THE PROBLEM OF LIMITED RECOVERIES IN THE PIDGEON PROCESS FOR MAGNESIUM PRODUCTION

Hypotheses are proposed concerning the failure of the reaction 2MgO(s) + 2CaO(s) + Si(s) --> 2Mg(g) + Ca2SiO4(s) to go to completion in the industrial retort. One is a "quasi-equilibrium" established towards the later stages of the reaction as a consequence of the mutual solid solubility in the MgO/CaO/Ca2SiO4 system. The other is the effect of temperature gradients which exist within industrial retorts. These cause the transfer of magnesium from the hotter to the colder portions of the charge. The local MgO excess so created fails to react. The local transfer of magnesium is via the reaction of the product vapour (Mg) with calcium silicate (formed by the primary reaction between lime and silicon) to form MgO. Experimental evidence is presented to substantiate both of the above hypotheses.