EXTRACTION OF METALS FROM ORES

"EXTRACTION OF METALS FROM ORES" TECHNICAL FIELD This invention relates to a method for. the extraction of metals from ores, and particularly to the recovery of nickel, cobalt and other associated minerals from various nickel and/or cobalt ores, in which they occur either singly or together.

BACKGROUND ART Most of the nickel and cobalt in the world today has come from ores in which the metals are found as sulfides. Sulfide ores make up about 2/3 of the world's deposit of these metals. In the other 1/3 of the world's deposit, these metals are found as oxides, and to date, there has been little development of methods for the recovery of nickel or cobalt from oxide ores. In addition, it has been found uneconomical to extract nickel and cobalt from low grade ores, either sulfide or oxide ores.

It is an object of the present invention to provide a method of extracting metals from ores containing the metals as oxides or sulfides. It is a further object of the present invention to provide a method of extracting nickel, cobalt and other associated minerals from ores in which they are found as oxides or sulfides.

DISCLOSURE OF INVENTION The invention provides a process for the extraction of metals from ores, such as nickel, cobalt and associated minerals from ore which contains the nickel, cobalt and associated minerals as oxides or sulfides, which comprises contacting the ore with hydrogen fluoride, and/or with fluorosilicic acid to convert at least some of the metals such as nickel, cobalt and associated mineral oxides or sulfides to fluorides, and then separating and collecting the resulting metal fluorides.

The method of the present invention can be applied to a variety of different types of ores. For example, the present invention is applicable to a wide range of metal oxides such as nickel, copper, chromium, magnesium, silver, tin, and titanium. The present process can be used with other minerals as well, provided they exist in a form in which acid action can transform the oxide (err sulfide) to a fluoride. The process of the present invention may also be applied to ores where the metals occur as oxides or sulfides. Despite this however, in some ores the sulfides exist in a complex ore body, closely aggregated with iron or carbon, which can interfere in the transformation of the sulfides to the fluorides. Despite this however, some sulfide ores can be treated effectively using the present method.

Preferred types of ores with which the present extraction method can be used are the nickel-cobalt Lateritic ores which occur in large quantities in Australia and throughout the world. These Lateritic ores are considered to be untreatable by prior art processes for both economic and technical reasons.

A typical Laterite ore would have the following analysis: Analysis (by Weight)

40 - 50 % Fe₂O₃

16 - 35 % SiO₂

8 - 25 % Al₂O₃

5 - 25 % MgO Normally under 1 % CaO and Na₂O

The laterite contains 0.2% to 1.2% Co O or similar cobalt oxide and 1% to 2.5% NiO and 1% to 3% Mn₂O₃ and 0.5% to 1.5% Cr₂O₃

BRIEF DESCRIPTION OF DRAWINGS The present invention is now discussed with reference to the drawings in which:

Figure 1 shows a flowchart detailing the extraction process using hydrogen fluoride and optionally fluorosilicic acid for hydrogen fluoride makeup. Figure 2 shows a flowchart for an alternate embodiment of the invention, detailing the extraction process using fluorosilicic acid. MODE(S) OF CARRYING OUT THE INVENTION The present invention is now described with reference to a preferred embodiment concerning recovering nickel, cobalt and associated minerals from the lateritic one described above.

One embodiment of the process of the invention utilizing hydrogen fluoride is now discussed, as represented in the flowchart shown in Figure 1.

A plant for carrying out the method of the invention can be designed and constructed in the usual manner. The plant is constructed of mild steel and lined with heat transferring materials for temperatures under 150°C. Such a material could be teflon ABS, carbon block, or natural hard rubber. All lines, valves, pipes, etc are lined with these materials which are unaffected by reagents at temperatures of 150°C. All other vessels and tubes where temperature is less than 150°C can be lined with polypropylene or polyethylene.

A preferred method according to the invention using HF gas is now described. The ore is reduced in size to below 3/8 of an inch particle size by conventional means. These reduced particles are then dried to about 0.5% moisture. Both crushing and drying are preferably undertaken in order to reduce costs in the process. Crushed, dried ore is fed into the primary reaction chamber where a constant temperature of 105 C is maintained and which is at slightly above ambient pressure.

The design of the vessel is such that an even temperature distribution is achieved. Inside the chamber the ore is fluidised by the passage of HF gas counter current to the movement of the ore. The gas reacts immediately on contact with all of the oxides other than iron (which it pacifies). The other minerals react with the HF gas. These metal fluorides have properties which differ slightly from each other. This allows for separation at a later stage. Silica, (SiO₂) is a major constituent of the ore. The SiO₂ structure will be attacked violently by the HF gas in an exothermic reaction forming SiF₄. This vigorous reaction assists in separating the

SiF₄ out of the iron structure. The silicon tetrafluoride and the HF both readily attack Al₂O₃ and the other metal oxides causing the total conversion of the oxides to fluorides and at the same time so permeates the iron structure that most oxides are released from the iron structure, which itself does not react.

As the chemical reactions are exothermic and occur in a body of ore which is maintained at a temperature in excess of 105°C and just above ambient pressure, the oxygen liberated from the metal oxides combines with the hydrogen liberated from the HF gas forming water vapour, which under these conditions does not combine with the SiF₄. The SiF₄ is a gas and it travels with the gas stream of excess HF along with the water vapour which is produced in the process.

It is possible to vary the quantity of HF gas introduced to the reaction chamber so that almost all the HF is converted to water vapour or SiF₄ prior to reaching the outlet of the reaction chamber so that there is minimum of excess HF.

Exhaust from the reaction chamber will contain a small excess of HF but predominantly comprises water vapour and SiF₄ in stoichiometric amounts. The water vapour would probably be slightly above the stoichiometric requirements according to the amount of residue moisture in the raw feed material.

