Reactors for simultaneous dissolution and precipitation

There are some technical processes where one solid is dissolved by a chemical reaction, while a precipitate is formed by the same reaction. Well known examples are the conversion of ilmenite (iron II titanate) to titanium hydroxide in the sulfuric acid process, or the conversion of zinc oxide to jarosite (iron II zinc sulfate). Such reactions can be sununarized by 

When a process of this type has to be developed, it is important to select such conditions that complete conversion of the solid reactant B) is possible. In the first place, it is essential to ensure that the solid reaction product P) does not completely envelop the solid reactant B). Preferably conditions should be such that P does not even adhere to R, but forms separate particles. 

The dissolution process is mainly characterized by the time needed for complete conversion of the R-particles, which is of course a function of dissolution conditions. This can be found from bench scale studies. It is necessary to find the dissolution time as a function of the original particle size, and the conversion as a function of time in a batch process  

The calculations shown in section 11.2.1 can help find the residence times needed in one or several CSTR s to reach a desired conversion. Complete conversion of B may be desired when it would prove difficult to separate B-particles from P-particles. As was pointed out in section 11.2.1, complete conversion in a continuous process can only be obtained when the last part of the reactor system is a tubular reactor with a residence time of at least die dissolution time. If this is not practical, a process has to be designed for separation of unconverted 5-particles from the reaction product (P-particles). 

The agitation in the stirred reactors has to be aimed at complete suspension of the 5- and P-particles. 

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