282 MANUFACTURE OF NITRATE OB SODA. 



is on oneside, and is always open to tlie rails which lead to the 

 two last boiling tanks, so that in case of the winding-engine 

 over-hauling, the cars cannot he derailed. The boiling tanks are 

 heated by a 3 -inch steel spiral pipe six rows deep, 1 foot off the 

 sides, with steam from the lioilers at a pressure of 6.0 lbs. to the 

 square inch. The steam enters at the top of the spiral, and 

 leaves at the bottom of the tank by a return pipe, which leads 

 to the return valves of the boilers, thus forming a circuit. 



The boiling is effected by the well known Shanks' s lixiviat- 

 ing system, introduced ia nitrate manufacture by Mr. J. T. 

 Humberstone, causing a continual circulation of the lighter li- 

 quid to the other boiling tanks by following the denser and 

 heavier solution. As soon as the solution, which has now be- 

 come " caldo " stands at 110° by Twaddell's hydrometer, it is 

 allowed to settle for a short time and is then drawn off into the 

 first canal, from which it runs into the crystallizing tanks by 

 means of other canals. The caldo is run off at a temperature 

 of 240" Fahrenheit. 



The "ripios, " or refuse, in the boiling-tanks is then washed 

 by well water and the washings are run off iuto the washing 

 tank, taking in solution nearly all the nitrate of soda which may 

 remain in the refuse. The washings are pumped up by a centri- 

 fugal pump, and used over again in the next boiling tank. 

 When all the washings have run off, the doors at the bottom of 

 the boiling tank are opened, and the refuse falls into cars placed 

 beneath, and is drawn away and dumped on the refuse heap. 



After the nitrate solution has become cool, and the nitrate of 

 soda has crystallized in the tanks, the "agua vieja," or mother 

 liquor, is run off into a set of return canals, by which it flows 

 into the bottom mother-liquor well, which is a round tank 25 feet 

 in diameter and 12 feet deep, sunk level with the surface of the 

 ground. Erom here it is pumped up by two of Tangye's 4-inch 

 special pumps to the top mother-liquor tank, from whence it is 

 run into the boiling tanks to undergo a similar process. The 

 mother-liquor stands at 90° Twaddell. 



When the nitrate in the crystallizing taijks is fairly drained, 

 it is shovelled out on to the drying-floors, where it soon becomes 

 perfectly dry in the tropical sun, and is put in sacks, weighing 



