Study On Decomposition Process Of Zircon Sand With Mixed Alkali

Zircon sand is the most important source of zirconium minerals. Its chemical properties are very stable. At present, it is mainly sintering of zircon sand with sodium hydroxide to decompose it as temperature of700～750℃, mole ratio of alkali to ore of6:1in the industry. In this paper, mixed alkalis of NaOH-KOH and NaOH-CaO have been selected as the decomposition agent. The sintering process and water leaching process have been investigated.To simplify the process, adopting equimolar ratio of NaOH-KOH with a low eutectic melting point (185℃) for the NaOH-KOH mixed alkali system, and maintaining [N(NaOH)+2N(CaO)]:N(ZrSiO4)=6:1for the NaOH-CaO mixed alkali system. Through experiment research, conclusions have been obtained as follows:(1) The decomposition rate of zircon sand is seriously affected by mole ratio of alkali to ore, temperature for both the mixed alkali systems. For the NaOH-KOH mixed alkali system, the decomposition rate of zircon sand is about95%as mole ratio of total alkali to ore of5:1, alkali fusion temperature of600℃, alkali fusion time of90min, and average particle size of zircon sand of141.41μm. For the NaOH-CaO mixed alkali system, the decomposition rate of zircon sand can also reach about95%as mole ratio of sodium hydroxide to calcium oxide to zircon sand of4.5:0.75:1, alkali fusion temperature of800℃, alkali fusion time of1h, and average particle size of zircon sand of141.41μm.(2) Obvious agglomeration is found during the alkali fusion according to the SEM images of the frit of the NaOH-KOH mixed alkali system. The main phases of the frit of the NaOH-CaO mixed alkali system are Na2ZrO3, Na2CaSiO4, ZrSiO4and CaO. Intermittent stirring during the alkali fusion can reduce the agglomeration and segregation of alkalis, improve the frit adherence to the internal side wall of the reaction vessel, and do good for the decomposition reaction.(3) The removal rate of alkalis of the frit during water leaching is seriously affected by L/S (ratio of liquid to solid) for both the mixed alkali systems. Given both alkali removal and silicon removal, the NaOH-KOH mixed alkali system can achieve better results:the alkali removal rate for the first water leaching can reach above60%, and the silicon removal rate is about81%as L/S of8mL/g, temperature of90℃, time of30min, and stirring speed of200r/min. For the NaOH-CaO mixed alkali system, the alkali removal rate also can reach about64%as L/S of6mL/g, temperature of90℃, time of30min, and stirring speed of200r/min, but for the existence of CaO, the silicon removal rate is very low, about30%～40%, which is detrimental to the after treatment of the product.This paper also compares the two systems, and consider that if the soure of the mixed alkali can achieve good solutions, the NaOH-KOH system can be competitive.