Research On Sodium Phosphate Roasting To Decompose Scheelite And Recycling Of Excess Sodium Phosphate

With the continuous depletion of wolframite recent years,scheelite has become the main raw material for tungsten metallurgy in China.China adopts the alkaline autoclave process as the main method for decomposing scheelite,which needs to be operated under a pressure of about 8-12 kg.However,it may not only lead to some security risks,but cause low productivity as it adopts a single-pot intermittent operation.For this reason,the research group has developed a series of methods to decompose tungsten ores,such as baking sodium hydroxide and calcining sodium silicate on the basis of overcoming the disadvantages of the conventional soda sintering method,by which the extraction rate of tungsten was stable at over 99%.Therefore,the tricky problem to completely decompose scheelite that cannot be solved by the roasting method gets fixed.Thermodynamically speking,sodium hydroxide and sodium silicate both can be used to decompose scheelite in the process of hydrometallurgy,and sodium phosphate also works in the same way.Thus,on the basis of the successful decomposition practice of scheelite by baking sodium hydroxide and sodium silicate,this paper presents an approach to decompose scheelite by roasting sodium phosphate,and systematically studies the related basic theories,new processes and the recycling of excess sodium phosphate.And it mainly includes:?1?Exploitation of scheelite by roasting sodium phosphate shows that sodium phosphate can completely decompose scheelite under certain calcination conditions.And this paper,combined with typical modern methods of XRD,SEM and EDS,proves that the calcined products are mainly sodium calcium phosphate and sodium tungstate,thus infers the main reaction of roasting process:Na₃PO₄+CaWO₄=NaCaPO₄++Na₂WO₄On this basis,this paper has studied the thermodynamics of the roasting reaction and draws the relationship diagram between Gibbs free energy and temperature in the reaction.The results show that the Gibbs free energy of the reaction decreases with the increase of298K to 1000K.?2?The effects of six required conditions were systemetically investigated,which are dosage of sodium phosphate,roasting temperature,roasting time,water immersion temperature,water immersion time and liquid-solid ratio on the leaching rate of tungsten,and the optimal process conditions were set,where the dosage of sodium phosphate is 2.0 times the theoretical amount,a high of 700°C in baking temperature,2h of baking,80°C in water immersion temperature,2.5h of flooding,and the liquid-solid ratio is 2.5:1.In this condition,the leaching rate of scheelite is as high as 99.78%.?3?In order to eliminate the competitive adsorption of phosphorus in the aqueous leaching solution with tungsten in the subsequent ion exchange process and ensure the adsorption capacity of the ion exchange resin,the excess sodium phosphate in the aqueous leaching solution was recovered by the same ion effect.And the study shows that under the conditions of 1h's crystallization,0 r/min's stirring speed,30°C in room temperature,and a concentration of sodium hydroxide of 4 mol/L,the phosphorus removal rate in the water immersion liquid was 97.18%,and the phosphorus removal solution was configured to be 20g/L pre-delivery solution?calculated by WO₃?,the phosphorus concentration can be reduced to 0.0475g/L<0.1g/L?according to industrial operation requirements?,and the precipitated crystals are mainly sodium octahydrate and sodium tungstate dihydrate.Meanwhile,under the conditions of 1h crystallization time and 0r/min stirring speed,the concentration of sodium hydroxide as a function of room temperature under different room temperature conditions was established as follow:y=0.009X²-0.205X+2.299Based on this functional relationship,it is certain that the concentration of sodium hydroxide under variable room temperature conditions at a range of 5-40°C can be figured out,so that the phosphorus concentration in the pre-completion solution after dephosphorization is less than 0.1 g/L.The results turn out to be ideal when the above-mentioned precipitated crystals were recycled to the calcining decomposition of scheelite,and finally recycling of surplus phosphorus in the aqueous extract was realized.