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Nambucca Pebble characteristics

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The Nambucca pebbles steadily change in overall makeup from the smallest to the largest gradings. The sands and fine pebbles (1.6mm and 3mm) are composed almost entirely of quartz. As the size of the pebbles increase the quantity of other rock types also increases, particularly the percentage of greywacke (dark grey, highly indurated fine sandstone and siltstone.) As a result the appearance of the material changes from a dominant single colour of yellow in the finer sizes through an increasingly speckled appearance in the middle ranges to predominantly grey above 50mm.

The pebbles are particularly well rounded with a tendency for the grey particles to become more disc shaped in the largest sizes. The pebbles rate 7 on the Mohs scale of hardness. They are competent and there are very few broken particles. Thousands of years of percolating rain water and moving ground water has dissolved the shell and other solubles.

Range of Sizes

Austone produces a standard eight sizes. 1.6mm; 3mm’ 6mm; 10mm; 14mm; 20mm; 50mm and plus 50mm. Each of these gradings actually passes through mesh of the nominated size and is retained on the next size below. In the case of the 1.6mm the pebble is retained on a mesh of 0.8mm. Special sizes can be produced.

How the pebbles are processed

The raw mixture of sand and pebbles is extracted from the ‘fossil’ sand dune and trucked a few kilometres to the washing and screening plants. Trommels – or barrel screens – are used where the lower section of the barrel rotates through separated bins of water. Inside the barrel there are a series of low retarder plates set to obstruct the smooth flow of material. These retarders cause the pebbles to ‘scrub’ on themselves as a cleaning procedure and also allow any undersized particles an additional means of escape into the appropriate bin. The screening equipment was designed and built in house in order to produce the consistent quality of sizing and cleanliness required.

Packaging

All sizings are available in bags of 20kg and 40kg, palletised and wrapped. Sizings of 10mm and upwards are available in either bulk bags- containing 1.2 tonnes – or loose in bulk.

Most material in sold in a ‘damp’ state, which means the pebbles have not been deliberately dried. Bagged ‘dry’ product is also available. It is fed through rotary gas fired dryers for those customers who require moisture free material – dust filter, resin bonded applications etc.

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