TECHNICAL FIELD

This invention relates to a process for converting waste aluminum dross residue into useful products. More particularly, the invention relates to the conversion of aluminum dross residue into soluble alkali metal aluminates and products derived therefrom. BACKGROUND ART Aluminum dross is formed whenever molten aluminum or aluminum alloy encounters an oxidising atmosphere, such as air, and consists of compounds of aluminum and any reactive alloying elements that may be present (e.g. magnesium) , and a percentage of unreacted metallic aluminum or alloy that is trapped in the form of small particles or droplets dispersed throughout the dross. Dross of this kind is produced in large quantities in aluminum production and fabrication plants and represents a significant loss of the original metal. In order to minimize this loss, the unreacted metal content of the dross is partially recovered either by heating and agitating the dross in the presence of a salt mixture, in order to reduce the surface tension of the metal droplets dispersed throughout the dross so that the droplets may coalesce and form a recoverable pool or layer or molten metal, or more recently by heating the dross by means of a plasma in the absence of salt additions in order to achieve the same effect (see for example, U.S. Patent 4,952,237 issued August 28, 1990; U.S. Patent 4,960,460 issued October 2, 1990; and U.S. Patent

4,959,100 issued September 25, 1990. However, once the unreacted metal content of the dross has been removed in this way, the remaining dross residue presents a serious disposal problem because the residue is generally unsuitable for dumping in land fill sites without expensive preliminary treatment. The residue from the

salt process contains a large proportion of water-soluble salt, which can leach from dump sites and pollute the water table. On the other hand, residue from the plasma process may contain aluminum nitride which reacts with water to produce ammonia, which is again an unacceptable pollutant.

There is therefore a need to find ways of using or recycling dross residues of this kind that are both economically attractive and environmentally acceptable. Dross residue contains a large percentage of alumina, a material which is used in various industrial processes, but the dross is too impure for its alumina content to be used without some kind of purification or extraction treatment. Numerous attempts have been made in the past to recover alumina values from waste alumina-containing refractory material, such as fly ash, by treatment with carbonates and oxides of various metals. For example, a reference to such processes is provided in U.S. patent 4,254,088 to McDowell et al, Column 2, lines 33 to 53, which mentions a "lime sinter" process wherein lime (CaO added as CaC0 ₃ ) is reacted with fly ash at elevated temperatures to form a water-soluble calcium aluminate compound. A dilute sodium carbonate solution is then used to dissolve the resulting fly ash/lime sinter and the aluminum is recovered by contacting the solution with C0 ₂ to form a precipitate of an aluminum hydroxide product. A second method is also disclosed in which the fly ash is sintered with a mixture of lime and soda (Na ₂ C0 ₃ ) to produce water-soluble sodium/calcium aluminate, and the sinter is again leached with sodium carbonate solution. Unfortunately, processes of this kind tend to be uneconomical when carried out on a large scale.

In U.S. patent 5,132,246 to Brisson et al, an aluminum dross residue is mixed with a particular metal oxide or oxide precursor and the mixture is calcined at high temperature. However, the purpose of this process is to

produce insoluble refractory products, but such products have limited markets and thus the disclosed process has not yet provided an effective way of utilizing large volumes of dross residue. Furthermore, International (PCT) patent publication WO 92/00246 discloses a process for producing particles of magnesium spinel from dross residue by mixing the residue with magnesium oxide or a precursor and heating the mixture at high temperature. As in the process mentioned immediately above, however, this does not represent a means for utilizing large quantities of dross residue because the market for such products is relatively small.

Despite prior attempts of this kind to convert dross residues into useful products, the quantities of unused dross residues continue to increase and represent a burden to aluminum producers and fabricators.

There is accordingly a need to find a way of utilizing aluminum dross residues in an economic manner that allows large scale consumption of the material without giving rise to harmful environmental consequences. DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a process for converting waste aluminum dross residue into one or more useful products so that the residue can be consumed and thus eliminated.

