the invention, the slag properties in the Smelting furnace and / or the slag furnace are controlled with the addition of flux.

Advantageously, the metal alloy is tapped from the Smelting furnace, after which the PGMs are recovered from the metal alloy by hydrometallurgical processes or a combination of pyrometallurgical and hydrometallurgical processes.

According to one embodiment of the invention, molten metal alloy from the Smelting furnace is tapped to a Peirce-Smith Converter, after which the converted metal alloy is subjected to atomization and hydrometallurgical process steps.

According to another embodiment of the invention, molten metal alloy is tapped from the smelting furnace directly to an atomizer, after which the atomized metal alloy is subjected to hydrometallurgical process steps.

The new apparatus comprises a DC Smelting furnace for producing a molten metal alloy containing the PGMs of the feed and a molten slag containing the chromium of the feed and an AC slag furnace for producing a Ferrochrome alloy from the DC smelting furnace.

According to one embodiment of the invention, the apparatus further comprises a heating unit for drying and / or preheating the Concentrate before it is fed to the Smelting furnace. The heating unit is preferably selected from the group consisting of a fluidized bed Reactor, a rotary kiln, a drying tower, or the like.

The slag furnace can be an open bath alternating current furnace, or similar.

According to one embodiment of the invention, the apparatus further comprises a Peirce-Smith Converter for removing iron from the molten metal alloy tapped from the Smelting furnace.

According to another embodiment of the invention, the apparatus further comprises an atomizer for atomizing the molten metal alloy tapped from the Smelting furnace or from the Converter.

Instead of using tailings from a PGM concentrator, the present invention proposes using a molten slag from a PGM smelter as a raw material in Ferro-chrome production. According to the new method both PGMs and Ferrochrome are produced at the same time, which gives flexibility for the use of raw material and makes the concentration of PGM and chromite containing air easier. The process also saves energy compared to current recovery processes. The ferrochrome containing slag fraction need not be cooled and reheated before introduction into the ferrochrome process.

The present invention allows adjusting the Cr / Fe ratio in the Ferrochrome by controlling how much iron is reduced in the Smelting furnace. Typical use of pure UG2 in air results in a Cr / Fe ratio of about 1.35, which means that the Cr content in the Ferrochrome is below 50%. Higher Cr contents are preferred by end users of Ferrochrome, i.e. stainless steel industry.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing, which is included to provide further understanding of the invention, and is intended to provide a portion of this Specification, illustrates an explanation of the invention and together with the description helps to explain the principles of the invention.

The enclosed Figure 1 is a flow chart illustration of one embodiment of a process according to the present invention.

DETAILED DESCRIPTION OF AN EMBODIMENT PGM bearing chromite air is finely ground to liberate the PGM particles. The finely ground air is concentrated in a concentrator 10, where the target is to remove gangue while keeping iron, chromium, base metals and PGMs in the concentrate. The process is simpler than the concentration processes currently used in PGM recovery because there is no need to separate chromium and iron from the base metals and PGMs.

The Concentrate is subjected to a heat treatment in a heating unit 12, where the Concentrate is dried, if necessary, and possibly preheated before it is fed to a Smelting furnace 14. The heating unit 12 can, for instance, be a fluidized bed reactor, a rotary kiln, or a drying tower. If the raw material contains lots of sulfides and / or volatiles, roasting can be carried out in the heating unit 12 to oxidize the metal sulfides. CO gas generated in subsequent Smelting and Slag Furnaces 14, 16 can be used as a heat source in the heating unit 12.

The preheated Concentrate is charged as a feed into a DC Smelting furnace 14. At the same time, carbonaceous reductant such as anthracite or Coke is charged to the Smelting furnace 14. Also some flux may be charged, if necessary.

In the DC Smelting Furnace 14 the Concentrate is melted and the PGMs, base metals and part of the iron contained in the feed are reduced to the elemental metal which is separated as a molten metal alloy below the lighter slag phase. However, most of the feed goes into the slag phase. For instance, all Cr and most of Fe, Al 2 O 3, SiO 2, MgO and CaO of the feed go into the slag phase. Reduction In Smelting Furnace 14 Is Limited By Controlling The Amount Of Carbon Charged In Furnace 14. Target Is Only To Get The PGMs Into The Metal Phase Together With Just A Part Of The Iron. Iron droplets Capture the PGMs and other base metals forming molten metal alloy. Ni and Cu can also be present in the molten metal alloy produced in the Smelting furnace 14.

In the direct current (DC) Smelting furnace 14 the charged material is directly exposed to the electric arc, and the current between the cathode and the anode passes through the charged material. Energy is supplied by open plasma arc. The temperature in the smelting furnace 14 is relatively high, which is why reactions are quick. The plasma arc agitates the slag phase and creates strong currents, which further improves the reactions. The carbon monoxide atmosphere is created in the closed furnace. One more advantage of using a DC Smelting Furnace is that it allows for Charging fine grinded material.

