44 



MARINE PRODUCTS OF COMMERCE 



separating the magnesium hydroxide. The filter cake, which is a mixture of sodium 

 chloride and calcium sulfate, is discarded. 



Next the filtrate, containing 35 per cent magnesium chloride, is evaporated to 

 50 per cent concentration. This is accomphshed in open brick-lined steel kettles 

 and finally in shelf driers provided with raker arms which slowly agitate the 

 product with circulating hot air, first at a temperature of 482° to 572° F (250° 

 to 300° C) and then at 842° F (450° C). Finally, a free-flowing white granular 

 solid, of the approximate composition MgClo.lJ2H20, is obtained. 



(^Courtesy Dow Chemical Co.) 



Fig. 4-5. Precipitated magnesium hydrate is filtered out by means of these Moore 

 filters in Dow's process for extracting magnesium from sea water. When the thick layer 

 of hydrate has built up on the filter leaves, unit is moved by 100-ton crane to an ad- 

 joining compartment where the cake is removed by air-blast. 



The magnesium chloride then passes to the electrolytic cells, each of which is 

 full of a fused salt mixture composed of approximately 25 per cent magnesium 

 chloride, 15 per cent calcium chloride, and 60 per cent sodium chloride at a 

 temperature maintained between 1292° to 1382° F (700° and 750° C) by con- 

 trolled gas firing. Magnesium chloride is continuously fed into the cells. The feed 

 is melted by the heat of the bath and the residual water is evaporated. Passage 

 of the 6-volt electric current between the graphite anode and the steel pot cathode 

 electrolyzes the magnesium chloride, yielding magnesium and chlorine. The molten 

 magnesium rises to the top of the cell where it is trapped by inverted troughs and 

 directed to metal storage wells at the front. The hot chlorine gas and some of the 

 hydrogen chloride produced by the reaction of chlorine with water are cooled and 

 piped to the hydrochloric acid plant where they are converted to hydrogen chloride 

 by reaction with steam and natural gas. As a small amount of chlorine does not 

 reduce to hydrogen chloride, a controlled amount of sulfur dioxide is added to 

 complete the conversion. The acid gases are absorbed in water in a scrubbing 



