GB2067596A

GB2067596A - Process and apparatus for the separation and re-use of dust material from the effluent gases of aluminium fusion electrolysis furnaces

Info

Publication number: GB2067596A
Application number: GB8101186A
Authority: GB
Original assignee: Vereinigte Aluminium Werke AG
Current assignee: Vereinigte Aluminium Werke AG
Priority date: 1980-01-19
Filing date: 1981-01-15
Publication date: 1981-07-30
Also published as: CH652317A5; FR2474062B1; DE3001910A1; FR2474062A1; DE3001910C2; GB2067596B

Abstract

In aluminium fusion electrolysis processes, valuable constituents namely cryolite, chiolite and aluminium oxide in the effluent gases are recovered and re-used by separating dust from the effluent gases and separated dust is subjected to a flotation step wherein the valuable constituents contained in the flotation slurry are separated and dried and recycled. The dust may be separated by a wet separation process using a scrubbing zone, or in a dry separation process in a filter unit.

Description

SPECIFICATION Process and apparatus for the separation and re-use of dust material from the effluent gases of aluminium fusion electrolysis furnaces This invention relates to a process and apparatus for the separation and re-use of cryolite, chiolite and aluminium oxide contained in dust material from the effluent gases of aluminium fusion electrolysis furnaces.

Modern plants for the production of aluminium include, for reasons of clean air maintenance, fully integrated electrolysis cells so as to retrieve effluent gases therefrom as completely as possible. Purification of the effluent gases then is usually effected in wet scrubbers, or more recently with dry purification units involving adsorption on aluminium oxide.

Valuable aluminium-containing compounds, particularly cryolite and chiolite (and possibly other fluorine-containing compounds) and alumina, are present as dust in the effluent gases, and the effluent gases have hitherto been recycled to the electrolysis furnaces to re-use such compounds. With such total recycle, impurities present in the gases e.g. carbon and compounds of phosphorous, vanadium, titanium, iron, silicon and sulphur, are also recycled. Such impurities rapidly build up in the furnaces and lead to diminution of yields and to deteriorations of the quality of the aluminium primary product.

It has therefore been advisable to remove the whole of the dust separately before treatment in wet scrubbers or dry purification units. However the fluorine-containing compounds present in the effluent dust has hitherto required high-cost elimination on a special refuse dump, resulting in loss of valuable raw materials such as cryolite, chiolite and alumina.

The object of the invention is to provide for separation of dust material from the effluent gases of aluminium fusion electrolysis furnaces in such a way that the fluorides and the alumina content can be largely recycled, the impurities, together with the carbon being substantially separated before recycling. By the practice of the invention and impurities may be removed to a content of about 5%.

The invention provides a process for the separation and re-use of cryolite, chiolite and aluminium oxide fusion electrolysis furnaces, characterised in that dust is separated from the said exit gases and the separated dust is subjected to a flotation step to form a flotation froth containing impurities and a flotation slurry containing the said cryolite, chiolite and alumina, and the said cryolite, chiolite and alumina is separated from the said flotation slurry and after drying is returned to the said electrolysis furnace.

The mixture of cryolite, chiolite and aluminium oxide, obtained in the process of the invention has such a low content of troublesome impurities that, after drying it can be re-used in the fusion electrolysis process. The lower amounts of flotation froth in which the impurities are contained, can be either burned or otherwise disposed of.

The process of the invention may be effected either in a wet or dry separation process, as hereinafter described.

Thus the said dust is removed from the effluent gases in a wet separation process by contacting the effluent gases with a countercurrent flow of washing water discharged through nozzles into a fluidized bed of plastic ball to provide a good aeration of the washing water, to produce in a tank the said flotation slurry, and the said flotation froth containing impurities.

Preferably Ai(OH)3 and NaOH or Na2CO3 are added to the washing water to maintain optimum precipitation and flotation conditions to form cryolite and chiolite in the washing water, and the flotation froth, namely a pH value between 0.5 and 6, preferably between 1 and 3, a fluoride concentration from 0.5 to 20 gF/litre and a sulphate concentration from 0.5 to 40 gS04/litre.

