Phosphorus slag extraction of silica and calcium hydrogen phosphate
|School||Kunming University of Science and Technology|
|Keywords||yellow phosphorus slag phosphoric acid leaching purification precipitated silica dicalcium phosphate dihydrate|
The yellow phosphorus slag is a solid waste of producing yellow phosphorus in electrical furnace,There are 8-10 t slag produced when 1t yellow phosphorus is produced in industry.According to the present status of producing yellow phosphorus in China,Chinese yellow phosphorus plants may discharge 6-7.5 million tons yellow phosphorus slag per year,in which there are 3-4 million tons yearly in Yunnan.At present,yellow phosphorus slag is mainly used in producing silicon-calcium fertilizer,cement,concrete,phosphorus slag brick and so on.But both the value of the product are lower and the utilizing rate of the slag are limited, the majority of them are accumulated and it cause resources waste and environment pollution.Yellow phosphorus slag contains a lot of CaO,SiO2 and a few of Fe,Al,Mg and other elements,it may be applied to producing Ca and Si product.The paper studies technology of producing the high quality precipitated silica and CaHPO4·2H2O,in which yellow phosphorus slag is leached by phosphoric acid, calcium and the other element impurities is separated in form of Ca（H2PO3）2, CaHPO4 and impurity phosphates,then purified and calcined.The new technology of processing yellow phosphorus slag is obtained.The silicon and calcium exist mainly in form of CaO·SiO2,3CaO·2SiO2 and 2CaO·SiO2 and so on in yellow phosphorus slag.The thermodynamics study presents that the react between these compounds and phosphoric acid can carries on spontaneously when yellow phosphorus slag is leached by phosphoric acid and form SiO2·nH2O,Ca（H2PO3）2 and CaHPO4 respectively,the impurity element such as Fe （exception phosphorus-iron）,Al,Mg and so on in the yellow phosphorus slag react also with phosphoric acid to form their phosphate simultaneously.According to solubility of these phosphate in water and phosphoric acid solution,the solubility of Ca（H2PO3）2 and CaHPO4 and the impurity element phosphate in the lixivium may are enhanced when the excessive phosphoric acid are used in the course of leaching and maintains enough amount of phosphoric acid in the lixivium and can guaranteed them dissolving in the lixivium,so the silicon is separated in form of SiO2·nH2O precipitation from the lixivium.The SiO2·nH2O and Ca（H2PO3）2·H2O or CaHPO4· 2H2O can be obtained simultaneously by separation silicon and calcium in yellow phosphorus slagThe experimental study indicated that the precipitated silica with SiO2 95%can be obtained after yellow phosphorus slag is leached by phosphoric acid solution and the calcium is separated in form of Ca（H2PO3）2 and CaHPO4,then leached dregs are washed,dried and calcined.The optimum technical conditions of leaching yellow phosphorus slag with phosphoric acid are:phosphoric acid 80ml,natural reaction temperature,reaction time 0.5 hours,fluid solid ratio 5:1 and stirring speed 400rpm. The leaching process of yellow phosphorus slag with phosphoric acid may be described by the unreacted shrink core model with solid resultant and the invariable particle size and the apparent rate of leaching process is controlled by diffusion process in liquid film.Its diffusion coefficient in liquid film is 1.68×10-4m2/s at 303.15KBecause C and the Fe in form of phosphorus-iron in the yellow phosphorus slag do not react with the phosphoric acid,they will remain in the leaching dregs when yellow phosphorus slag is leached by the phosphoric acid,and some of the impurity phosphate formed will also remain in the leaching dregs simultaneously,thus the leaching dregs contain many impurities.The appearance of precipitated silica sample obtained by leaching yellow phosphorus slag with the phosphoric acid is grayish white with slight red and white degree reach only 61.51%,so it must be treated further.According to the thermodynamic analysis,when the leaching dregs are purified by the nitric acid solution the impurity element C,Fe（exception FePO4）,Al,Ca,Mg and so on contained in the dregs can react with the nitric acid into the carbon dioxide and the corresponding soluble nitrate respectively and are separated from the leaching dregs.The experimental study indicats that when precipitated silica prepared from yellow phosphorus slag by the phosphoric acid leaching is purified by nitric acid solution,Fe content in the precipitated silica purified can decrease to about 0.02%,the removal rate of iron reach 99%and white degree is above 90%.The optimum purification technical conditions are:the nitric acid concentration 8%,reaction time 2.0 hours,reaction temperature 70℃,fluid solid ratio 4:1,stirring speed 300 rpm.The purification process may be described by the unreacted shrink core model with solid resultant（inert material）and the invariable particle size and the purification process rate is controlled by chemical reaction step on interface.Its apparent activation energy Ea is 30.662kJ/mol and its reaction order is 0.6746.After yellow phosphorus slag is leached by phosphoric acid solution and the calcium is separated in form of Ca（H2PO3）2 and CaHPO4,then leaching dregs are purified,washed,dried,the other item of the precipitated silica product obtained can all reach the quality requirement of the precipitated silica product,but its appearance is still grayish white and white degree reach only 67.83%.But the precipitated silica products reach A grade of the HG/T3061—1999 standard when the sample is calcined 0.5 hours at 700℃.The product is amorphous silicon dioxide, average grain diameter is 66～68μm,the silicon dioxide of particle size 2～200μm account for above 90%by X-ray diffraction and electron microscope analysis. The silicon dioxide content of the product is 97%,white degree 88%,specific surface area 205cm2/g,DBP absorbability value 2.01ml/g and the content of Fe,Cu, Mn can achieve the quality requirement of the precipitated silica product.The lixivium obtained from leaching yellow phosphorus slag by phosphoric acid contains a lot of Ca（H2PO3）2,CaHPO4,phosphoric acid and impurity element fluorine.In order to use phosphoric acid in the lixivium effectively and obtain qualified CaHPO4·2H2O product,the lixivium is treated by the vacuum evaporation,the concentration crystallizing,separation,circulation use of the mother liquor by returning to leaching stage.In order to preparation CaHPO4·2H2O product by using the calcium element fully,the mixture of the lixivium and first wash solution is concentrated by vacuum and then crystallized and separated,the mother liquor is used in leaching the yellow phosphorus slag by returning to leaching stage.When intermixture solution is concentrated to 50%of original liquid volume and is used in leaching the yellow phosphorus slag by returning to leaching stage,amount of using the industry phosphoric acid may be decreased 37.5% compared with the direct leaching,the industry phosphoric acid used is reduced obviously.The feed grade-CaHPO4·2H2O with F 0.08～0.10%can be obtained after the mixture of the lixivium and first wash solution is concentrated by vacuum and crystallized and separated,then the mixture crystal of Ca（HPO4）2·H2O and CaHPO4·2H2O obtained is dissolved again,neutralize.The content of Ca,P,F in the product can reach quality requirement of HG2636-2000 feed grade-calcium hydrogen pyrophosphate The paper researches a technology which yellow phosphorus slag is leached by phosphoric acid solution,then leaching dregs are purified,washed,dried,calcined and the mixture of the lixivium and first wash solution is used circularly and obtain precipitated silica and calcium hydrogen pyrophosphate simultaneously,so the paper has the innovation in the application of yellow phosphorus slag and the processing technology.The project is a technology which use the solid waste resources circularly,may use the waste resources fully,achieved the goal of the continuable development and solve waste treatment question of yellow phosphorus enterprise,so it will has the obvious social efficiency and the economic efficiency.