On the ash of time: waste from coal-fired power plants will make aluminum production cheaper

Scientists have developed a new technology for producing alumina from ash, a by-product of coal-fired power plants. Since it is needed for aluminum production, the proposed approach will make it cheaper to produce this metal, and will also make it possible to utilize the waste generated in huge quantities. The alumina obtained by the authors meets the quality standards established in Russia, China and India, so it can potentially be used in both domestic and foreign production facilities.

How aluminum is mined

Alumina is an aluminum oxide that is widely used in various industries, but is mainly used in the production of aluminum. Studies show that various industrial wastes can serve as raw materials for its production, including ash and slag (coal ash) - a by-product of coal combustion at thermal power plants. Every year worldwide power plants produce more than 1 billion tons of coal ash, which is stored in sludge fields and can get into soils and water bodies, thereby polluting them. Therefore, the use of coal ash for alumina production will help to preserve the environment around power plants.

Scientists from the Vernadsky Institute of Geochemistry and Analytical Chemistry (GEOCHI) of the Russian Academy of Sciences (Moscow), Ural Federal University (Ekaterinburg) and Tongji University (China) have developed an improved technology for obtaining high-quality alumina from ash and slag, in which the feedstock is sequentially treated with several chemical reagents.

The authors used ash from the Reftinskaya GRES - one of the largest coal-fired power plants in Russia, located in the Sverdlovsk region.

First, the ash was dissolved for three hours in a mixture of sulfuric acid and ammonium bisulfate at 200°C. The resulting solution was cooled, resulting in the production of alum-ammonium alum - a complex salt of ammonium, aluminum and sulfuric acid. Then they were additionally purified from the impurity of iron, dissolved in water and deposited in the form of the mineral alunite on the surface of the seed, which served as the mineral bemite. In this case, the scientists changed the volume of reagents used, temperature and duration of sedimentation to determine what conditions allow you to get the largest amount of alunite (salt mineral of volcanic origin, consisting mainly of potassium sulfate and aluminum). In the final stages of synthesis, the alunite was washed with ammonia to remove sulfur and fired in a furnace at 950°C.

Dmitry Valeev, Leading Researcher of the Laboratory of Sorption Methods of GEOHI RAS, in the laboratory of Tongji University

Photo: Dmitry Valeev

It turned out that the maximum amount of alunite - an intermediate product of synthesis - can be obtained within eight hours when heating the solution to 90 ° C. In this case, the deposition efficiency is 88%.

- The main innovation of the proposed method was the possibility of precipitation of aluminum in the form of alunite without the use of additional reagents, such as gaseous ammonia. The use of bemite (mineral) instead of traditional gibbsite as a seed allowed to reduce the temperature of alumina calcination by 300°С. Thus, energy costs are reduced by 30%, " Dmitry Valeev, the project manager, candidate of technical sciences, leading researcher of the laboratory of sorption methods of GEOKhI RAS, told Izvestia.

Where alumina is used

Chemical analysis showed that the particle size and purity of alumina produced by the proposed method meet the requirements of state standards of Russia, China and India. Therefore, the resulting product can be used by domestic and foreign plants.

- In the future, we plan to optimize the duration of the precipitation process and obtain pure bemite powder without sulphur impurities in order to eliminate the stage of additional purification," said Dmitry Valeev.

Wagons with coal on the territory of Reftinskaya GRES

Photo: TASS/Donat Sorokin

Vladimir Pinaev, associate professor atthe Patrice Lumumba Institute of Ecology at the Patrice Lumumba University of Russia, and member of the Public Council under the Ministry of Natural Resources, told Izvestia.

- Alumina is widely used not only for aluminum production, but also for other purposes. It should be noted that alumina production is a rather complicated and energy-consuming process. Its production in laboratory conditions is one thing, so to speak, to prove the efficiency of reactions, but the production of industrial batches on a permanent basis is another process. Therefore, industrial testing of the method is necessary," the expert emphasized.

Pipe belt conveyor leading to the ash dump at Reftinskaya GRES

Photo: TASS/Donat Sorokin

Irina Golovacheva, Deputy Director of the Institute of Ecology of PFUR, also emphasized the importance of reducing coal combustion waste.

- It is not only favorable for the environment, but also stimulated by the Russian legislation. Reducing waste and getting a new product out of it is a step towards a closed-cycle economy," said the specialist.

The results of the study, supported by a grant from the Russian Science Foundation (RNF), were published in the Journal of Cleaner Production.