In a particular embodiment of the wet separation process purification of the effluent gases is effected in a two-step wet scrubbing process at a flow-rate of from 2 to 10 litres of washing water per cubic metre of exit gas and at a solids content of from 5 to 35 g solids per litre, and the washing water is sprayed by way of nozzles in counterflow to the effluent gases onto a fluidized bed of plastic balls. The washing water containing the dust of the said effluent gases produces in the tank of the first stage the said flotation slurry and the said flotation froth containing impurities. This flotation takes place due to the thorough aeration of the washing water in the fluidized bed by the exit gas.

This procedure has the advantage that separatiohn of the dust material can be effected without special additives within the tank of the fluidized bed-scrubber.

Another embodiment of the invention uses a dry separation process wherein the said dust is removed from the effluent gases in a dry separation process in a filter unit, and the thereby separated dust is contacted with flotation medium and a flotation aid and the mixture thus obtained is agitated in a flotation machine with air to produce the said flotation slurry and the said flotation froth by means of the flotation aid and the air.

Preferably the said flotation aid comprises butanol and decanol with additions of 2ethylhexanol and heptanol isomers, introduced at a rate between 0.01 and 10 g per litre of said mixture.

Apparatus for carrying out the wet separation process comprises aluminium fusion electrolysis furnaces, a dust separation zone for separating dust from the effluent gases of the said electrolysis zone, a flotation zone for producing a flotation slurry of cryolite, chiolite and aluminium oxide from the said separated dust and a flotation froth containing impurities from the said separated dust, froth separating means for removing flotation froth from the said flotation slurry, and separating and drying means for separating and drying cryolite, chiolite and alumina from the said flotation slurry, and recycling means for recycling to the said electrolysis furnaces the separated and dried product obtained from the said separating and drying means.

Apparatus for carrying out the dry separation process comprises a filter unit for removing dust from effluent gases from the said electrolysis furnaces mixing means for producing a mixture of the separated dust with flotation medium. and a flotation machine for producing flotation slurry from the aforesaid mixture using a flotation aid and air, comprising a flotation machine with a plurality of standpipes for the introduction of air, each provided with an impeller, and separating and drying means for separating and drying cryolite. chiolite and alumina from a flotation slurry produced therein, and recycling means for recycling to the said electrolysis furnaces the separated and dried product obtained from the said separating and drying means, The invention is hereinafter further described and illustrated in the accompanying drawings of which Figure 1 is a diagrammatic representation of apparatus for the wet separation of the gases, and Figure 2 Is a diagrammatic representation of apparatus for the dry separation of the gases.

Referring to Fig 1, the effluent gases from an aluminium fusion electrolysis furnace 21 are removed therefrom by a fan 2 and are washed with water in a single-step, or two-step wet scrubber 1 as shown in Fig. 1. The washing water is recycled by pumps 3 into upper and lower portions of the scrubber through nozzles 4, into the second and first stages as shown.

Separation of the acidic gaseous HF component of the effluent gases and of the dust is effected within the two fluidized beds 5 and 6 by intimate contact with the washing water. The fluidized beds are formed of balls of plastics material for example hoilow balls of propylene having a diameter less than 50 mm maintained in movement by the flow of gas therethrough and which spin in the ascending gas stream and are wetted by the washing water, which trickles down in counterflow, and thus form a large phase interface which is constantly broken and renewed by the collision of the balls. By this means good adsorption efficiency is attained and so much gas is entrained in the discharged washing water 7 that a flotation froth 8 containing carbon and impurities is formed on the surface of the tank of the first stage.The underlying flotation slurry contains particulate cryolite, chiolite (and other solid fluorine-containing compounds), and alumina. When the flotation froth is taken off for example by jet aspirators 9, and removed fro the system, only aluminium oxide, cryolite and chiolite still remain in the circulating washing water. Ideal flotation and precipitation conditions are maintained by addition thereto of NaOH and Al(OH)3. These conditions may be described as follows:- pH value: between 0.5 and 6, preferably between 1 and 3; fluoride concentration: from 0.5 to 20 g F/litre; sulphate concentration: from 0.5 to 40 g SO4/litre; solids content: from 5 to 35 g solids/litre; quantity of washing water: from 2 to 10 litres/m3 of exit gas.

