Problems of phosphoric acid manufacture

In one process used for the manufacture of phosphoric aicd, ground phosphate rock containing calcium phosphate is reacted with dilute sulphuric acid. A chemical reaction takes place and phosphoric acid and gypsum (calcium sulphate) are formed. This process is carried out by ICI Fertilizers in their phosphoric acid plant at Billingham in Cleveland. The phosphoric acid is made in a large reactor which has seven large agitators to move the solids from stage to stage and prevent them settling. Traditional materials of construction for use in this process have included alloy 20 type stainless steels. When this material was used for the manufacture of agitators it was found that failure was occurring too frequently, often as a result of weld failure at the top flange and where the blades join the shaft.