The stream of SiF₄, water vapour and minimum HF at 105°C is cooled so that the water condenses. The condensed water will solubilise a small amount of HF and combine with the SiF₄. to form H₂SiF₆, or fluorosilicic acid.

At this stage about one third of the silica, less the amount which has reacted with excess HF in the solution to form fluorosilicic acid, will be precipitated. This silica is removed physically and the fluorosilicic acid is piped to a re-boiler where it is brokendown, and HF and silica are recovered.

The reacted ore passing from the reaction chamber is free of silica. Substantially all the oxides other than the pacified iron oxide are converted to fluorides. A second vessel is then used to solubilise the reacted ores.

The liquid in this vessel is 15% to 20% HF in an aqueous solution. This is passed counter current through the ore body at slightly above ambient pressure and at a temperature at which none of the HF is evaporated out of the stream.

The metal fluorides which will be easily solubilised in this tank are primarily AlF₃, NiF₂, CoF₂, CuF₂,

CrF₂, and MnF₂ and various other trace metal fluorides. Two metal fluorides will remain inside the iron structure and will not be solubilised by the passage of HF. These are MgF₂ and CaF₂. The aqueous HF solution now containing the metal fluoride is taken to a stirred reaction chamber in which Al₂O₃ is added. (The Al₂O₃ can be obtained from a later stage in the process if this is desired.)

The Al₂O₃ neutralizes the remaining HF to AlF. and water. This solution then passes to a crystallising tank in which AlF₃ is crystallised. The other metal fluorides contained in the liquor are obtained by boiling off the water in a separate vessel leaving the residue of valuable metal fluorides. These are then taken for further treatment. The rejected solids from the solubilising tank are mainly Fe₂O₃ and MgF₂ and CaF₂. These are passed to a second solubilizing vessel where the structure is washed in a weak solution of hydrochloric acid. This operation is strictly controlled to ensure the minimum reaction between HC1 and the iron structure thus maximising the amount of MgF₂ and CaF₂ solubilised.

The leach liquor stream is then taken to a separate vessel where the water is boiled off along with the excess HC1. The residual solids are predominantly MgF₂ and CaF₂ plus some iron fluorides. These are then passed to a vessel to be reacted with H₂SO₄ in order to regenerate HF.

The Fe₂O₃ is now of high iron purity but will contain some residue of HC1. This can be removed by washing. The iron is then of commercial value.

Another embodiment of the invention using fluorosilicic acid is now discussed, as represented in the flow chart shown in Figure 2. The plant for this second process method would be constructed using mild steel with linings of polypropylene or polyethylene.

The ore is reduced in size to below 3/8 of an inch maximum particle size and the moisture is reduced to .5% by conventional methods. The main reason for preferably crushing and drying is to reduce costs in the process. The ore is fed into the reaction vessel where a temperature of 70°C is maintained, at slightly above ambient pressure. The ore is placed in a counter current flow of 20% to 22% H₂SiF₆ (i.e. fluorosilicic acid).

The silica contained in this ore will not react and will remain in the structure. If the iron structure is very porous and these pores are blocked by the silica then the fluorosilicic acid cannot contact the complete structure and will, of course, convert less of the available metal oxides to fluorides. In ores where this becomes a problem, some aqueous HF can be added to the fluorosilicic acid stream.

The percentage of aqeous HF to be added is dependent on three factors: (1) The level of recovery required.

(2) The amount of silica in the structure in comparison to the other elements.

(3) The physical porosity of the iron structure.

The H₂SiF₆ on leaving the primary reaction vessel will contain excess H₂SiF₆ plus, AlF₃, NiF₂, CoF₂, CaF₂, CrF₂ and H₂O.

This stream passes into a stirred reaction vessel. This then has Al₂O₃ added to it, (which can come from a stage later in the process). The aluminium will neutralize the acid, forming AlF₃ and silica and some H₂O. The silica will be released in this tank and can be removed separately to the liquor. The liquor from this tank will then be passed to a crystalliser to form AlF. crystals. Liquid which does not precipitate as aluminium fluoride crystals will pass on to a distillation vessel for the recovery of metal fluorides. HF and H₂O can be recovered from this vessel. The AlF crystals formed in the crystalliser pass on to a pyrohydrolization process where HF and H₂O are boiled off and may be recovered, and Al₂O₃ is formed. This can be used in the process and may be of high enough quality to be saleable. The solid ore which passes through the main reaction vessel contains Fe₂O₃, CaF₂, and MgF₂ and some SiO₂ which is trapped in the pores of the iron. The iron, because of the silica content, will be of less commercial value. This process is represented in the flowchart in Figure 2.

One of the problems in the past of using HF for the reduction of minerals has been the cost of HF and the disposal of HF mineral residues. In this invention the HF used is converted at the end of the process to metal fluorides such as MnF₂, CuF₂, MgF₂ and Ni F₂.

These can be pyrohydrolized thereby converting them to oxides and regenerating the HF as a gas or liquid which is then returned to the reaction or solubility stages as make up, or used in any other way. As further options, and as represented in Figure 1, in the first of the two preferred processs described above the HF gas can be added to the reaction stage as make up for loss out of the system, or aqueous HF can be added to the solubility stage as make up. However, make up with fluorosilicic acid is a further preferred option to both these options. Fluorosilicic acid can be added to the silica recovery circuit after the removal of the SiO₂ formed by the condensing of the gases from the reactor. This is indicated in Figure 1.

The fluorine in the fluorosilicic acid can be returned to the start as gas, or condensed and returned in aqueous form to the solubilising tanks.