In order that the concentrations remain in equilibrium, fluoride and solids are removed in a branch line by the programme-controlled valve 11.

The further treatment of the partial stream produced by control valve 11 comprises concentration unit 12, filtration unit 1 3 and drying unit 14. The final product contains cryolite, chiolite and alumina and is returned to the electrolysis furnace through line 33.

The solids-free liquid from the concentration tank 1 2 can be returned through the pipeline 1 8 to the tank of the first stage of the wet scrubber. The solid sludge of solids is removed from the base of the concentration unit 1 2 to the filtration unit through stirring vessel 1 2a. The filtrate from the vacuum drum filter 1 3 can be returned to the concentration tank 1 2 through the pipeline 1 9 and another stream is removed from the drum filter by the vacuum pump through pipeline 20 as fluorine-free outgoing air. From the dryer 14, which is shown in the drawing as a revolving tubular furnace, the exit gas can be carried off to the scrubber by pipeline 32. The product is circulated to the electrolysis furnaces by way of pipeline 33, possibly by way of a mobile silo.

The flotation froth is removed by suction pump 9 from the tank of the first stage of the scrubber to neutralisation vessel 1 5 wherein it is treated with NaOH, and from thence to concentration unit 1 6 and to filter unit 1 7.

Thus the solids-free liquid from the concentration tank 1 6 is passed through pipeline 35 into the tank of the first stage of the wet scrubber. The filtrate is returned from the vacuum drum filter 1 7 by the pipeline 34 into the concentration tank and a further stream is removed from the vacuum pump by pipeline 35 as fluorine-free outgoing air. The impurities pass through line 36 to be burnt or otherwise disposed of.

Referring to Fig. 2, the effluent gases from the electrolysis furnaces 21 are freed from dust in an electro-filter or in a fabric filter 22 and subsequently hydrogen fluoride gas in the exit gas is adsorbed on Al203 in a dry gas purification unit 23, and subsequently discharged.

The dust from the preliminary dust separation is admixed with flotation medium from pipeline 24 in a mixer 37, and this slurry having aof example a solids content of from 50 to 200 g/litre, is passed into the flotation machine 24. By the action of the impellers 26ato 26dair passes down by the standpipes M and is mixed with the pulp in the aerating zone. The impellers are designed to create a thorough mixing of the air and slurry together with the flotation aid, to form small bubbles to produce the flotation froth containing impurities.

With the aid of these small air bubbles and by addition of the collector/foaming agent (flotation aid) through pipeline 27 into mixer 37, the impurities together with carbon pass into the flotation froth.

Discharge of the flotation froth is effected through discharge channel 28 and is disposed of through line 29. The cryolite, chiolite and alumina in the sediment after treatment in drying unit 31 can be directly returned to the electrolysis furnaces.

The flotation medium perferably has the following composition to provide optimum conditions of flotation and precipitation: Solids content: from 50 to 200 g/litre fluoride content: from 0.5 to 20 g F/litre sulphate content: from 0.5 to 40 g (SO4)/litre pH-value: from 0.5 to 6, preferably from 1 to 3 The collector/foaming agent combination (flotation aid) may consist of a mixture of butanol and decanol, with additional of 2-ethyl hexanol and heptanol isomers. This is added to the liquid entering the flotation unit, through pipeline 27, in a quantity of from 0.01 to 10 g/litre, so as to accelerate separation into flotation froth and slurry.

The fresh water, used in both methods for compensating for evaporation losses, can be normal industrial water. If the undustrial water is heavily contaminated, it is advantageous to improve flotation in the fluidized bed scrubber by addition of a collector/foaming agent (flotation aid). This is appropriately effected in a cell, placed in succession behind the valve 11 and before the settler 1 2.