Another object of the invention is to provide a process of converting dross residue to alkali metal aluminate and products derived therefrom.

Another object of the invention is to recover alumina values from waste aluminum dross residue.

Yet another object of the invention is to derive alumina and caustic values from bauxite and waste aluminum dross residue by means of the Bayer process.

According to the present invention there is provided a process for producing a solution of aluminum-containing compounds from aluminum dross residue which comprises mixing the dross residue with solid sodium oxide or a

precursor thereof to form a mixture; heating the mixture in an oxidizing atmosphere to a temperature in the range of about 800-1300°C for a period of time long enough to form a solid material containing sodium aluminate; partially dissolving the resulting solid aluminate- containing material in an aqueous liquid to produce a solution and undissolved solids; and separating the solution from the undissolved solids.

The invention also provides a process for recovering alumina values from aluminum dross residue, which process comprises: mixing the dross residue with solid sodium oxide or precursor thereof to form a mixture; heating the mixture to a temperature in the range of about 800-1300°C for a period of time long enough to form a solid material containing sodium aluminate; partially dissolving the resulting solid aluminate-containing material in an aqueous liquid to produce a solution and undissolved solids; separating the solution from the undissolved solids; and treating the solution according to the Bayer process to recover alumina values therefrom.

Furthermore, the invention provides a process of recovering alumina values from bauxite and dross residue, which process comprises: digesting bauxite by the Bayer process to produce a sodium aluminate solution and precipitating alumina from said aluminate solution while producing salt cake and spent Bayer liquid as by-products; mixing the dross residue with said salt cake from the Bayer process to form a mixture; heating the mixture to a temperature in the range of about 800-1300"C for a period of time long enough to form a solid material containing sodium aluminate; partially dissolving the resulting aluminate-containing material in said spent Bayer process liquor to produce a resulting solution and undissolved solids; separating the resulting solution from the undissolved solids; and adding the resulting solution to said sodium aluminate solution produced during said Bayer process to recover alumina values therefrom.

BEST MODES FOR CARRYING OUT THE INVENTION

Stated in simple terms, the process of the present invention involves heating and reacting dross residue with sodium oxide or, more usually, a precursor of sodium oxide (generally in the absence of other metal oxides or precursors, particularly CaO) to form a solid product (referred to as a calcinate) containing a large percentage of sodium aluminate. The calcinate is then partially dissolved in (referred to as being "leached with") water or an aqueous alkaline solution (referred to as the leachant) in order to form a solution (referred to as the leachate) containing NaOH and A1(0H) ₃ and undissolved solids. The leachate can then be used, often without purification, in various processes and the undissolved solids can be disposed of without causing environmental hazard.

Dross residue of any kind may be used in the present invention but plasma dross residue is preferred because, if salt dross residue is employed, the salt content must first be removed, e.g. by washing with water, which results in greater inconvenience and expense.

It is particularly advantageous to use a dross residue having a high content of aluminum nitride (A1N) and/or metallic aluminum (Al) . This is because these ingredients oxidize exothermically during the heating step of the process and thus contribute heat to the aluminate-forming reaction and reduce the amount of heat which has to be added from the exterior. This can represent a consider¬ able cost saving and makes the process much more economically attractive compared with the treatment of alumina from other sources, e.g. fly ash.

To indicate the magnitude of the heat saving that can be obtained, it is noted that when the dross residue contains significant amounts of A1N and/or Al, as soon as the reaction temperature reaches about 850°C a violent oxidation reaction commences and the temperature quickly increases to more than 1000°C. The heat input can then

often be reduced by as much as 80% without the reactants cooling undesirably.

Plasma dross residue is particularly preferred in the process of the invention because it usually contains a large amount of alumina (generally at least 50% by weight and often at least 75% by weight) as well as A1N and metallic Al and only a small amount of undesirable silica (less than 1% by weight, if present at all) .

A typical composition of dross residue suitable for use in the present invention is shown in Table 1 below.

