GB2109008A - Refining non-ferrous alloys - Google Patents

Abstract

The invention relates to a method and apparatus for refining non-ferrous metals, such as tin, lead and alloys thereof, and more particularly for the recovery or removal of indium therefrom. A tubular reaction vessel (4) is supported above a reservoir (7) and a shower of impure molten tin containing indium is introduced by means of a perforated plate (3). Chlorine gas is introduced via inlet (5) and flows countercurrently with respect to the droplet flow in the vessel (4). The chlorine reacts selectively with the indium contained in the impure tin and a tin-indium chloride slag is formed on the surface of the tin in reservoir (7). Refined tin is removed from an underflow weir (12) and the molten slag from overflow weir (9). The slag may be further treated for the recovery of indium values. <IMAGE>

Description

SPECIFICATION Refining of non-ferrous metals This invention relates to the refining of non-ferrous metals and more particularly to the removal or recovery of indium from tin, from lead, or from alloys thereof.

Indium occurs in complex zinc, tin and lead ores, and is concentrated in smelting circuits into byproducts such as fume, dross or slag which can in turn be smelted to produce an indium enriched alloy of tin or lead.

Infium has been removed from these alloys by chlorinating it using a chloride slag. Coyle, J.

Electrochem Soc. 1944 85, 223, and Lebedeff U.S.P. 2433770, 1947 described processes using molten Zn 012-PbCI2 mixtures. Coyle describes experiments using many possible chlorides.

It is necessary to keep the temperature of the fluid slag as low as possible because indium chlorides are resonably volatile, and 4000C was chosen. The indium is removed by a reaction of the type PbCI2+M < MC12+Pb (1) and mixed zinc-lead chloride gave the best results. These showed indium in slag: indium in metal=100:1. The selectivity of the reaction 3PbCI2+21no21nCI3+3Pb (2) over the reaction 2PbCi2+SneSnCI2+Pb (3) was observed to be higher the lower the temperature.

To remove indium from otherwise pure tin, for example, the use of lead chloride is very undesirable, as lead may be introduced into the tin.

The use of zinc chloride presents problems in the subsequent treatment of the slag with either tin or lead metals or their mixtures. If the slag is treated by dissolving in water, the zinc chloride reagent is lost. If the slag is treated by cementation in the liquid phase it is not possible using zinc metal to completely recover the indium and tin from the slag; aluminium can be used but the reaction then is dangerously rapid and can explqde.

Thus it is highly desirable to eliminate zinc chloride and another chlorinating agent is needed which will result in the formation of a fluid slag which contains substantially all of the indium originally present in the tin, lead or alloys thereof.

In one aspect this invention provides a method of refining a molten non-ferrous metal which consists of, or includes tin, or lead or both, and which further contains indium as an impurity, comprising dispersing the molten metal into a droplet phase and contacting the droplet phase with elemental chlorine gas to react selectively with the indium, and recovering a first phase of refined, substantially indium free metal, and a second fluid slag phase containing substantially all of the indium.

In view of the fact that the free energies of the reactions: In +CI2=lnCI2 Sn+CI2=SnCI2 in the dilute solution of indium in tin are very close to each other over the range 2300--2800C (i.e.

about 1 K cal apart), it would not be expected that the chlorination reaction would strongly promote the selective removal of indium from tin. However, this is not in accordance with the fact, and we have surprisingly discovered that the method according to the invention promotes a strongly selective removal of indium from tin.

Experiments were carried out using a chlorine tube immersed in a molten tin bath, containing .20 tonnes of tin stirred by a metal stirrer. At 2500C the following results were obtained.

Chlorine added Ibs. In. in tin % In. in slag % 0 0.033 1 0.019 2.0 2 0.012 1.3 3 0.008 0.8 The In in slag: In in metal ratios are 105:1, 108:1, 100:1.

In the reaction between tin and chlorine two products are possible, SnCI2 and SnCl4. It is well known in chlorine de-tinning by the Goldschmidt process that tin will reduce SnCI4 to SnCl2, and therefore at the surface of liquid tin the fluid chloride will be SnCI2. It is possible that In may react the same way so that the reverse reaction 2InCI3+In3lnCI2 (4) could occur or the reaction 21nCI3+3Sne3SnCI2+21n (5) The thermodynamic functions of indium chlorine reaction show that when the indium species is trichloride, in the presence of a lot of tin, selective chlorination of indium is even less favoured than with the dichloride.

However, it is now established that the selectivity is improved if the chlorination reaction is carried out in the gas phase between a spray of metal droplets and chlorine gas, and the slag produced being Preferably removed continuously to prevent the back-reaction (5) occurring. This may be obtained by the use of a spray-tower for metal-gas reaction, and a settlement tank from which the slag is drawn continuously. The surface area of the spray of metal droplets is much greater than that of the settling tank, so the forward gas reaction is not reversed as effectively in the slag phase.

At 2500C the following results were obtained.

Chlorine added (Ibs.J In in tin % In in slag % 0 0.033 1 0.013 3.5 2 0.004 2.4 3 .004 1.8 with In in slag: In in metal ratios: 270:1, 600:1,450:1.

These results show a great improvement over the stirred pot process and a great improvement over the ZnCl2/PbCl2 slag used by Coyle for tin-lead alloys.

Average results on batchwise indium removal operations using tons tin per batch, at 2500 C.