Claims

1. A process for the separation and re-use of cryolite, chiolite and aluminium oxide contained in dust from the effluent gases of aluminium fusion electrolysis furnaces characterised in that dust is separated from the said exit gases and the separated dust is subjected to a flotation step to form a flotation froth containing impurities and a flotation slurry containing the said cryolite, chiolite and alumina, and the said cryolite, chiolite and alumina is separated from the said flotation slurry and after drying is returned to the said electrolysis furnaces.

2. A process according to Claim 1, wherein the said dust is removed from the effluent gases in a wet separation process by contacting the effluent gases with a counter-current flow of washing water discharged through nozzles into a fluidized bed of plastic balls to provide good aeration of the washing water, to produce in a tank the said flotation slurry, and the said flotation froth containing impurities.

3. A process according to Claim 2, wherein Al(OH)3 and NaOH or NaCO3 are added to the washing water to maintain optimum precipitation and flotation conditions to form cryolite and chiolite in the washing water, and the flotation froth, namely a pH-value between 0.5 and 6, preferably between 1 and 3, a fluoride concentration from 0.5 to 20 gF/litre and a sulphate concentration from 0.5 to 40 gS04/litre.

4. A process according to Claim 1, wherein the said dust is removed from the effluent gases in a dry separation process in a filter unit, and the thereby separated dust is contacted with a flotation medium and a flotation aid and the mixture thus obtained is agitated in a flotation machine with air to produce the said flotation slurry and the said flotation froth by means of the flotation aid and the air.

5. A process according to Claim 4, wherein the said flotation aid comprises butanol and decanol, with additions of 2-ethylhexanol and heptanol isomers, introduced at a rate between 0.01 and 10g per litre of said mixture.

6. A process according to Claim 4, wherein the flotation medium has a solids content of from 50 to 200 g/litre and a pH-value between 1 and 3, and a fluoride concentration from 0.5 to 20 g F/litre and a sulphate concentratio n from 0.5 to 40 gSO3/litre.

7. A process according to Claim 1, substantially as hereinbefore described with particular reference to Fig. 1 or Fig. 2 of the accompanying drawings.

8. Apparatus for the separation and re-use of cryolite, chiolite and aluminium oxide from the effluent gases of aluminium fusion electrolysis furnaces comprising aluminium fusion electrolysis furnaces a dust separation zone for separating dust from the effluent gases of the said electrolysis furnaces, a flotation zone for producing a flotation slurry of cryolite, chiolite and aluminium oxide from the said separated dust and a flotation froth containing impurities from the said separated dust, froth separating means for removing flotation froth from the said flotation slurry, and separating and drying means for separating and drying cryolite, chiolite and alumina from the said flotation slurry, and recycling means for recycling to the said electrolysis furnaces the separated and dried product obtained from the said separating and drying means.

9. Apparatus according to Claim 8, wherein the said dust separation zone comprises a scrubbing zone containing a moveable bed of inert particles wherein scrubbed gases from the said electrolysis furnaces are contacted with water to produce a washing water which is separated in the tank of the first stage in flotation slurry containing the wanted materials as cryolite. chiolite alumina and the flotation froth containing the impurities.

10. Apparatus according to Claim 8, wherein the said dust separation zone comprises a filter unit for removing dust from the effluent gases from the said electrolysis furnaces, mixing means for producing a mixture of the separated dust with flotation medium, and a flotation machine for producing flotation slurry from the aforesaid mixture using a flotation aid and air, comprising a flotation machine with a plurality of standpipes for the introduction of air, each provided with an impeller, and separating and drying means for separating and drying cryolite, chiolite and alumina from a flotation slurry produced therein, and recycling means for recycling to the said electrolysis furnaces the separated and dried product obtained from the said separating and drying means.

11. Apparatus according to Claim 8, substantially as hereinbefore described and illustrated in Fig. 1 or Fig. 2 of the accompanying drawings.