CHALK INDUSTRY

Press, Volume LXXXIV, Issue 25554, 23 July 1948, Page 9

CHALK INDUSTRY

CHRISTCHURCH FIRM SUPPLIES N.Z. INCREASING DEMAND The use of chalk is not confined to the schoolroom. It is used extensively in the manufacture of putty, dyes, paints, glue, plasticene, window cleaners, insecticide, rubber and cement mixtures for spraying buildings. Although there is an increasingly heavy demand for chalk there is only one source of supply in New Zealand. At Chalk Hill, Oxford, is the only known deposit of pure chalk in the Dominion and a Christchurch firm which works the quarry there is the sole supplier of chalk in. New Zealand. Pure chalk is the non crystallised form of calcium carbonate. Fully crystallised calcium carbonate, or marble, is found in various parts of New -Zealand, and Oamaru stone is probably the most well-known example of the compound in its partly crystallised form. It is only from pure chalk that true whiting can be obtained. True whiting has very great ability to absorb and to adhere, and these qualities give it its particular value in various products. In its finished state it is used as a filler in the manufacture of many rubber articles such as gumboots and bicycle tubes. In insecticides such as derris dust it is used as a conveyor. School chalk is not pure chalk, but chalk with gypsum added, to make it set. Chalk Seam at Oxford The chalk seam at Oxford is about 80 feet thick and varies only infinitesimally in quality. If it is conserved for industrial purposes there is no likelihood of a shortage occurring. Cleanliness is one of the first essentials in the production of whiting. No foreign matter must become mixed with it. To eliminate this danger the chalk is quarried and put into bags, in its clean state on the spot. It is processed in Christchurch to its powdered form as whiting, and about eight tons a day are manufactured. The chalk in its lump form has a water content of 30 per cent. All the water must be removed by heat, and this is a slow process, as no greater heat than 420 degrees centigrade may be used without altering the chemical composition of the chalk.

When it arrives at the factory in Christchurch the wet chalk is first partly pulverised and rotary paddle fed into the rotary drier. This drier is unlike most of its type, in that it revolves inside an oven. An oil fuel burner combustion chamber reaches the whole length of the drier in a tunnel underneath it. This allows the heat to plav on the outer surface of the drier. The heat from the combustion chamber is then picked up by a large heat fan and is used again by passing it through the internal cylinder of the drier. In this way the maximum use of heat is made, and radiation is cut down to a minimum.

Dust-Free Factory The steam caused by the heat in the drier is sucked off as quickly as possible, .and any dust that comes off with the steam is "killed” by a fog making device consisting of a series of five spray jets. The steam and fine chalk particles are passed through this fog. The success of this device is evident in the almost complete absence of chalk dust in the factory. When the chalk is thoroughly dried it is elevated into a 100-ton hopper which stands over flour high-speed centrifugal force air separating pulverisers. These four machines use about 100 horsepower. The finished material, which is ground to a fineness of 40,000 particles to a cubic inch, is then bagged for shipping to all parts of New Zealand. Canvas bags are used whenever possible‘because of the fineness of the whiting, which would readilv pass through jute material. The' whiting industry, which began in Christchurch about 16 years ago in a small way. was built up during the war when many products containing chalk could no longer be imported into New Zealand, and had to be manufactured locally. As there is no immediate likelihood of these overseas manufactured products becoming available here the demand for chalk will have to be met in New Zealand, and the Christchurch firm is confident that it can fulfil all the requirements of the Dominion.