In in slag/ Initial Indium Final lndium Average In in metal range % range % slag In % ratio range 0.035-045 0.0055-0.015 2.83 514-188 Preferably, indium is recovered from the slag by leaching the slag with an aqueous medium suitable water, to give an aqueous solution of mixed chlorides. The chloride solution is neutralised, suitably with NaOH to precipitate stannous oxy-chloride, which is filtered for the recovery of tin values.

Lead ions in the filtrate are precipitated, suitably by treatment with sulphuric acid to precipitate lead sulphate. The solution is then oxidised, suitably with hydrogen peroxide or sodium chlorate to convert any residual stannous tin to stannic acid. The pH of the solution is then preferably adjusted to pH 1.5 to 2 to precipitate hydrated stannic acid leaving indium in solution for recovery by cementation.

The invention further provides apparatus for refining a molten non-ferrous metal comprising a reservoir for molten metal, an elongate reaction vessel arranged in an upright position with the lower end portion disposed within the reservoir, means for introducing a dispersed molten metal phase into the upper end portion of the vessel and means for introducing a chlorine gas into a lower portion of the vessel whereby countercurrent flow of the gas and the dispersed metal phase is obtained.

Preferably, the reservoir incorporates an underflow weir for the removal of refined molten metal contained in the reservoir and an overflow weir for the removal of a molten slag phase floating on the molten metal in the reservoir.

Preferably a baffle member having an upstanding sidewall is disposed beneath the lower end of the vessel to deflect slag droplets upwards as they leave the vessel.

The invention will be further described by reference to the accompanying single figure diagrammatic drawing which shows an apparatus suitable for removing indium from tin by reaction with chlorine gas.

In the drawing, molten tin is introduced via duct 1 into a reservoir 2 at the top of the vertical cylindrical vessel 4. It passes through perforated asbestos cement plate 3 and is dispersed into droplets which fall, by gravity, through the space within vessel 4, countercurrent to a flow of chlorine gas which enters via duct 5.

At the bottom of the vessel 4 there is a turbulent slag metal interaction zone from which slag droplets separate and pass under the bottom rim of vessel 4. A layer of slag forms on top of the purified tin in reservoir 7 and runs off continuously over the weir 8. Any entrained tin is separated in a small catchpot 10. Ciean slag overflows down the spout 9 into a collecting device. Purified tin from the column 4 follows a similar route into the reservoir 7 and is taken off continuously via an under-flow weir 12 and launder.

In this way at least 80% of the indium present in the tin can be removed. The chlorine flow can be adjusted so that virtually no chlorine reaches the top of the column 4 and hence there is substantially no gaseous effluent to deal with. The following examples illustrate the invention:- Example 1 40 tonnes of tin containing 0.032% In were pumped through the apparatus at a temperature of 3200C over a period of seven hours. In was reduced to 0.007% in the tin. 1 89 Kg of Cl2 gas was used and 509 Kg of stannous chloride slag was produced containing 1.96% In.

Example 2 55 tonnes of tin containing 0.044% In were treated at a temperature of 2850C over a period of seven hours. In. was reduced to 0.009% in the tin. 259 Kg of C12 gas was used and 698 Kg of stannous chloride slag was produced containing 2.75% In.

Claims (11)

Claims

1. A method of refining a molten non-ferrous metal which consist of, or includes tin or lead or both, and which further contains indium as an impurity, comprising dispersing the molten metal into a droplet phase and contacting the droplet phase with elemental chlorine gas to react selectively with the indium and recovering a first phase of refined, substantially indium free metal, and second fluid slag phase containing substantially all of the indium.

2. A method as claimed in claim 1 in which the dispersed droplet phase and the chlorine gas are caused to flow countercurrently in a vertical reaction vessel.

3. A method as claimed in claim 1 or 2 in which the indium-containing slag phase is removed continuously.

4. A method as claimed in any preceding claim in which the initial temperature of the molten droplet phase is between 2600C and 3400 C.

5. A method as claimed in any preceding claim in which the molten slag phase is separated, cooled, and leached to provide a solution of chlorides of the non-ferrous metal and of the impurity metal, the solution being neutralised to precipitate stannous oxy-chloride, which is filtered for the recovery of tin values therefrom to leave a filtrate containing indium in solution.

6. A method as claimed in claim 5 in which the slag phase is a tin-indium chloride slag further containing lead, in which the filtrate is treated for the precipitation of lead ions as lead sulphate, the solution then being treated with an oxidising agent to convert residual stannous tin to stannic acid.

7. A method as claimed in claim 6 in which after oxidation the pH of the solution is adjusted to pH 1.5 to 2 to precipitate hydrated stannic acid leaving indium in solution for recovery by cementation.

8. Apparatus for refining a molten non-ferrous metal comprising a reservoir for molten metal, an elongate reaction vessel arranged in an upright position with the lower end portion disposed within the reservoir, means for introducing a dispersed molten metal phase into the upper end portion of the vessel and means for introducing chlorine gas into a lower portion of the vessel whereby countercurrent flow of the gas and the dispersed metal phase is obtained.

9. Apparatus as claimed in claim 8 wherein the reservoir incorporates an underflow weir for the removal of refined molten metal contained in the reservoir and an overflow weir for the removal of a molten slag phase floating on the molten metal in the reservoir.

1 0. Apparatus as claim in claim 8 or 9 in which a baffle member having an upstanding sidewall is disposed beneath the lower end of the vessel to deflect slag droplets upwards as they leave the vessel.

11. A tin refining process, or an apparatus, substantially as described with reference to the accompanying drawing.