TABLE 1

While the dross residue may contain a large amount of insoluble magnesium spinel, it is found that the process of the present invention converts the alumina content of the spinel to soluble aluminate and leaves an insoluble magnesium oxide (as periclase) . More than 90% of the spinel is usually converted in this way. For this reason, the solid dross residue is converted in high yield to soluble aluminate. For example, while the average dissolution percentage of the starting dross residue in caustic soda solution is typically about 25%, the dissolution product of the calcinate produced according to the invention may be as high as about 78% or more, thus

making the process economically attractive.

It has also been found that a further improvement in the dissolution percentage of the calcinate may be achieved by decreasing the particle size of the starting dross residue. Particles of size smaller than 25 mm [1 Tyler mesh] are preferred for this reason, and ideally particles of less than 0.83 mm or even less than 0.35 mm [i.e. -20 or even -40. Tyler mesh] are employed.

The particle size of the other starting material, i.e. the sodium oxide or precursor, is not as critical to the percentage solubility of the calcinate, but it is preferably similar to the particle size of the dross residue for ease and thoroughness of mixing so that a substantially homogenous reaction mixture can be prepared. A precursor of sodium oxide is normally employed in the process, e.g. sodium carbonate, sodium hydrogen carbonate, sodium oxalate, sodium hydrogen oxalate, sodium hydroxide or sodium sulphate. Most advantageously, however, so- called "salt-cake" (a by-product of the Bayer process containing sodium carbonate, sodium oxalate, sodium sulphate, etc.) is employed as the sodium oxide precursor. This material is preferred because it is itself a material that is difficult to dispose of in an economical manner and there is thus a double benefit in using it as a starting material in the process of the present invention.

Typical compositions of salt cake (from the Bayer plant at Auginish, Ireland) are shown in Table 2 below.

TABLE 2

The dross residue and sodium oxide or precursor are generally mixed together in a ratio by weight of 1:0.2 - 1.2, respectively, depending on the alumina content of the residue (the precursor being calculated as the oxide) .

The dry mixture of dross residue and sodium oxide or precursor is heated in an oxidizing atmosphere (usually air at atmospheric pressure) at a temperature in the range of 800-1300°C, more preferably about 900-1000°C, for a period of time preferably between 10 minutes and two hours.

Using sodium carbonate as an example of the sodium oxide precursor, the reactions taking place during the heating step can be represented by the following formulae:

A1 ₂ 0 ₃ + Na ₂ C0 ₃ → 2NaA10 ₂ + C0 ₂

MgAl ₂ 0 ₄ + Na ₂ C0 ₃ → 2NaA10 ₂ + C0 ₂ + MgO

2A1 (metal) + 3/20., + Na ₂ C0 ₃ → 2NaA10 ₂ + C0 ₂

2A1N + 3/20 ₂ + Na ₂ C0 ₃ → 2NaA10 ₂ + C0 ₂ + N ₂

During the period of heating, the mixture is preferably agitated to allow ingress of air and egress of reaction gases. This can be achieved, for example, by carrying out the reaction in a rotary or rocking arc furnace operated continuously or intermittently at a speed of about 2-4 rpm. However, the heating step may alternatively be carried out in a vertical shaft furnace or other furnace or oven.

While the reaction may be virtually complete after less than 30 minutes at l000 ^β C or more, it has been found that the reactants and calcinate may tend to attach themselves to the furnace walls during the initial reaction periods. After longer heating times, however, e.g. 60 minutes or more, a free-flowing calcinate (unsintered product) may be obtained, which is easier to deal with. The optimum retention time in the furnace or oven is usually 45 to 60 minutes in order to balance heating costs against ease of use of the calcinate.

As indicated above, after allowing the calcinate to cool to ambient temperatures, the solid (which contains a high percentage of sodium aluminate) is leached with (i.e. partially dissolved in) water or, more preferably, an aqueous alkaline solution to generate NaOH and A1(0H) ₃ , i.e. both caustic and alumina values. Alkaline solutions are preferred because they promote the precipitation of impurities from the sodium aluminate solution and produce solutions that are of greater commercial value and which can be used or sold without a preliminary concentration step.

