Study On The Scale Formation Mechanism And Inhibiting Methods In The Vacuum Filtration System In The Production Of Wet-process Phosphoric Acid

In the wet-process phosphoric acid production from the Hubei phosphate ore, one of the bothersome problems is that potassium fluosilicate and sodium fluosilicate scales are liable to form on the filtration system owing to its high potassium and sodium content, which greatly restrains the application of Hubei phosphate ore. Some factories have thought out many methods to solve this problem through changing the process, equipments, pipe material and so on,but they didn' t work very well. Therefore, it is very important to study the scale formation mechanism systematically in order to solve the scale problem.Firstly, we studied the crystallization mechanism of the scales formation by means of simulating the cooling process of the vacuum filtration in the wet-process phosphoric acid production in laboratory, the experiment results show that: the main components of scales are sodium fluosilicate and potassium fluosilicate, the former is segregated out earlier than the latter, and has an inducing effect on the latter, the third main component of the scales is gypsum which is the lastest to precipitate; we made scale-segregating experiment on the surface of the three common pipe materials PVC, PTFE and Mo2Ti, then found that the scales is most easily crystallized on Mo2Ti and not easily on PTFE.Secondly, the interface energy principle and the crystallization principle were integrated and applied to this system, the interfaceparameters between the above three scales and pipe materials were got by means of Van Oss et al's interface energy principle (that is LWAB principle), according to the scale formation mechanism integrated with crystallization principle, it is found that the mechanism is accordant with the simulation experiment results satisfactorily. All these show that the used method is feasible.We have found several additives to change surface characters in the system and try to inhibit the scale crystal on the pipe. Their effects on the interfacial energy and metastable zone width of the three scales in phosphoric acid were studied, the results show that the additives increase the interfacial energy , therefore decrease the rate of nucleation and make crystal process harder, at the same time the additives increase metastable zone width , consequently makes the critical super saturation larger and the solution system more stsLle, so the scales is more difficult to crystal. The experiments' results also verified these analyses. Based on the above two methods, we can choose appropriate additives in industry.