TECHNICAL FIELD

This invention relates to the treatment of oxidic mineral ores or concentrates.

BACKGROUND ART

In Australian Patent Application 15299/88 proceeding in the name of the same applicant disclosure .is made of the use of microwave energy to heat active carbon in "composites" to achieve rapid reduction of oxidic and sulphidic mineral ores and concentrates. Also disclosed in that application is the use of slightly moist composites in the form of pellets, briquettes or extruded shapes which are rapidly dried in the microwave cavity before being rapidly heated to temperatures where chemical reduction reactions begin. The results of this process are then fed directly into leaching vats or into the molten baths of smelt-reduction furnaces.

DISCLOSURE OF THE INVENTION

It has now been discovered that, particularly with particulate oxidic minerals that are themselves good receptors of microwave energy, it is not necessary to compress the carbon material and particulate mineral ore or concentrate into composites before feeding them into . the microwave cavity. Provided, of course, that a sufficiently powerful microwave field is utilized in the cavity the reduction or metallising reactions will take place in a dry intimate mixture of particulate ore, or concentrate, with particulate carbon.

Accordingly, a first aspect of this invention consists in a method of treating oxidic mineral ores or concentrates comprising the steps of metallising by microwave irradiation particulate ores or concentrates which have been previously intimately admixed with particulate carbon to produce metallic droplets and subsequently collecting said droplets.

According to a second aspect of this invention there is provided an apparatus to treat oxidic mineral ores or concentrates comprising feeding means to deliver an intimate admixture of particulate ore or concentrate and particulate carbon to an inclined column through which the admixture is passed, microwave generating and guiding means to generate and direct microwave energy into said column, and metal droplet collecting means disposed adjacent an outlet from said column.

As used herein the term "oxidic mineral" is intended to include any oxygen containing mineral whether or not it is considered a true oxide.

Not only has microwave energy been found to rapidly heat the admixtures, but it has also been found to facilitate reduction reactions within the mixtures. .This method is preferably used with metals or alloys of sufficient value since it requires the prior carbonization of a carbon-containing fuel, such as low rank coal or peat or wood or other carbonizable plant products. All forms of particulate carbon, including many fine naturally occurring graphites, are suitable for use in the present invention.

The beneficial rapid reduction or metallising reactions which are initiated by the method according to the invention are of the type:- MO + C —■> M + CO MO + CO-→ M + CO„

C0 ₂ + C—■> 2CO

Such reactions are initiated by microwave energy preferably as the mixture of particulate carbon and particulate mineral matter is conveyed through a microwave field for example a conveniently designed oven. Such continuous microwave ovens are now commercially available.

It is preferred that the method according to the invention is carried out in a refractory pipe or other

suitable conveying system, and preferably in an inclined pipe or column, made so that gravity alone will enable the mixture of particulate carbon and mineral matter to flow at an appropriate and controlled rate through the microwave field.

At least the part of the refractory pipe or other containing and conveying system in which microwave irradiation takes place must be of high quality and freely allow microwave energy into the particulate mixture slowly moving through the system. High grade alumina rich refractory is an appropriate container-conveying material.

It is preferable for the particulate ores or concentrates and particulate carbon to be dried before irradiation. Drying can be achieved for example, using a conventional oven or heating or using a low power microwave system.

Preferably, the admixture of particulate ore or concentrate and particulate carbon contains an excess stiochiometric proportion of carbon to oxygen. More preferably, the admixture has an amount of carbon in excess of twice the stiochiometric carbon requirement. The resulting excess char is recycled.

When microwave energy of high power is used it is possible to rapidly achieve not only metallising but melting of the metal or alloy in the microwave cavity. In such cases, molten metal or alloy will flow from the

refractory pipe into a collecting system.

It is preferred that the apparatus according to this invention comprises a column having an inner wall formed from a substantially microwave transparent material. Preferably an air gap spaces the inner wall from a surrounding insulating layer. The air gap provides additional thermal insulation and appropriate distribution of the microwave energy.

Microwave energy is preferably directed into the column by waveguides which extend through the insulating layer and terminate adjacent or within the air gap. The waveguides are preferably terminated by microwave transparent windows to prevent hot gases entering the waveguides. Additional thermal insulation can be provided within the waveguide adjacent the window.

Some embodiments of this invention will now be described, by way of example only, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a block diagram of an apparatus according to this invention;

Figure 2 is a sectional elevation of an apparatus according to this invention;

Figure 3 is a sectional plan view of the apparatus Shown in Figure 2;

Figure 4 is an enlarged sectional view of part of the apparatus shown in Figure 2; and

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Figure 5 is a schematic sectional elevation of an apparatus according to another embodiment of this invention.