Sodium hydroxide solution may be used, for example, as the aqueous alkaline solution used for the leaching step. However, it has unexpectedly been found to be particularly

advantageous to use sodium aluminate solution or spent Bayer liquor (which usually contains sodium aluminate, sodium carbonate and dissolved degraded humate materials) . When such solutions are used, it is found that impurities in the calcinate, especially compounds of Fe, Mg, Si, etc. , then tend to accumulate or precipitate in the undissolved solids and can be removed by a solid/liquid separation step (e.g. filtration) and so do not contaminate the leachate. As much as 90% of the calcinate may dissolved in the leaching step leaving a small quantity of undissolved and/or precipitated solids containing oxides of Mg, Si, Al, Fe and Ca, etc.

The leachate is generally sufficiently pure to be used without further treatment for a variety of processes, e.g. for papermaking or water purification. It is particularly noteworthy, however, that the leachate may be used in the Bayer process without causing operational problems, particularly if the leachant used for dissolving the calcinate contained little or no sodium carbonate which could interfere with the subsequent recovery of alumina values. Briefly stated, the Bayer process is described as follows. In a typical example of the Bayer Process, the following operations are performed in turn: (1) the bauxite ore is treated with a caustic solution at elevated temperature and pressure to dissolve the alumina values in the ore, thereby producing a slurry comprising a solution of saturated sodium aluminate and suspended insoluble material; (2) separation and washing of the insoluble residue (called red mud) remaining after the dissolution;

(3) cooling the saturated solution of sodium aluminate to supersaturation and causing the precipitation of the alumina hydrate on added seed crystals of alumina hydrate;

(4) separating the precipitated alumina from the solution by filtration producing a filtrate which is known as spent liquor; (5) regeneration of the spent liquor for reuse by evaporating the water thereby precipitating the sodium

salts of the impurities such as oxalate, sulphate, carbonate, etc. , and filtering off this solid, referred to as "salt cake"; (6) calcining the alumina hydrate to drive off the water of crystallization and convert it to anhydrous alumina.

The leachate produced according to this invention may be added to the sodium aluminate solution of the Bayer process and results in the precipitation of relatively pure alumina from the solution. It will therefore be seen that the process of the present invention can be integrated with the Bayer process, and with an aluminum fabrication process, in a most convenient and economical way. In particular, aluminate solutions resulting from the use of spent Bayer liquor as a leachant are compatible with the Bayer process and, as a result, the leachate may be added to the Bayer circuit without incurring a dilution penalty. Thus, impure aluminum dross residue and impure Bayer salt cake can be reacted and leached with spent Bayer liquor to produce a relatively pure aluminate solution which can be cycled to the Bayer process and eventually converted into aluminum by the Hall-Heroult process. In turn, dross residue from the Hall-Heroult process and other aluminum fabrication operations can be recycled to the process of the invention.

This internal recycling procedure can be optimized by associating the process of the invention with a Bayer process plant and bringing in dross residue from an associated Hall-Heroult plant. Further dross residue could also be brought in from other plants. By using dross residue of high A1N and/or Al content, the process is made even more cost-effective.

It has furthermore been found that the undissolved solids of the process of the invention may be disposed of in the "red mud circuit" of the Bayer plant, which removes and disposes of waste solids from the Bayer process, without adversely affecting flocculation or precipitation

rates which are critical to the successful operation of the circuit. The entire product of the invention may therefore be utilized or disposed of in a Bayer plant. However, if it is not desired to dispose of the undis- solved solids in this way, they can be disposed of in conventional land-fill sites because they do not contain harmful pollutants.

