Use of Different Types of Phosphate Rock in Single and Triple Superphosphate Production

Keywords: Phosphate rock reactivity, Phosphate rock solubility, Superphosphate production.

Introduction:

In introducing the paper Mr. T. P. Dee said he thought it was fairest to indicate where the responsibility lay. Mr. Sharples had provided a script on the technical considerations involved, both general and in relation to different phosphate rocks. Mr. Nunn had been responsible for the more recent work in the laboratory. He, himself, had aimed to produce a coherent paper by writing an introductory section based on published articles and a little work of their own, followed by sections based on reports by Mr. Nunn and two of their earlier colleagues, and Mr. Sharples’ script. The draft had been read through by his co-authors and some modifications were agreed on. Of their earlier colleagues, Mr. Knight was mentioned in the paper and Mr. R. N. Woodward, now in New Zealand but still a member of the Society, was responsible for the first experiments.

Until recently, ideas on differences between phosphate rocks had lacked correlation. It was known that rocks from some deposits were a little slower or perhaps a little faster to react with sulphuric acid than that from Morocco, but there was no real basis for a quantitative comparison, nor was it known what were the main factors (assuming the same degree of fineness) which decided the reactivity of a given rock. The position had recently, however, been considerably clarified by the work of Mr. W. L. Hill and his colleagues in the Fertiliser Section of the Agricultural Research Service of the United States. Mr. Hill’s interest was in the use of phosphate rock as a fertiliser, but it was found that the correlations which he and his colleagues had established applied also to the behaviour of the rocks towards mineral acids.

The American workers had found the best correlation to exist between the reactivity of the rock and its solubility (at a given degree of fineness) in 2% citric acid. This was in line with a statement given on page 60 of a booklet published by the O.E.E.C. in 1953 (‘Sulphuric acid and the manufacture of phosphatic fertilisers’) to the effect that in France importance in regard to the agronomic value of a phosphate rock is attached to its solubility in citric acid. The highest values were obtained with South Tunisian and Curação phosphates; Algerian and Morocco phosphates followed closely; Florida phosphates were less soluble, and crystallised apatite phosphates, such as Kola, gave much lower values.

The information set out in the paper showed that in regard to the investigation of a new phosphate rock there now existed:

1. A useful tool, namely extraction with 2% citric acid, to determine its reactivity.

2. Criteria, namely CO₂ content and determination of certain physical properties, which gave further evidence as to reactivity and would probably explain the behaviour of the rock.

3. The possibility that even a very unreactive rock could be reacted successfully with sulphuric acid to make phosphoric acid.

It was pointed out in a paper by two of the authors in 1954 that the factors which determine the speed of interaction between phosphate rock and sulphuric acid to make superphosphate, and the composition of the product, are (i) the nature of the rock, (ii) the degree of fineness to which it is ground, (iii) the concentration of the sulphuric acid, (iv) the proportions in which acid and rock are mixed, which may be stated in lb. of H₂SO₄ per 100 lb. of phosphate rock, and (v) the temperature of reaction.

T P Dee BSc. MIChemE. FRIC., R J Nunn BSc. ARIC. and K Sharples ARIC., Fisons Ltd, UK.

52 Pages, 5 Figures, 13 Tables, 22 references.