Preparation And Performance Of The Crystal Seed For Phosphate Removal By Crystallization Based On Hydrothermal Modification

Phosphorus (P) is one of the major nutrient elements which could cause water eutrophication, however, it is also an important nonrenewable nature resource which is faced with exhaustion. So, it is extremely urgent to develop efficient phosphorus removal technology and new phosphorus resource facing the shortage of phosphate rock resources all over the world. Phosphorus removal by crystallization is of great research value which has been recognized as an effective way to prevent the water eutrophication and realize the sustainable utilization of phosphate rock resources.This study was investigated to recovery phosphorus from artificial low concentration phosphate wastewater. The character of phosphate removal with quartz sand, ceramsite, steel slag and blast furnace slag were compared and the favorable materials were screened out which could be in favor of phosphate crystallization. Different active methods for screened materials which could promote the phosphate crystallization were discussed. And the basis materials of crystal seeds were prepared by powdery steel slag and blast furnace slag. Then the crystal seeds could be obtained by the hydrothermal modification of basis materials, which have the ability of adsorption and release alkalinity and calcium. A method based hydrothermal modification to prepare an efficient crystal seeds for phosphate removal was extracted. The performances of crystal seeds for phosphate removal by crystallization were evaluated. The reaction products of phosphate crystallization with crystal seeds were analysed by XRD and SEM-EDS, and then the mechanism of crystal seeds in phosphate removal were also discussed preliminarily.The results showed that the performances of phosphater removal by quartz sand, ceramsite, steel slag and blast furnace slag were different, and steel slag was much better than blast furnace slag and that of quartz sand and ceramsite was very little.The phosphate remove efficiency of low P concentration wastewater by crystallization could be improved by increasing initial Ca/P,pH,P concentration and reaction time. When the Ca/P>2.0, the efficiency of phosphate removal increased slowly with increasing Ca/P. When the initial pH>9 or P concentration was higher than 10mg/L, the efficiency of phosphate removal was highly improved. When the crystallization was processed for more than 24 hours, the efficiency of phosphate removal was almost steady even if the reaction time was prolonged continually.In this experimental condition, the quartz sand and ceramsite could not promote the phosphate crystallization. But steel slag could remove the phosphate effectively. The results showed that the material which could release alkalinity and calcium can promote the phosphate crystallization and also the favorable basis material for prepare efficient crystal seed.The results showed that performances of phosphate removal by steel slag and blast furnace slag could improve by hydrothermal modification. So, the steel slag and blast furnace slag were crushed and grinded into smaller grain size firstly, and the granular steel slag and blast furnace slag with grain size at 3mm - 5mm could be obtained through sieveing. Then the powdery steel slag and blast furnace slag could be obtained through futher grinding, and the hybrid seed media with steel slag and hybrid seed media with blast furnace slag could be prepared by pelleting, molding and curing. The granular steel slag, granular blast furnace salg, hybrid seed media with steel slag and hybrid seed media with blast furnace slag as the basis material would be modified by hydrothermal method.The ability of phosphate removal by four basis materials could be improved by hydrothermal method, which was related to modification temperature, modicicaiton time, modification NaOH concentration and the ratio of liquid to solid. The efficiencies of phosphate removal by granular steel slag, granular blast furnace slag, hybrid seed media with steel slag and hybrid seed media with blast furnace slag were increased with increasing modification temperature, modicicaiton time, modification NaOH concentration and the ratio of liquid to solid. The optimum modification temperature, modicicaiton time, modification NaOH concentration and the ratio of liquid to solid of granular steel slag, granular blast furnace slag and hybrid seed media with blast furnace slag were 200℃, 24h, 6M, 1 and that of hybrid seed media with steel slag were 105℃, 6h, 6M, 1.A method based hydrothermal modification to prepare an efficient crystal seeds for phosphate removal was extracted. The ability to remove phosphate of modified granular steel slag, granular blast furnace slag, hybrid seed media with steel slag and hybrid seed media with blast furnace slag were 6.88mg/cm³, 4.67mg/cm³, 5.63mg/ cm3 and 4.19mg/cm³, and the phosphate remove rates were increased by 60.8%, 45.2%, 54.8%, 39.9%, respectively.The character of phosphate removal by crystal seeds were bound up with the ability to release alkalinity and calcium. When the initial solution Ca/P=2 and pH=8.0, the alkalinity and calcium release rate of modified granular steel slag, granular blast furnace slag and hybrid seed media with blast furnace slag was very fast. However, the modified hybrid seed media with steel slag could be reused for longer times for phosphate removal.XRD and SEM-EDS patterns of the recovered products which generated from the phosphate removal by crystal seeds showed that the degree of calcium phosphate crystallization enhanced, the amount of crystal nucleus increased and the crystal was growing up with the increasing reaction time. Then the calcium phosphate precipitation turn into global hydroxyapatite crystal with poor crystallization, the Ca/P of which was 1.66, which was very close to purified HAP crystal. The hybrid seed media with steel slag prepared based on hydrothermal modified had amounts of C-S-H and Ca(OH)₂, which could provide alkalinity and calcium to promote the phosphate crystallized as the form of calcium phosphate, and the hydroxyapatite could be obtained by a futher convertion.