The following Examples illustrate the invention but are not intended to limit its broad scope. EXAMPLE 1

One part of dross residue containing 8 % of metallic aluminum, 16 % of A1N and 30 % of spinel was sieved to a particle size of less than 25 mm [minus 1 Tyler mesh] and mixed with one part of sodium carbonate. The mixture was heated at 1300°C for one hour' in air. An X-ray analysis indicated that NaA10 ₂ was the main product of the reaction. MgO was detected as a minor phase in the product. The product of the reaction was digested in a solution containing 100 g/1 of NaOH. The digestion was done at 143°C for 30 minutes. About 80% of the solid was digested by the caustic solution. The remaining 20% of the solid was composed of MgO as the major impurity.

EXAMPLE 2

One part of dross residue containing 8 % of metallic aluminum, 16 % of A1N and 30 % of spinel was sieved to a particle size of less than 25 mm [minus 1 Tyler mesh] and mixed with one part of sodium carbonate. The mixture was heated at 800°C for one hour in air. An X-ray analysis indicated that NaA10 ₂ was the main product of the reaction. MgO was detected as a minor phase in the product. The product of the reaction was digested in a solution containing 100 g/1 of NaOH. The digestion was done at 143°C for 30 minutes. About 70% of the solid was digested by the caustic solution. The remaining 30% of the solid was composed of MgO as the major impurity.

EXAMPLE 3

Dross residue (100 kg) from the plasma dross treatment plant in Jonquiere, Canada, was used in this Example and contained 16% by weight of A1N, 8% by weight of residual metallic Al and 30% by weight of spinel. The dross residue was sieved to a particle size of less than 25 mm [minus 1 Tyler mesh] and mixed with 100 kg of light soda ash (Na ₂ C0 ₃ ) . The mixture was heated at a temperature between 950"C and 1050°C for one hour in air. After the reaction, 150 kg of a calcinate was obtained.

An X-ray analysis indicated that the product consisted mainly of NaA10 ₂ with traces of an unknown compound.

A 300 g sample of the product was taken and digested in a sodium aluminate solution in a bomb simulating the Bayer process. From the sample, only 33 g of the product was not digested. The non-digested product was sent to the red mud circuit of a Bayer plant. The analysis of the remaining material is shown in Table 3 below.

TABLE 3

It was found that more than 95% of the sodium of the calcined product dissolved, more than 65% of the Al

dissolved, more than 80% of the Si impurities remained in the undissolved solid, more than 95% of the Mg remained in the undissolved solid, more than 94% of the Ca remained in the undissolved solid and more than about 40% of the iron remained in the undissolved solid.

The digested part of the 300 g sample increased the alumina and caustic concentration of the liquor. The product gave an equivalent of 190 g of caustic as Na ₂ C0 ₃ and 165 g of alumina to the liquor. These two products are useful for the Bayer plant.

EXAMPLE 4

Four different kinds of dross residue were used in this Example. The compositions of these residues is shown in Table 4 below. TABLE 4

The typical size distribution of this material is shown in Table 5 below.

TABLE 5

Only material smaller than 25 mm [minus 1 Tyler mesh] was used in the Example, so the product was sieved as required.

The dross residues contained magnesium oxide in the form of spinel, aluminum nitride and residual metallic aluminum. These dross residues included two high magnesium content materials (Bag #s 784 and 4885) , a low magnesium content material (Bag # 908) and an intermediate magnesium content material (Bag # 2220) so that the influence of magnesium on the sodium aluminate formation could be investigated.

Sodium carbonate (soda ash) was obtained from three different sources (Tennaco Minerals Inc. , FMC Corporation, and General Chemicals Canada Ltd.). The chemical and physical properties of these 'materials are shown in Table 6 below.

TABLE 6

The size distribution of this material is shown in Table 7 below.

TABLE 7

Three of these soda ash materials are dense and two are light materials.

A rotary batch furnace having a capacity of 500 kg, operated at a rotational speed of 3 rpm and heated by a propane burner having a calorific capacity of 378 Kcal was used to heat mixtures of 40 kg of screened dross residue smaller than 25 mm [-1 Tyler mesh] and 40 kg of sodium carbonate, which was tumbled in the furnace for 5 minutes to effect mixing prior to heating. The mixture was then heated at maximum burner output up to a temperature of about 1000'C. Because of the A1N and residual metallic Al components of the dross residue, it was found almost impossible to vary the temperature. When the temperature reached 850°C, a violent oxidation reaction took place and the temperature increased suddenly to 1000"C.

