Superphosphate In The Making

Press, Volume XCVIII, Issue 29021, 9 October 1959, Page 12 (Supplement)

Superphosphate In The Making

SULPHUR from Texas, Florida and Mexico, phosphate rock from Nauru, Ocean and Christmas Islands and nitrate of soda from Chile come by sea to the port of Lytfelton as the raw materials foe the manufacture of superphosphate at the Hornby works of Kempthorne, Prosser, Ltd.

The artificial fertiliser industry takes a foremost place among those which serve the New Zealand farmer. Hundreds of thousands of tons of its products are used each year on the land.

Phosphate rock as it is mined in the Pacific Islands is relatively insoluble but by treating the pulverised rock with strong .sulphuric acid made from the sulphur a soluble compound is formed which readily provides the growing plants with phosphate, calcium and sulphur.

The greater part of a fertiliser works is taken up with the making of sulphuric acid. Burners convert the powdered sulphur into oxide gases which are absorbed in water to form the acid und&r rigidly controlled conditions. Oxides of nitrogen formed from the nitrate of soda aid in the conversion of the sulphur oxides.

Rigid Control—

Huge lead chambers nearly the size of a two-storey house where the gases are absorbed by water dwarf a small room which is the nerve centre of the process. A system of continuously recording pyrometers and thermometers mark on graphs the various key stages watched over by the control staff. The acid runs down to lead tanks at ground level to be pumped to the superphosphate converter.

From the stockpiles of rock bulldozers push their loads into the hoppers of the grinding and crushing plant. Pneumatic conveyers lift the fine powder to the metering plant at the top of the building.

High among the rafters, on catwalks and steel platforms, the operators watch the raw materials automatically measured as they pass into the long reaction chamber.

The rock powder is fed through a unit which weighs the load carried on a moving belt and red and green lights flash if the set amount is exceeded or reduced. The acid pours through a calibrated orifice from a supply tank.

The reaction chamber consists of a long box-like structure with a moving conveyer floor. At the mixing end the acid and phosphate boil together in a sloppy slurry which gradually thickens in consistency as the reaction takes place. The conveyer moves the solidifying mass along to the cutters.

By this stage the superphosphate is a crumbly cake about five feet thick which steams with a hot vitriolic vapour. The cutters slice through the cake and it falls

to the elevators which carry it to the maturing area.

Different grades of fertiliser are • stored in a huge building in piles the size of small hills. As the superphosphate ■ matures the acidity is reduced and the conversion is completed.

Bulldozers and front-end loaders tear at the bottoms of the piles and replenish the hoppers of the screening and packing plant. Bulk deliveries are made direct from the screens along a belt conveyer to the railway truck or motorlorry standing outside on a weighbridge. The weighbridge operator controls the conveyer and as the load is completed a docket is automatically stamped by the weighing mechanism.

For bagged superphosphate there is a unit which prints the sacks with the brand and description before they go to the automatic bagging and sewing plant.

Special Mixes—

Special mixes incorporate serpentine rock from Nelson and Marlborough, lime from North Canterbury quarries, and trace elements, added at different stages of manufacture. D.D.T. superphosphate is mixed in a gigantic mixer to ensure perfect blending.

In 1958 the Hornby works used 50,000 tons of rock phosphate and 10,000 tons of sulphur to make 84,000 tons of superphosphate.

About half of this was used as “straight super” and the balance was processed further with additives such as lime, serpentine rock, D.D.T. and trace elements to make the host of special mixes used by the farmer.