Thermodynamics Study On Scheelite Decomposition By Alkalic Method

The industry has been seeking to ways to enhance the technology of scheelite decomposition using alkali method; increasing concentration in order to reduce reagent consumption is one of the ways. But there is a strange phenomenon in the industry:when using soda to leach sheelite, the concentration should not be too high and it is generally less than 200 g/L, Soviet scholars even has pointed out that once it is more than 230 g /L, it will lead to a sharp decline in the leaching rate. Using sodium phosphate or sodium fluoride to leach scheelite also exists in the industry, and the concentration is also in a low level. The existing theory can not provide a reasonable explanation of this phenomenon.Using pseudo-ternary diagram as an analysis means, the author has made a systematic and in-depth study about the process of alkali-by scheelite decomposition from the thermodynamics direction. The common law hidden behind different systems has been found, based on it, a unified theory which can describe the thermodynamics issue of leaching reaction has been established. The specific contents are as follows:(1) The pseudo-ternary diagrams of soda-by scheelite decomposition system under 75℃and 225℃have been drawn. Detailed analysis of diagrams has been done, the prediction has been proved and some conclusions have been obtained:in the process of soda-by scheelite decomposition, the reason why leaching agent concentration should not be too high is that:too high concentration will lead to "salting out effect", which making the leaching agent separate out from the solution in the crystallization, then causing the leaching reaction reverse, thereby reducing the leaching rate;(2) The experimental results of scheelite concentrate decomposition have been analyzed using the obtained pseudo-ternary phase diagram of soda system. It has been found that the diagram is reasonable to interpret the experimental results and it thereby proves the correctness and effectiveness of the diagram.(3) The thermodynamics information of the reverse reaction can also be obtained from the pseudo-ternary phase diagram of soda system, and the reverse reaction—decomposition of calcium carbonate using sodium tungstate has been studied experimentally. It has been found that:high concentration of sodium tungstate benefit calcium carbonate decomposition which is consistent with the information obtained. It further proves the correctness of pseudo-ternary phase diagram.(4) Following the study method of soda system, the pseudo-ternary diagrams of sodium phosphate-by scheelite decomposition system under 230℃have been drawn, by which, the thermodynamics analysis about leaching reaction has been done. It has been found that there exists a similar law in the sodium phosphate system:high sodium phosphate concentration will also lead to "salting out effect", which making the leaching agent separate out from the solution in the crystallization, then reducing the leaching rate. The experimental results of scheelite concentrate decomposition have been reasonably explained using the obtained pseudo-ternary phase diagram of sodium phosphate system and it thereby proves the correctness and effectiveness of the diagram.(5) The pseudo-ternary diagrams of sodium hydroxide-by scheelite decomposition system under 90℃have been drawn. Comparing different scheelite leaching systems by alkali method, it has been found that "salting out effect" is the common law hidden behind different systems. A unified theory which can describe the thermodynamics issue of leaching reaction in a wide concentration scope has been established. Using the obtained theory, the sodium fluoride-by scheelite decomposition system has been predicted, it is found that high sodium fluoride concentration will also be harmful to the leaching reaction, which is corresponding to the actual situation.(6) Using pseudo-ternary phase diagram, the behavior of phosphorus in the process of sheelite leaching by sodium hydroxide has been studied. It has been found that it is difficult to extract phosphorus from the ore using sodium hydroxide. In the crystallization process of leaching solution, the majority of phosphorus remains in the mother liquor. The remaining phosphorus will combine existing calcium to form calcium hydroxyapatite; thereby it will be discharged from the system in the form of slag. So it will do not accumulate in the solution.