Although FeAsO4 can be precipitated at temperatures less then 473.15 with initial solution of As(V) and Fe(III), experimental testwork has showed that a stable crystalline ferric arsenate is not synthesized at ternperatures below 483.15 from liquors containing Fe(III) and As(III), through in-situ oxidation. Precipitations have been conducted at temperature range 483.15-553.15K. From these investigations, a crystalline form of ferric at-senate was possible only if the reaction was been carried out at temperatures above 483.15K. The XRD patterns of the compounds obtained revealed an unknown iron arsenic phase. The major- considerations of this paper concern: The mixture of ferric (III) sulfate and arsenic (III) oxides made up the initial solutions. The unexplored range of precipitation temperatures which are investigated. The study of in-situ oxidation of arsenic(III) at high temperatures with hydrogen peroxide. The use of solutions with concentrations similar to hydrometallurgical liquors. The effect of some impurities to the ferric arsenate as well as materials choices for the reactor This paper points out also some engineering considerations regarding reactor design.