Method of manufacturing NP- or NPK-containing fertilizers from magnesium containing phosphate

In a method of manufacturing NP- or NPK-containing fertilizer from phosphate ore having a magnesium content of more than 0.3 percent by weight, which comprises subjecting said ore to digestion with nitric acid, removing calcium, as calcium nitrate-tetrahydrate, from the digestion mixture by means of crystallization and filtration until the filtrate resulting from said filtration has a Ca/P-ratio of about 0.35, subjecting said filtrate to neutralization with ammonia, directly subjecting the neutralized filtrate to evaporation, and prilling or granulating the product resulting from said evaporation, the improvement wherein the method is carried out without causing viscosity problems related to the high magnesium content in the ore, by (1) reducing the Ca/P-ratio in said filtrate resulting from said filtration from about 0.35 to 0.01-0.15 by adding sulphuric acid or a sulphate to said filtrate to precipitate calcium in said filtrate, as calcium sulphate, and removing said calcium sulphate from the resultant mixture, and (2) subjecting said filtrate having a Ca/P-ratio of 0.01-0.15 to said neutralization with ammonia until said filtrate has an N/P.sub.2 O.sub.5 - ratio of 0.5-2.