Liquid slag is tapped from DC Smelting furnace 14 to an AC slag furnace 16. Liquid metal alloy is tapped from bottom to DC Smelting furnace 14 to further Refining steps in pyrometallurgical and / or hydrometallurgical processes.

The slag furnace 16 is preferably an open bath alternating current furnace where electrodes are buried in the burden of lumpy materials comprised of molten slag received from DC Smelting furnace. Carbonaceous reductant and flux are charged to the AC furnace to control the reduction reactions and to optimize the amount and quality of the slag. Typical Ferro-chrome furnace operations include reduction of oxides of iron and chromium into metal phase. The resulting slag mainly contains A1203, MgO, CaO and SiO2. Metal alloy received from the slag furnace 16 contains Fe,

Cr, some C and Si. All the rest of the feed is retained in the slag. Products received from the slag furnace 16 are Ferrochrome metal and slag. Typically, the temperature of the slag tapped from the AC slag furnace 16 is 1650-1750 ° C and the temperature of the ferrochrome tapped from the AC slag furnace 16 is 1550-1600 ° C. PGM rich metal alloy tapped from smelting furnace 14 can either be directly passed to hydrometallurgical treatment steps or it can be converted in a Peirce-smith converter 18 before passing to hydrometallurgical treatment. The purpose of Converting is to remove iron and other impurities from the metal alloy. The recovery of PGMs can comprise, for instance, atomization in an atomizer 20 and leaching.

The basic idea of present innovation is to smelt the Concentrate in a DC Smelting Furnace 14, where PGMs are reduced, and then to produce FeCr alloy from the DC Smelting Furnace in a separate AC slag furnace 16. This gives flexibility as regards the raw materials and simplifies the concentrating process 10.

Benefits of the new process include simplicity of the previous concentration process as there is no need to remove the chromite at an early stage. As FeCr and PGMs are produced at the same time, less concentration, cooling and melting are needed and the process is more energy efficient. The safety of the process is improved as there is no risk of crust formation or explosion. There are fewer restrictions on raw materials and no limits on the Cr content of the feed. Neither Cr nor PGMs are lost in the process.

It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of invention can be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.

Claims (14)

A process for recovering platinum group metals and ferrochrome from platinum group metal containing chromite ore comprising the steps of: - preparing an concentrate containing most of the ore platinum group metals and chromite, subjecting the concentrate to a heating step for drying and / or preheating the concentrate; 14) for producing a molten alloy containing the platinum group metals in the feed and molten slag containing the feed chromium, characterized in that the molten slag is discharged from the smelting furnace (14) to the AC slag furnace (16), and - reducing the iron and chromium oxides in a slag furnace (16) to produce a ferrochrome alloy. The method of claim 1, wherein the heating step also comprises roasting the concentrate to remove the sulfur and / or volatiles present in the concentrate. The method of claim 1 or 2, wherein the slag properties are controlled by flux. A method according to any one of claims 1 to 3, comprising adding a flux and / or a reducing agent to the melting furnace (14) and / or to the slag furnace (16). The method of claim 4, wherein the reducing conditions in the melting furnace (14) and / or the slag furnace (16) are controlled by adding a reducing agent. A method according to any one of claims 3 to 5, wherein the slag properties in the melting furnace (14) and / or the slag furnace (16) are controlled by adding fluxes. The method of any one of claims 1 to 6, comprising lowering the molten alloy from the smelting furnace (14) and recovering the platinum group metals from the alloy. The method of claim 7, comprising lowering the molten alloy from the melting furnace (14) to a Peirce-Smith converter (18) and / or subjecting the converted alloy to atomization and hydrometallurgical process steps. The method of claim 7, comprising lowering the molten alloy from the melting furnace (14) to the atomizer (20) and subjecting the atomized alloy to the hydrometallurgical process steps. Apparatus for recovering platinum group metals and ferrochrome from an ore concentrate containing platinum group metals and chromite, comprising a DC smelting furnace (14) for producing a molten metal alloy containing platinum group metals and a molten slag containing feed chromium, characterized in that the apparatus also comprises a slag furnace (16) for producing a ferrochrome alloy from molten slag calculated from a DC smelting furnace (14). Apparatus according to claim 10, comprising a heating unit (12) for drying and / or preheating the concentrate before feeding it to the melting furnace (14), which heating unit (12) is selected from the group consisting of a fluidized bed reactor, rotary kiln, drying tower or the like. Apparatus according to claim 10 or 11, wherein the slag furnace (14) is an open bath alternating current furnace or the like. The apparatus of any one of claims 10 to 12, further comprising a Peirce-Smith converter (18) for removing iron from the molten metal alloy lowered from the melting furnace (14). The apparatus according to any one of claims 10 to 13, further comprising an atomizer (20) for atomizing the molten alloy calculated from the melting furnace (14) or the converter (18).