THE MANUFACTURE OF PHOSPHORIC ACID. 153 



pressure until the liquid which flows away only tests 0*25° B., a 

 density equal to that of gypsum -saturated water. The wash water 

 then contains on an average 3 per cent P^O:, ; it is used, as already 

 seen, to reduce the sulphuric acid to 16" B. The phosphoric acid 

 collected in a "weak solution " tank tests about 12" B., say about 

 1° B. less than the sulphuric acid used ; there is reason to conclude 

 that a part of the latter has combined with the lime present in the 

 phosphate as CaCOgCaFU, CaSiO^. A weak phosphoric acid 

 solution of average quality has the following composition : — 



Per cent. 



PA ^*^ ■ 



so, 0-2 



CaO 0-4 



(FeAl)oO, 0'3 



and small quantities of HF, Si, MgO, etc. For most applications, 

 especi:dly for the manufacture of double superphosphates, the 

 phosphoric acid must be concentrated to a certain extent. It 

 w^as at first attempted to concentrate it by evaporation by lead 

 steam coils, but the pipes became rapidly incrusted with lead salts 

 and lost their conductivity. This process was therefore abandoned, 

 and attempts made to concentrate the acid by bottom heat. Great 

 flame ovens were constructed heated by combustion gases. These 

 gases are led underneath the liquid contained in a pan sur- 

 mounted by an arch. The pan is of strong wa^ought-iron, lined 

 inside with stones which resist acid, and thus was protected from 

 direct contact with the flame and the x^hosphoric acid. On the other 

 hand, the exterior side of the pan is naked. The dilute solution of 

 phosphoric acid runs in continuously in the back part of the pan, 

 until after two or three days an average concentration of 50° B. is 

 obtained. The concentration is then finished and the phosphoric 

 acid, making 56' to 58° B., is finally run out by a central gutter, 

 whilst the contents of the pan are continuously agitated so that no 

 mud remains. It is collected in wooden vats, or in lead-lined iron 

 ones. In this condition the phosphoric acid is turbid and blackish 

 owing to the presence of soot deposited by the combustion gases. It 

 contains about 54 per cent P.p.,, I'S per cent of FeAip^, and 

 variable amounts of gypsum, phosphate of lime, calcium fluoride, 

 hydro-fluosilicic acid, arsenic, etc. 



Up to 1890, the factory of H. & E. Albrecht treated about 60 tons 

 of phosphate in twenty-four hours. Five large evaporation ovens, 

 analogous to those described, were installed, which evaporated the 

 enormous amount of 200 tons of water in twenty-four hours. The 

 largest of these evaporation pans was 16-5 metres (say 54 feet) 

 long, and 5-5 metres (say 18 feet) wide, but it was divided into two 

 compartments by a longitudinal diaphragm. It had a capacity of 

 36 cubic metres, and yielded at one operation 55-8 tons of 54 per 