Reactions were carried out for 15, 30 and 60 minutes at a temperature of 1000+25"C.

After furnace discharge and cooling of each product, a representative sample of 25 kg was taken and homogenized by grinding to less than 3 mm [-1/8 inch]. This material was then split into various fractions for analysis and

further ground to less than 0.15 mm [minus 100 Tyler mesh] and digested in caustic soda solution in the following conditions:

Sample weight 130 g Caustic concentration . 100 g/1 as NaOH

Caustic temperature . . 143°C

Caustic volume 1 L

Digestion time 30 minutes.

The digestion liquor was then filtered and the residue was weighed after drying.

Analyses were then carried out on the sintered products, on the liquors and on the residues after filtration, as follows:

1. The product of the heat treatment (calcinate) was analyzed by X-ray diffraction (XRD) for phases and compounds, for the concentration of elements by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP) , and fluoride by Technicon analyzer.

2. The liquor was analyzed by ICP, and thermometric titrationε, and by ion chromatography for NaCl, Na ₂ S0 ₄ and Na ₂ C ₂ 0 _A .

3. The material remaining after digestion and filtration was analyzed by XRD, ICP and by Technicon analyzer.

The results are shown in Tables 8 to 16 below.

TABLE 8 RESULTS OF XRD ANALYSIS

DROSS RESIDUE

It should be noted from Table 8 above that while corundum and spinel are the dominant compounds in the dross residue before the reaction, these compounds are found in only minor proportions in the calcinate, and that sodium aluminate is the major phase.

The average yield of the reaction for more than 12 different tests was 77%. The loss in weight corresponds essentially to the C0 ₂ loss.

In these tests it was found that the exothermic nature of the oxidation of AIN and metallic Al allowed the heat input from the burner to be decreased by as much as 80% once the oxidation reaction was initiated. The energy content of the dross residue from its metallic Al content was evaluated at 4000-8000 MJ/Tonne, so much less energy was needed during the operation of the process. After the reaction was completed, the material was discharged into cast iron pans to cool down. The colour of the material was light yellow to beige with glints of green.

The different tests carried out during this Example are shown in Table 9 below. For' convenience, the test numbers shown in Table 9 serve as a reference in succeeding Tables.

TABLE 9 SUMMARY OF TESTS CARRIED OUT

Table 10 below shows the ICP results for the calcinate before digestion. The sodium varies from 19.2 to 23.5%, while the Al content ranges from 28.6 to 32.7%. Test No. 13 was a sample of unreacted dross residue analyzed under the same conditions as the calcinate. Along with these data, the residual AIN and metallic Al are given.

TABLE 10 ICP RESULTS ON THE CALCINATE

As a comparison, test # 13 was a sample of the # 2220 bag untreated dross

Table 11 shows the dissolution percentage of the calcinate in the 100 g/1 caustic NaOH solution (equivalent to Bayer process spent liquor) at 143°C for 30 minutes. Table 12 shows the results of ICP analysis on the leachate obtained by digesting the calcinate. Table 13 shows the total titratable soda (TTS) , sodium carbonate, alumina, and the NaCl, Na ₂ SO^ and Na ₂ C ₂ 0 ₄ content of the leachate.

