42 



of the plates can be quite readily controlled. In the first part of 

 the process the mass loses water and turns darker until it becomes a 

 deep brown and evolves a peculiar odor. When the temperature 

 reaches about 180 C., the mass begins to lose color again and becomes 

 a greenish yellow. The temperature is then gradually raised to 

 240 C., at which point it is held until the mass no longer contains 

 particles of wood and is of a greenish white color, the total time 

 required being about six hours. The mass is then removed from the 

 plates and cooled or immediately dissolved in hot water and the 

 liquid concentrated to about 38 B., when it is run into small crystal- 

 lizing pans in which, on rapid cooling, nearly all of the sodium oxalate 

 separates, leaving potassium carbonate, caustic soda, caustic potash, 

 humus compounds, and a little oxalate in solution. The sodium 

 oxalate may be freed from the mother liquor by draining and washing 

 or by centrifuging. 



The mother liquors are evaporated to dryness, roasted in air, and 

 causticized as before for use again. The sodium oxalate is dissolved 

 in a very little boiling water, and sufficient thin milk of lime is run 

 in, with constant stirring, to change all of the sodium oxalate to cal- 

 cium oxalate and caustic soda. The mixture is run into settling 

 tanks, where the oxalate of lime settles out and the supernatant 

 caustic soda is run off and concentrated with the first wash water 

 from the oxalate, mixed with caustic potash, and used for treating 

 another lot of sawdust. After washing, the oxalate of lime is run 

 into a lead-lined tank, stirred to a paste with water, and treated 

 with dilute sulphuric acid of from 15 to 20 B., in such quantity 

 that the mixture contains two equivalents of sulphuric acid to one of 

 lime. The whole mixture is kept hot until a test shows no calcium 

 oxalate present, when the calcium sulphate is allowed to settle, the 

 clear solution containing the oxalic acid is drawn off, and the calcium 

 sulphate washed with water. The first washings are added to the 

 oxalic acid solution and the remainder is used to mix with the oxalate 

 of lime in a subsequent decomposition. The calcium sulphate, or 

 gypsum, thus produced may be disposed of as a fertilizer or, as it is 

 very pure, may be used as a filler in paper making, or dehydrated 

 and used as plaster of Paris. The solution of oxalic acid is concen- 

 trated in shallow lead pans until it is, in summer, 15 B., and in 

 winter 10 B. It is then cooled to ordinary temperature, when the 

 dissolved gypsum separates in crystals. The liquid is further concen- 

 trated to 30 B. and crystallized in shallow lead pans. These crystals 

 are washed in a minimum quantity of cold water to remove adhering 

 mother liquor, dissolved in boiling water, and cooled rapidly, when 

 small crystals form. The product is commercial oxalic acid, and 

 contains small quantities of sulphuric acid and oxalate of soda or 



[Cir. 36] 





