N.Z. FERTILISER PRODUCTION

Press, Volume LXXXIX, Issue 27094, 18 July 1953, Page 5

N.Z. FERTILISER PRODUCTION

Serpentine Phosphate

MANY ADVANTAGES CLAIMED

The recommendations made on the New Zealand fertiliser industry by Dr. G. L. Bridger, the American authority who was brought to New Zealand three years ago to report on the New Zealand fertiliser industry, included the establishment of works to make triple superphosphate and fused serpentine phosphate, as well as the equipment of existing works to burn pyrites as a source of sulphur. In a paper which won the Templin Prize this year, Mr D. R. Teplitzky, a senior chemical engineering student in the Engineering School at Canterbury College, developed the theme that discussion since Dr. Bridger’s visit had concentrated on pyrites, and that the other two recommendations, which had a great deal of merit, had been largely forgotten. In recent months much has been written on the manufacture of phosphorus fertiliser in this country, with particular emphasis on superphosphate, Mr Teplitzky said. This emphasis is natural, because New'Zealand manufactures straight superphosphate only, or derivatives of it. In these discussions, the name of Dr. Bridger, of lowa State College, has figured frequently for his proposal to convert, and expand, the present industry to one using pyrites instead of sulphur, which is now in short supply. It is regrettable that Dr. Bridger’s other proposals, particularly those for fused serpentine, or olivine, phosphate, and for triple superphosphate, have been overlooked, because they have more merit than the pyrites proposal. At present all the raw materials for the phosphate industry are imported. The rock phosphate imported from Nauru and Ocean Islands has an average content of 37.2 per cent, of insoluble P 205, The only significant phosphatic deposits in New Zealand are the Clarendon sands, present in vast quantities, but averaging only 3 per cent, of P 205. Methods of concentrating this are at present being investigated in New Zealand, and indications are that a concentration to about 25 per cent P 205 will be possible. Ordinary superphosphate made in a modern plant by reacting rock phosphate with sulphuric acid has an average P 205 content of 16 to 18 per cent., and Dr. Bridger estimated that a plant designed to produce 100.000 tons a year could deliver P 205 at £57 17s Id a ton at present costs of imported rock and sulphur. If after benefication the Clarendon sands contain only 25 per cent. P 205, it is doubtful if they could compete with Nauru rock, because the sands would give only 10 to 12 per cent, available P 205, would use increased amounts of imported sulphur during manufacture, and the cost of the available P 205 would be much higher. Aerial Topdressing Superphosphate is in very fine particles, and its use for aerial topdressing is restricted to windless days. It has the disadvantage of caking in damp weather, and so is difficult to store for any time. Its main advantage is that the consumer knows it well and is familiar with its characteristics, and how to apply it best. The Department of Agriculture could easily impart this advantage to other types of fertiliser by issuing information based on experiments it is well equipped to carry out.

If existing plant was converted to use pyrites instead of sulphur the product would still be straight superphosphate of 16 to 18 per cent, available P 205, and of the same physical* properties. The cost of production would rise to £53 12s Id a ton of available P 205 at the works against £5l 7s Id for superphosphate made with sulphur. Agronomicaly and economically the conversion has no advantages. Pyrites has to be imported. Australia has large undeveloped deposits which might be made available so that the import problem would be slightly lessened. It should be noted clearly that straight superphosphate manufacture in New Zealand is entirely dependent on imported raw materials. The only by-product from superphosphate manufacture is .sulphuric acid regardless of whether it is made from sulphur or pyrites, and as this could be made quite independently of a superphosphate industry, it hardly be called a direct by-product. Triple superphosphate is made by electrothermal decomposition of rock phosphate to produce phosphorus which is converted into phosphoric acid. The acid is reacted with more, rock to produce a fertiliser which contains 45 to 50 per cent, available P 205, about two-and-a-half times the amount in straight superphosphate. The only raw material used in the process is rock phosphate, and though Nauru rock is excellent, the Clarendon sands might prove ideal. Thus triple super manufacture could possibly be developed in New Zealand to be entirely independent of imported raw materials.

Triple super is excellent for aerial topdressing. One of the most striking features of the process of manufacture ir that it produces as by-products phosphorus, a powerful chemical in modern industry, and hydrogen, an essential material for the manufacture of ammonia.

A plant to produce 100,000 tons of triple super, with no by-products, would cost £2,600,000 at present costs, and its product would cost £5l 16s 6d a ton of available P 205. The main disadvantages of triple super are that the phosphoric acid used in the process is highly corrosive and leads to many production plant difficulties; that power consumption is high; and that for application to some types of soil addition agents must be incorporated to give high solubility. Serpentine phosphate fertiliser was developed as recently as 1943 by Dr. Bridger and a team of chemical engineers for the Tennessee Valley Authority. It has many of the advantages of triple super, with none of its disadvantages. It contains about 24 per cent, available P 205. that is, 50 per cent, more than straight superphosphate. As it also contains soluble magnesium compounds, it would be specially adaptable to soils that respond to magnesium, such as there are in North Auckland, Taranaki, and Southland.

To produce 80.000 tons of serpentine phosphate a year, a plant costing only £700,000, a great reduction on the other methods, would give fertiliser at £56 3s 4d a ton of available P 205. The-raw materials are all available in New Zealand. There are two very large deposits of serpentine ore of high grade in New Zealand, and the Clarendon sands, concentrated to about 20 to 25 per cent. P 205. are likely to prove ideal for this electric furnace process. A silicate slag is produced during the process, and this is showing great promise as a raw material for- cement manufacture, and could be a valuable by-product. Serpentine phosphate needs no additives to improve its solubility. As neither sulphuric nor phosphoric acid are used in its manufacture, corrosion is not a problem. In addition to these advantages, it is easy to handle. It does not cake, and can be transported in open trucks and dumped to store outside. It requires less electric power in manufacture—lo,ooo kilowatts a ton, against 25.000 kilowatts for triple super, which is the capacity of Highbank. To summarise:—All four processes give a product at about the same price at the plant. Triple super has the advantage of low transport costs because of its concentration. Serpentine phosphate, with* 50 per cent, more available P 205 than straight superphosphate for the same weight, can be agglomerated into nodules, and is therefore ideal for aerial topdressing. The cost of a serpentine phosphate plant iS low relative to the cost of plants for the other three processes. Triple super and serpentine phosphate are independent of imported raw materials, and both give valuable byproducts. Both would give secure, fruitful industries.