GULF OF MEXICO 



157 



hydroxide. Filter cake containing 25 percent of 

 magnesium hydroxide by weight is obtained. 



The third step provides for neutralization of the 

 alkaline filter cake. The cake is mixed with 

 previously prepared magnesium chloride solution 

 and agitated to make a slurry which caa be 

 pumped. It is transferred to a neutralizing tank 

 where an automatically controlled stream of 

 hydrochloric acid is added to exactly neutralize 

 the magnesium hydroxide. Thus, a 15 percent 

 magnesium chloride solution is obtained. 



The fourth step consists of evaporation of the 

 magnesium chloride solution to eliminate water 

 and to reduce the solubility of salts picked up 

 from the sea water. Evaporation is accomplished 

 in either of two ways. In the earliest or direct 

 fired method the magnesium chloride solution is 

 sprayed into gas fired chambers. The more re- 

 cent submerged combustion method accomplishes 

 evaporation by burning a carburated mixture of 

 natural gas and air below the surface of a pool of 

 magnesium chloride solution. In either case, 

 direct contact with the hot products of combus- 

 tion concentrates the solution to 35 percent mag- 

 nesium chloride by weight. Direct heating is 

 necessary because of the scaling tendency of the 

 solution. 



In the fifth step the unwanted calcium is pre- 

 cipitated from the solution by the closely controlled 

 addition of magnesium sulfate. The treated 

 liquor is held for 24 hours in an agitated tank to 

 encourage crystal growth of the salt and gypsum. 



The adjusted evaporator product is then fil- 

 tered, first tlu"ough Moore filters identical to those 

 used earlier for the magnesium hydroxide filtra- 

 tion, and then through plate and frame presses 

 for a final polish. 



The seventh step of the process is evaporation 

 of the filtered 35 percent magnesium chloride solu- 

 tion to a concentration of  approximately 50 per- 

 cent. Open top, brick-lined steel boiling kettles 

 heated by alloy steam coils are used for this 

 purpose. 



In the next stage, the concentrated magnesium 

 chloride liquor at a temperature of 170° C. is 

 transformed into a solid suitable for feeding the 

 electrolytic cells. The hot liquid is sprayed on 6 

 to 10 times its weight of previously dried solid in 

 a horizontal rotary mixer, producing a white 

 granular material containing abt)ut 68 percent 



magnesium chloride. This is dried with hot re- 

 circulated air in a multi-shelf drier, similar in 

 design to a HerreghofT furnace, and becomes a 

 free-flowing granular cell feed of approximate 

 composition MgClj.l.SHzO. Part of the dry 

 granular material is returned to the rotary mixer 

 and part is conveyed to the magnesium cells. 



The ninth step is electrolysis of the cell feed 

 (Hunter 1944). The electrolytic cells used for 

 this operation are bathtub shaped steel pots of 

 approximately 2,500 gallons capacity, filled with 

 a fused salt mixture consisting of 25 percent 

 MgCU, 15 per(;ent CaClj, and 60 percent NaCl at 

 700° C. Graphite electrodes suspended in the 

 bath serve as anodes; the pots and their internal 

 baffles act as cathodes. Passage of a high am- 

 perage, direct current between the electrodes and 

 the pot decomposes the magnesium chloride of 

 the bath to elemental magnesium and chlorine 

 gas. Cell feed is added continuously to maintain 

 the proper bath composition and level. The hot 

 gaseous products are collected under a tightly 

 fitting refractory cell cover, cooled, and piped to 

 the hydrochloric acid plant. The molten mag- 

 nesium metal rises to the top of the bath where it 

 is trapped by inverted troughs and conveyed to 

 the storage wells in the front of each cell. 



The metal is hand dipped from the cells three 

 times daily and cast into the familiar 18-pound 

 notched ingots. Each cell, operating at 60,000 

 amperes, produces approximately 1,200 pounds 

 of magnesium per day having a purity in excess 

 of 99.8 percent. No other refinement is necessary 

 to meet the specifications for commercially pure 

 magnesium. 



The final step of the process consists of con- 

 verting the chlorine from the cells to hydrogen 

 chloride by high temperature reaction with steam 

 and natural gas in a regenerative furnace. A 

 small amount of unreacted clilorine is reduced by 

 the controlled addition of SO2 supplied by con- 

 ventional sulphur burners. The hydrogen chlo- 

 ride and the small amount of H2S04 are absorbed 

 in water, and the resulting acid solution is re- 

 cycled to the neutralizers for the reaction with 

 Mg(0H)2 previously mentioned. 



Since process losses are inevitable, it is necessary 

 to replenish the recycled hydrochloric acid to the 

 extent of about one-half pound per pound of mag- 

 nesium produced. This may be added as chlorine 