TABLE 11

DISSOLUTION PERCENTAGE OF THE CALCINATE /1 caustic solution at 143°C for 30 minutes

TABLE 12

ICP RESULTS ON THE LEACHATE AFTER DIGESTION OF THE

CALCINATE IN 100 g/1 Caustic Solution at 143°C for 30 minutes

TABLE 13

TOTAL TITRATABLE SODA, SODIUM CARBONATE, NaCl, Na ₂ SO and Na ₂ C ₂ 0 CONTENT OF THE LEACHATE AFTER DIGESTION OF THE CALCINATE in 100 g/1 caustic solution as NaOH at 143°C for 30 minutes

The caustic solution used to dissolve the sintered material (tests #1 to 12) has been evaluated at:

Na ₂ C0 ₃ = 127.5 g/1 The spent liquor used to dissolve the test #13

(as-produced bag # 2220) has been evaluated at:

Na ₂ C0 ₃ = 188.58 g/1

A1 ₂ 0 ₃ = 66.43 g/1

It was found that the finer the particle size of the dross residue before the reaction, the better the disso¬ lution after the reaction. The rate of dissolution increases linearly using particles from size smaller than 25 mm to smaller than 0.35 mm [-1 to -40 Tyler mesh]. Clearly, it is therefore better to use smaller particles

Tests on the reaction time in the furnace the extent of dissolution of calcinate show that as the reaction time increases, the sodium and aluminum concentration in the leachate increases, but that the reaction was essentially complete after 30 minutes at 1000°C.

It was found that there was essentially no difference in the reaction according to the different sources and grades of the sodium carbonate.

Table 14 below shows a comparison between impurities measured in the leachate produced according to the invention and in the spent liquor from a Bayer plant (at Jonquiere, Canada) . The invention produces a liquor of composition which compares very favourably with the composition of Bayer spent liquor.

TABLE 14

COMPARISON OF IMPURITIES IN LEACHATE OF THE CALCINATE VS THE TYPICAL IMPURITIES FOUND IN SPENT LIQUOR FROM VAUDREUIL PLANT

average of 12 tests

A test was carried out to determine the solubility of the calcinate in hot water at 95°C for 30 minutes, or in comparison with the regular caustic digestion at 143°C for 30 minutes. The weight loss percentage was 5.5% lower

than that in the caustic solution. This satisfactory solubility in water as opposed to caustic indicates that the calcinate may be suitable for applications other than recycling to Bayer plants.

TABLE 15

DISSOLUTION PERCENTAGE OF THE CALCINATE IN CAUSTIC AND IN WATER

Table 16 shows the elemental composition of the final residue produced by caustic digestion of the calcinate, filtering and drying.

TABLE 16

CHEMICAL ANALYSES OF THE SOLIDS FROM DISSOLUTION OF THE CALCINATE IN CAUSTIC SOLUTION

The Al content varies from 17.1 to 33.8% and the recovered weight of the residue ranges from 15.5 to 31.4 wt.% which gives an Al content lost of 2.65 to 10.6% in the residue and an efficiency of Al recovering of 65.0 to 91.7%.

The retention of impurities by the undissolved solids is shown in Tables 17 and 18 below. In these Tables the "actual" figures are taken from Table 16 above and the "calculated" figures were derived from the formula "100/wt. residue x element composition in Table 10 above." The "weight residue" is taken from Table 11 above.

TABLE 17 ACTUAL AND CALCULATED COMPOSITION OF RESIDUES AFTER DISSOLUTION

EXAMPLE 5

A further experiment was carried out using a continuous rotary kiln. The kiln employed was a direct fired counter current flow kiln of one foot in diameter and 12 feet in length fired by an oil burner. The kiln was rotated at 1.5 rpm and the kiln sloped down by 6 inches over 12 feet.

A mixture of plasma dross residue and sodium carbonate was fed to the furnace at a rate of 25 kg/h. The retention time in the furnace was about 1 hour.

The resulting solid was leached with 100 g/1 caustic soda solution at 143°C for 30 minutes.

The results obtained were similar to those of Example 4.

TABLE 18

PERCENTAGE RECOVERY OF IMPURITIES IN INSOLUBLE RESIDUE RECOVERY CALCULATED AS "% ACTUAL/ % CALCULATED"

These results show that the leachate solution produced from the calcinate of the invention is suitable for recycling to a Bayer process since it contains few impurities. INDUSTRIAL APPLICABILITY

The invention is useful in aluminum metal fabrication and process industries for recovering metal values that would otherwise be lost.