BEST MODES FOR CARRYING OUT THE INVENTION

Figure 1 is a block diagram of an apparatus to treat oxidic mineral ores or concentrates according to this invention in general form. The apparatus comprises a feeding means which can take the form of a hopper 1 or other suitable system which delivers an intimate admixture (not shown) of particulate ore or concentrate into a substantially vertical column 2. Microwave energy is directed into column 2 from suitable sources to irradiate the admixture passed therethrough. The irradiation produces metallic droplets which with the other reaction products are directed through a microwave containing connector 3 to a collector 4. The collector can comprise a hearth or induction furnace or any other suitable collector for smelt products and any excess char. The slag and excess char directed to collector 4 are separated from the metal by known means and the char are recycled with the feed. A tap hole or other appropriate means is used to withdraw the liquid metal or alloy from collector 4.

Figures 2 to 4 show in more detail an embodiment of an apparatus to treat mineral ores or concentrates according to this invention. The apparatus comprises a refractory column 5 extending substantially vertically

above an induction smelting crucible 6. The column 5 has a cylinderical inner wall 7 of microwave transparent high-alumina refractory material. An air gap 8 spaces inner wall 7 from a surrounding refractory insulating layer 9. The air gap 8 is provided to give improved insulation and aid in distributing microwave energy. A . stainless steel shell 10 forms the outer covering of the column 5 to prevent microwave leakage.

A hopper 11 disposed above column 5 delivers the admixture 12. The hopper 11 is fitted with an appropriate form of feed control means 13, indicated schematically as rotatably mounted vanes, to control the rate of through flow of the admixture 12.

Microwave energy is generated by magnetrons 14 and directed into the column 5 by waveguides 15. The number of magnetrons and associated waveguide assemblies are determined by the required energy density. As best seen in Figure 3 the waveguides 15 are spaced around the column 5 to give a more uniform microwave field and enter the column 5 at an angle to reduce the likelihood Of reflections being transmitted along the waveguide in the reverse direction which could damage the magnetron.

Figure 4 shows an enlarged view of part of the column wall. As shown the waveguide 15 penetrates outer shell 10 and insulation 9 to terminate adjacent air gap 8. The end of waveguide 15 is sealed by a microwave transparent window 16 formed from a suitable refractory

which is preferably also abrasion resistant. Alumina refractories are suitable for this purpose. Additionally, insulation 17 is provided within the waveguide 15 to prevent heat being conducted to the magnetron 14. The insulation must be substantially microwave transparent and preferably comprises several centimetres of material known as "Fibrefrax" or "Kaowool". The wails of the waveguide are preferably provided with some form of cooling jacket 18 through which water or other suitable coolants are circulated.

Additionally, magnetron 14 may need to be protected from temperatures much above 100°C by provision for example low powered cooling fans (not shown) .

The induction crucible is of conventional design and only the heating element 19 is shown. The crucible 6 has an appropriate tap hole 20 at the bottom for removing molten metal or alloy at appropriate intervals. An argon purging and feeding system of substantially conventional construction indicated generally at 21 can be connected between the bottom and the top of the refractory column, through inlet 22 and outlet pipe 23. This is required for use with more reactive metals such as titanium or zirconium.

In use, particulate -mineral matter admixed with particulate carbon, preferably in excess of the stoichiometric proportion of carbon to oxygen, is admitted to the column from hopper 11. The mixture is

irradiated by microwave energy from one or more magnetrons 14 directed via wave guides 15 as it passes through column. The inner wall 7 of column is substantially transparent to microwave energy but refractory enough to withstand the temperatures involved in the metallising reaction. It is advantageous if the mineral matter is also a good receptor of microwave energy.

When refractory or more reactive metals or alloys are being produced, for example Fe-Cr-C or Fe-Ni-Cr-C or Fe-Ti-C or Fe-Zr-C alloys, it may sometimes be beneficial to inject a small amount of inert gas such as argon into the column. The argon is preferably injected at a point just above the induction furnace and at or near the bottom of the metallising column. The whole system is advantageously made gas tight by using suitable seals so that only very small volumes of inert gas need to be used and therefore cleaned. The upward moving argon, or other inert gas, helps sweep out at the top of the system air, N , CO and CO which might cause back reactions of the metals to oxides or nitrides. Some nitrides may, of course, be advantageously produced with certain alloys in which case the inert atmosphere need not be provided. As an alternative to injecting inert gas into the system near the base of the metallising column this gas may be bubbled through a porous plug (not shown) into the base

of the induction furnace crucible, and through the molten metal or alloy in the crucible.

It should be appreciated that many other systems of collecting holding and tapping the metals or alloys may be used.

