Effect Of Coagulant On Silica Removal And Analysis Of Reaction Mechanism In Heavy-oil Wastewater

It was the complicated composition of heavy oil waste-water and stringent standard for reuse as boiler feed water that determined the high challenge in waste-water treatment,particularly the problem of high content of silica. During the process of thermal recovery,super-heated steam and condensed fluid could react with stratigraphic rocks, thus releasing the contained silica into the produced fluid and finally they were brought into the waste-water.The silica concentration of treated water should be controlled below 50 mg/L based on Chines standard before reused in boilers to prevent the formation of silica scale for a long running operation. Chemical coagulation methods are the major techniques for silica removal. Many kind of metal oxide and metal hydroxides are efficient at silica removal, however the specific removal mechanism was still unclear, because of the diverse species of silica in water, besides,the formed hydrolysates after dosing were also quite complex.The paper introduced the source of silica in heavy oil waste-water, coagulation techniques for silica removal and relevant progress of theoretical research. The heavy oil waste-water in Liaohe Oil Field and stimulated water were used to analysis various silica species,with the help of previous literature, and the silica were divided into monomer silica(Sia), oligomer silica(Sib), polymer silica(Sic) and insoluble silica. The effect of temperature,pH and salts were investigated to explore the variation and conversion of silica speciation.Coagulants, involving calcium, magnesium and aluminum, were studied, and the effect of their dosage and composition on the removal efficiency of different silica species were experimented. The probably active ingredients were discussed and the main mechanism for silica removal during coagulation were summarized.The results showed that higher temperature promoted the change from Sic to Sia, and lead to a higher content of Sia. Under the condition of acidity, the formation of Sia was inhibited,while in the basicity that was promoted. The present of Mg2+, Al3+ and Fe3+ could significantly influence the soluble silica content, and remove them mainly by sedimentation, of which Al3+had the best effect, while Ca2+ was proved to inhibit the polymerization of Sia, besides, the anions such as Cl-, HCO3-, NO3- and SO42- had little influence in the process of equilibrium and conversion. For the studied waste-water which was rich in the content of HCO3-,coagulants of magnesium promoted the silica removal than the effevt of coagulants with calcium. As to coagulants of magnesium, MgCl2 added with alkali showed a best effectiveness on silica removal, and the removal efficiency would be 91.89% when the pH reached 12 withthe dosage of 800 mg/L, leaving a residual silica content about 38.4 mg/L, while the effect of granular MgO was worse and that of granular Mg(OH)2 was the worst, besides, a smaller particle size was.favorable to a better removal efficiency. However, the directed removed silica was mainly Sia by analyzed the variation of removal process, which was probably via surface adsorption of formed hydrated shell on added coagulants of calcium and magnesium.When using Al2(SO4)3 at 6 mmol/L, the silica concentration decreased to less than 50 mg/L,and the dosage just was 4 mmol/L to reach the similar effect when using commercial polyaluminum chloride(PAC). pH had an influence on the aluminum speciation of hydrolysate, and the effect of Al2(SO4)3 was optimal when pH was 5, and the result showed that the removal efficiency of Sic was better than Sia. In contrast, freshly prepared Al(OH)3had a better effect on Sia rather than Sic.Three PAC samples of specific aluminum speciation synthesized in the laboratory, that were PAC10, PAC22, and PAC25, were used. Results showed that PAC10 with higher nomonuclear aluminum(Ala) content was more efficient at Sic removal, and PAC25 with higher colloidal aluminum(Alc) content was more efficient at Sia removal, which meant Ala and Alc promoted the removal of Sic and Sia, respectively. An absorption-modification method could be applied to explain the interaction between silica and aluminum, and it was the formed aluminosilicate sites or precipitate films on the surface of polymer silica and polymer aluminum that decreased the solubility of original substance, thus leading them more easily to be removed during the coagulation of waste-water.