The paper presents a joint research made by the Siberian State Academy for Mining and Metallurgy and the Institute of Metallurgy and Concentration of the National Academy of Sciences, Kazakhstan, on separation of aluminum oxide from ash and slag of Kuzbass thermal power plants. This work is an integral part of 'The Development of a Technology for Deep Processing of Waste from Thermal Power Plants and Steel Plants' carried out in accordance with the program environment protection. A lime-free technology of extraction and concentration of high-silica aluminum raw materials by means of alkaline solution has been developed. A solid residue of a chemical concentration is a product which may be processed into alumina. This product contains up to 45 to 50% Al2O3 versus 25 to 28% for initial raw materials. The concentrate with an increased content of aluminum oxide can be processed either means of caking or hydrochemically. After the chemical concentration, the silicon-alkaline solution is supposed to be processed into silicate products such as cement, zeolite, silicate glass, soda.
Purpose of Study
As a result of investigations of wastes from thermal power plant and the development of a technology for separation of iron oxides from them, it has been found that up to 10% magnetic fraction can be extracted from ashes of Belovo and Tom-Usinsk power plants which produces an iron concentrate with 80 to 93% iron oxide contents after leaching. A technology for producing the concentrate with a high content of iron oxide has been developed from which steel can be melted missing agglomeration and blast-furnace processes (1,2). However, this concentrate only provides 10 percent utilization of ash, while up to 90% of it is a non-magnetic fraction containing 25 to 30% aluminum oxide and up to 50% silicon oxide. Therefore, it was necessary to create the technology for separation of aluminum oxide for its subsequent use as a raw material in the production of alumina and aluminium. The availability in Novokuznetsk of two large enterprises of ferrous metallurgy (Kuznetsk Steel Combine and West Siberian Steel Plant) and of aluminium plant made the taste easier. It is profitable to create the production of deep processing of ash-slag wastes for the above consumers in South Kuzbass. Since the aluminum oxide of the ash from thermal power plants is combined with silicon and iron oxides as well as with other minerals, its separation can be performed by extraction of minerals chemically. Conventional methods of the extraction of aluminium silicate materials for the selective transition of aluminium into solution involve the introduction of calcium into a charge in order to convert the silica of the charge into insoluble calcium silicates (3).
The Institute for Metallurgy and concentration of the Kazakhstan Academy of Sciences has developed an effective lime-free technology for extraction and concentration of such high-silica aluminium materials as bauxites, nephelines, coal clays, red mud and ash from thermal power plants (4). Under certain conditions, aluminium and silicon, dissolving instantly in an alkali is retained in the solution for a period during which the aluminate-silica solution is separated from the sludge concentrated with iron and rare-earth elements. A new way of decomposition of high-silicon rocks, ash from thermal power plants in particular, is based on this technique permitting the separation of ash components at the 'head' of the technological scheme by transition of aluminium and silicon into solution and iron into sludge. Thus, the technology for producing raw materials of alumina which maces their integration in a single complex possible. For this purpose, the possibility of processing tails from Belovo and Tom-Usinsk power plants after producing iron concentrates has been studied.
The presence of aluminum oxide in tails (non-magnetic fraction) provides for extraction of alumina though, without previous extraction of silicon oxide from the tails, the latter cannot be treated by conventional methods. Therefore, the authors performed the alkali chemical concentration for producing a silicon-alkaline solution and an aluminium concentrate containing little silicon oxide which may be processed into alumina, alkali and silicate products. This technique is based on the property of aluminiferous minerals to decompose under the action of alkalis.
First, chemical analysis of the tails of non-magnetic fraction treated by a magnetic separation was made
Chemical Analysis of Samples of Tails from Thermal Power Plants after Magnetic Separation
Test results of chemical concentration of ash tails are summarized in Table 2. The effect of the L:S (liquid-to-solid phase) ratio during leaching and duration of the process on the amount of extraction of SiO2 and Al2O3 into solution was studied at +100oC and the 200 g/dm3 Na2O concentration. The conclusions arrived at from the analysis of the test results may be stated as follows. Variations in a L:S ratio (L:S = 3 to 10:1) for the first sample (tests 8 to 11) reduce the SiO2 content of a negligible change to the Al2O3 content of the solution (1.7 to 0.9 g/dm3).