Figure 5 schematically shows an apparatus to treat mineral ores and concentrated according to another embodiment of this invention. The apparatus comprises a feed hopper 24 which can be of similar construction to that described above. The hopper 24 directs the ^' intimate admixture 25 into a cylinderical supply pipe 26 formed of stainless steel or other suitable heat resistant material. The column shown generally as 27 includes a frusto-conical inner wall portion 28 of microwave transparent refractory material. The supply pipe 26 feeds the admixture 25 into frusto conical portion 28 for irradiation by microwave energy in the manner described above.

Frusto conical portion 28 open at its open end so that water vapour and other gases produced by the irradiation can escape into a chamber 29 formed within the column 27. An exhaust vent 30 is provided to allow escape of these gases which are preferably ducted away in an appropriate manner. The bottom of frusto conical portion 28 directs the processed material into a collector 31 which can be of the type described above. The outer walls of column 27 can be constructed in an

identical manner to that described in more detail above. The outer surface of frusto conical portion 28 is preferably covered with refractory insulation (not shown) to reduce radiant heat transfer to the outer walls of the column.

The use of an open topped portion ^' for irradiation of the admixture which allows escape of gases and in particular water vapour has been found to provide a better through flow of the admixture. Additionally, this modified apparatus can reduce the need for complete drying of the feed material.

For safety reasons it is also necessary to ensure that leakage of microwave energy does not occur in either the feed or exit ends of the apparatus described above. Conventional chokes or wave traps can be used to ensure this is achieved.

The following examples serve to further illustrate this invention. Example 1

Gravity tin concentrates from northern New South Wales averaging 72.5% tin and containing minor gangue minerals as garnets and spinels were intimately mixed with finely crushed (minus 3mm) high grade char made from Victorian brown coal briquettes in 50:50 proportions. The dry basis carbon in the char was 98.5%.

The mix was fed into apparatus similar to that illustrated schematically in Figure 5 and the power in

two IKW 2450MHz magnetrons was progressively increased until the temperature in the char-tin concentrate mixture was above 900°C. Temperature rise was rapid as both the char and the tin concentrate minerals were excellent receptors. Within seven minutes tin metal began to collect in the collecting crucible in the bottom furnace. Smelting continued until the crucible was full.

Approximately 3kg of high grade tin was collected averaging 99.86% tin. The excess char plus a little unreduced concentrate was recycled for a second smelting run and a similar result was achieved.

It was noted that if the temperature in the smelting zone was allowed to climb to much above 950°C some iron began to enter the tin metal and it was turned into an alloy known in the tin trade as "hard lead" . Little slag was produced and this was preserved for a later "clean-up" smelt run. Example 2

Gravity chromite concentrates containing

FeO 18.3%

MgO 15.7%

SiO„ 4.2 % were blended with a high grade haematite ore from the Pilbara area in Western Australia (Fe 2O3 content

greater than 95%) in the ratio 2 of chromite to 1 of haematite. This blend was then intimately mixed with char of the same high quality as used in Example 1. A little CaO and fluorspar flux was added and the mixture fed into the apparatus shown schematically in Figure 5 using one controllable ^' 5KW ^' 2450MHz frequency magnetron for microwave power. The alloy collected in the collector induction furnace was found ^* to contain

When chill cast into a cold iron mould the alloy was extremely hard and could not be cut with a hacksaw. It was also found to be corrosion resistant. Example 3

Gravity and magnetically concentrated beach sand ilmenite containing

TiO„ 44.0%

FeO 39.4%

MgO 3.3 o_

was blended with the high grade haematite used in Example 2 in the proportions 1 of ilmenite to 1 of

haematite and then that blend was intimately mixed with the same high grade brown coal char as used in Examples 1 and 2. A little CaO flux was added and the mixture was fed to the apparatus shown diagrammatically in Figure 5. The power into a 5KW magnetron was slowly increased until melting temperature ^* was reached and maintained until the receiving crucible in the induction furnace was full of metal at the bottom and unused char on top. As in the earlier examples the unused char was scraped.off for re-use.

The alloy produced was cast into a cold iron mould and was found to be very hard. It was analysed and found to include

Carbon 6.1%

Chromium 5.8% Titanium 4.2%

Silicon 5.6% and there were smaller amounts of aluminium and manganese.

On microscopic examination it was apparent that the alloy had major amounts of carbides and carbo-nitrides in it and would be suitable applications requiring high hardness and abrasion resistance. Example 4

Another experiment was conducted in which zircon (as beach sand concentrate) was substituted for the ilmenite of example 3. In -this case it was verified

that zirconium was present in significant percentages in the alloy produced but a quantitative analysis was not conducted. The alloy produced was very hard as in the case of the alloy of Example 3.