Patent ID: 8435467

Claim:
A process for preparing ammonium metatungstate comprising the steps of: A. leaching of wet ammonium paratungstate by slowly adding wet ammonium paratungstate to stirred pure water while slowly adding a 1:1 to 1:4 nitric acid (HNO 3 ) solution to control the solution at pH 3 to 5 for the occurrence of the following reaction: 5(NH 4 ) 2 O.12WO 3 .5H 2 O+4HNO 3 ═(NH 4 )6H 2 W 12 O 40 .6H 2 O+4NH 4 NO 3 when WO 3 content of the solution reaches 130 to 160 g/L, the solution is stirred and boiled for 20 minutes or more, and then filtered to obtain a dilute solution of ammonium metatungstate; B. converting the dilute solution of ammonium metatungstate to a concentrated solution of ammonium metatungstate by standing and aging the dilute solution of ammonium metatungstate for 2 to 6 hours, filtering the aged dilute solution of ammonium metatungstate to obtain a first filtrate, concentrating the first filtrate by heating, in a reactor, with ammonia water having a concentration in the range of 1:4 to 1:6 to adjust the first filtrate to a pH in the range of 3-4, repeating the heating step until the first filtrate reaches a density in the range of 1.8-2.2, and thereafter cooling the first filtrate for 1-4 hours, filtering the first filtrate to obtain a second filtrate, standing and aging the second filtrate, in a settling tank, for 5-12 hours, and filtering, via a suction filter, the second filtrate to obtain a concentrated solution of ammonium metatungstate; and C. spray-drying of the concentrated solution of ammonium metatungstate into a powdery ammonium metatungstate with a high-speed centrifugal spray-dryer or a spray atomizing dryer.