GB817739A - Process of concentrating nitric acid and apparatus therefor - Google Patents

Abstract

<PICT:0817739/III/1> In a cyclic process for continuously concentrating nitric acid by means of a dehydrating inorganic nitrate, nitric acid at a concentration of e.g. 50-70 per cent is passed into the free space of rectifying column 13 from pipe 11 where it is admixed with an aqueous solution of a nitrate, e.g. lithium, magnesium, calcium, strontium or manganese nitrate, at a concentration of e.g. 65-75 per cent and a temperature of e.g. 135-145 DEG C. which is recycled via a return pipe 12. The ratio of solution of nitrate to that of nitric acid may be 5-8 to 1. The upper part of column 13 may be packed with e.g. Raschig rings or may contain bubble cap plates. Solution from the bottom of rectifying column 13 passes via pipe 19 to a stripping column 20 having a height equivalent to at least eight theoretically perfect distillation plates, a temperature of e.g. 80-90 DEG C. being maintained at the top of the column and a temperature of e.g. 125-150 DEG C. at the bottom of the column. The stripping column may be packed with Raschig rings or Berl saddles or may contain bubble cap plates or slotted plates, and may be in two or more sections. A dilute solution of nitrate substantially free from nitric acid is passed from the bottom of column 20 via pipe 22 to a boiler 23, heat being supplied by circulating the solution through heat exchanger 24, a temperature of e.g. 175-185 DEG C. being employed at atmospheric pressure. A lower temperature may be employed at lower pressures, e.g. 135-145 DEG C. at 24-26 inch of mercury below atmos. pressure. Concentrated nitrate solution is returned via pipe 29, cooler 31 (if necessary) and pipe 12 to rectifying column 13. Part of the steam produced in boiler 23 is introduced into the stripping column 20 via pipe 25 while the remainder, which passes through pipe 26, passes to waste. The proportion of steam returned to stripping column 20 is determined by the temperatures desired in the column. Acid vapour formed in column 20 is passed through line 21 to the free space of column 13 and thence to the rectifying section of the column together with vapour evolved from the liquid mixture introduced into the lower part of the column. Vapour from column 13 is passed via pipe 14 to a condenser 15 maintained at a reduced pressure of e.g. 20 inch of water, and part of the condensate formed is withdrawn through pipe 17 while the remainder is returned to column 13 as reflux via pipe 18. The ratio of reflux to product may be 1.5-3 to 1. Parts of the apparatus coming into contact with nitric acid are preferably of acid-resisting silicon iron alloy. In a modification (not shown) only sufficient steam is evaporated from the solution in vessel 23 for recycling to column 20, the solution being further concentrated in an additional evaporator at a reduced pressure of e.g. 24-26 inch of mercury below atmos. pressure.