Study On Secondary Reaction During Clinker-leaching And The Preparation Technology Of Pregnant Liquor With High Al₂O₃ Concentration In Alumina Production Process

Alumina is one of the most important raw materials in the world economics. With the development of national economy, the alumina industry of China has developed with high speed since 2000's in the 21^th century. The total output of alumina annually was nearly up to 20 megaton in year 2007. It is only inferior to the iron & steel yields in metallurgical industry. Due to the high content of silica in bauxite, sintering process and Bayer-Sintering combination process act special roles in our country's alumina production. Nowadays, some new intensifying technologies of alumina production were come forth, but the problems of high energy consumption and cost is still under solved, this greatly decreases the benefits of the alumina company by sintering process. An efficient means for reducing energy consumption is to increase Al₂O₃ concentration of wet method system, its pivotal technique involve the processes of clinker-leaching with high Al₂O₃ concentration and the separation of solid and liquid, however, the difficulties of these two processes are respectively the control of secondary reaction during clinker-leaching and the determination of suited flocculent. Therefore, it has great meanings to work over the secondary reaction mechanism, the technologies of clinker-leaching with high Al₂O₃ concentration and the sedimentation of leached slurry for increasing the technology economic index of alumina production by sintering process. In this case, the following aspects have been studied systematically in the dissertation:1. Thermodynamic calculation and analysis of clinker-leaching processThermodynamic computation and discussion of reactions involved in the clinker-leaching process are described in detail. The stability ofβ-2CaO·SiO₂ in three alkaline liquors and different types of calcium silicate in sodium aluminate liquor have been confirmed in thermodynamic terms, results are respectively as follows: NaOH>>NaAl(OH)₄>Na₂CO₃ and CaO·SiO₂>3CaO·2SiO₂>2CaO·SiO₂. Meanwhile, the results of investigation on the reactions between hydrogarnet with different saturation coefficient and NaOH or Na₂CO₃ show that Na₂CO₃, instead of NaOH, can easily decompose hydrogarnet and hydrogarnet in NaOH solution is more stable than in Na₂CO₃ solution. Finally, the hydration ofβ-2CaO·SiO₂ and reactions between calcium silicate hydrate and three alkaline liquors have all been analyzed. Its results indicate that hydration ofβ-2CaO·SiO₂ to afford 2CaO·SiO₂·1.17H₂O, 2CaO·SiO₂·0.5H₂O and 4CaO·3SiO₂·1.5H₂O is thermodynamically most favorable. NaOH contributes less dependence on the decomposition of calcium silicate hydrate, in contrast, Na₂CO₃ and NaAl(OH)₄ remarkably influence on the decomposition of calcium silicate hydrate. All these results lay a theoretic basis for the investigation of new clinker-leaching technology.2. The kinetics of secondary reaction during clinker-leachingThe kinetics of secondary reaction during clinker-leaching was researched in this part. Results demonstrate that secondary reaction process is divided into two sections according to the characteristics of dynamic process, namely, one is the process that SiO₂ dissolves into sodium aluminate solution and the other is the process about loss of Al₂O₃. After comprehensively treated, the two kinetic equations can be expressed as follows:It can be inferred from kinetic equation that reaction between calcium silicate and NaAl(OH)₄ is the main reason to result in SiO₂ entering into sodium aluminate solution and loss of Al₂O₃.3. Behaviors of calcium silicate in different alkaline solutionsIn this part:, the behaviors of calcium silicate in different alkaline solutions were systematically studied in order to further discover the mechanism of secondary reaction. The results illustrate that the stability sequence of calcium silicate in three alkaline solutions is NaOH>NaAl(OH)₄>Na₂CO₃, which is coincident with the thermodynamic results of this dissertation under the same leaching condition. Leaching in sodium aluminate solution, the prolongation of reacting time and elevation of reacting temperature will increase the decomposition of calcium silicate. The influence of Na₂CO₃ on the decomposition of calcium silicate in sodium aluminate solution is less than in Na₂CO₃ solution, which due to mutual action between NaAl(OH)₄ and Na₂CO₃. Attributing to decline of calcium silicate's reactive activity after decrease of CaO/SiO₂ of calcium silicate, 3CaO·2SiO₂ and CaO·SiO₂ in sodium aluminate solution are more stable than 2CaO·SiO₂.4. Technology of clinker-leaching with high Al₂O₃ concentrationBased on the mentions above, particular research on the technology of clinker-leaching with high Al₂O₃ concentration has been carried out. Experimental results display that SiO₂ concentration in leached slurry climbs down with the increase of liquor-solid ratio to a certain extent under assuring the Al₂O₃ concentration>180 g.L^-1, however, liquor-solid ratio exerts little influence on the leaching rate of Al₂O₃. Lower reacting temperature(about 75℃) and less reacting time(5-10 min) avail to reduce the extent of secondary reaction. Na₂Os, as well as Na₂O_c and free Na₂O_k concentration, have insignificant impact on the process of clinker-leaching with high Al₂O₃ concentration, but NaAl(OH)₄ has profound effect on the behaviors of SiO₂ and Al₂O₃ during clinker-leaching. Original liquor contains proper amount of SiO₂ is propitious to the elevation of leaching rate of Al₂O₃ and the reduction of leaching rate of SiO₂. Clinker-leaching with high Al₂O₃ concentration under suitable leaching conditions can make the extent of secondary reaction on the decline. It was also found that additive type A-l("-COOCa" as main functional group) can restrain secondary reaction from happening to some extent in this part.5. Technology of sedimentation of leached slurry with high concentrationIn the last part, the detailed investigation on sedimentation of slurry obtained after clinker leached with high Al₂O₃ concentration was completed. As seen from results, it was found that adding flocculent type A-15 can make rapid settlement of red mud come true under assuring the Al₂O₃ concentration>180 g.L^-1, its proper additive amount is 125-250g/t dry red mud. The sedimentation rate reduces and the compressibility of red mud deteriorates with the fall of clinker size. Na₂CO₃ concentration exerts remarkable influence on the separation process of solid and liquor, sedimentation performance of leached slurry becomes bad gradually with the elevation of Na₂CO₃ concentration, but Na₂O_c concentration in the range of 10-20g/L exposes inapparent impact on this process. During circulation leaching with the use of liquor which is used to extract clinker, the increase of circulation-leaching amount make against the settlement process, especially when clinker size changes into more fine. However, the deterioration of slurry's sedimentation performance can be greatly alleviated through combining flocculent type A-3 and A-15 according to the ratio by volume of 1:7- 1:9.