Test Results of Chemical Concentrations of Ash Tails from Belovo
and Tom-Usinsk Power Plants
Test No. Testing Conditions Solution, g/dm3
Time, hour Concentration Na2O, g/dm3 T, ºC L:S Na2O Al2O3 SiO2
The extraction of SiO2 and Al2O3 into the solution increases with the change in the L:S ratio up to 48...55% and 2.9...3.8%, respectively. The least extraction of SiO2 and Al2O3 is observed at the L:S=3:1 (tests 12 to 14) which indicates a negligible effect of a time factor on the extraction of SiO2 and Al2O3 into solution at 1 to 4 hours period.
Data on the second sample (tails from Tom-Usinsk power plant) approximate to the above. The maximum extractions of SiO2 and Al2O3 into solution at 3 to 4 hours were 48.6 to 49.6% (48 to 55% SiO2 for the tails of Belovo power plant) and 2.0 to 2.3% (2.9 to 3.8% Al2O3 for the tails of Belovo power plant), respectively.
Different data were obtained from the study of kinetic dependence (test 5 to 7). It was found that the silicon oxide extraction from the ash tail of this sample was not sufficient (~37%) at 1 hour. With the prolongation of the treatment to 3 to 4 hours, the extraction of SiO2 into solution increased by 7 to 11% amounting to 44.5 to 48.3% while the extraction of Al2O3 did not change being kept at 1.82 to 2.15% level. Thus, the investigation performed proved the possibility of a 50 percent extraction of SiO2 from the tails with a little Al2O3 extraction into a liquid phase under the above conditions. As to the solid residue from the chemical concentration, it was a product which could be processed into alumina. The main aluminaiferous phase was sodium hudroalumosilicate.
For comparison, the chemical analysis of a high-aluminate concentrate is also given here. A concentrate with a high content of aluminum oxide can be processed either by the method of caking used in industry (like the Kola concentrate) or by hydrochemical method. The method of caking is based on the solid-phase transformations of the aluminium concentrate containing lime and soda.
Caking is carried out at 1280 to 1300oC. As shown by the experiments, fusing of the cake does not decrease its contents of soluble alumina and alkali. At lower temperatures of caking, some indecomposed concentrate is produced which results in the underextraction of alumina during the subsequent leaching of the cake by the aluminate solution. The cake produced is ground and leached by a solution containing 25 to 30g/dm3 Na2O with a/q=1.85 to 1.95 at 80oC. The solution contains 90 to 95g/dm3 aluminum oxide after leaching. After separation, aluminium solution is used for the production of alumina by conventional methods.
After washing, the sludge consisting mostly of a silicate is used of the production of cement. The product of the extraction of SiO2 from the aluminium solution consist of the blend of salts: soda and potash which can be reused. After processing of an ash aluminium concentrate, 800kg of soda, 160 to 180kg of potash and 9 to 11 tons of cement are produced from a ton of alumina. A silicon-alkali solution from the chemical concentration of tails of an ash iron concentrate (non-magnetic fraction) will be processed into some silicate products. Utilization of the alkali-silicon solution is the main concern of all technologies dealing with an alkali chemical concentration.
As a result of a previous investigation, high-silicon zeolite has been obtained from these solutions. Zeolite is a valuable material used as an adsorbent and ion exchanger for cleaning facilities. The data form our laboratory investigation have shown that it is possible to produce zeolite with SiO2 : Al2O3 molar ratio of 10 at a 150oC temperature of crystallization and with the L:S ratio of 3. Further research is needed relating to the production of sodium metasilicate, calcium silicate and liquid glass.
The conclusions arrived at from this work may be stated as follows:
Separation of aluminum oxide from ash tails of Belovo and Tom-Usinsk power plants (after producing the iron concentrate for steelmaking) is possible by their chemical concentration similar to leaching of silica when producing a ferriferous concentrate but at a lower temperature (+100oC) and at a 200g/dm3 Na2O concentration. Lowering of the temperature and decreasing of the concentration of a solution reduces significantly the transition of Al2O3 into solution.
The aluminium concentrate obtained can be processed into alumina using the conventional technology of alumina plants (caking) with the subsequent extraction of iron oxides, which escaped a magnetic fraction, and silicon. At this stage, the extraction of a titanium dioxide is of a particular interest for an industry since its content exceeds 1% of the aluminium concentrate.
The sludge produced in the process of separation of silica from the aluminium concentrate is mostly in the form of calcium silicate used for the production of cement. From 1 ton of a concentrated alumina (raw material for aluminium) it is possible to produce 9 to 11tons of cement, 800kg of soda and 160 to 180kg of potash.
A soluble portion of an alkali chemical concentration of aluminium can be used for the production of high-silicon zeolit, a valuable material for chemical industry (adsorbent, ion exchanger), as well as for treatment of waste water.
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