of only about 2 5 the tests also show that the example 2 nozzle is a preferred configuration unless otherwise indicated herein all percentages are by weight as used herein the term 34 substantially solids free 34 means that the filtrate contains less than about 1 solids typically less than 0 5 in the above examples a layer of the initially 4 inch thick filter cake is continuously removed by the doctor blade until the cake reaches about 1 2 34 in thickness at which point the filtration of crude acid is stopped and a new layer of diatomaceous earth is applied in these examples no provision was made for adjusting the angle of the spray as the filter cake thickness changed the angle chosen was such that a uniform spray without substantial overlap was obtained when the cake was about 2 34 thick therefore at greater thickness there was a slight area between sprays where there was no overlapping of sprays and at thickness less than about 2 34 there was a slight overlapping better results than in these examples can be obtained by appropriate continuous adjustment of the spray angle and or pattern in the above examples the crude concentrated phosphoric acid had been treated with a calcium and fluoride containing solid in accordance with the teachings of u s pat no 4 136 199 and 4 243 643 the entire disclosure of said patents being incorporated herein by reference the present invention is especially useful for filtered hemihydrate process acid especially after concentration by evaporation which has been treated in acordance with the teachings of said patents table______________________________________ ex 2 ex 3 ex 4 ex 5______________________________________feed wt mgo 57 55 57 65wt.