Investigations into blast affected marble at the Penrice quarry

The Penrice Quarry in South Australia mines chemically pure marble for use in the chemical industry. The Quarry's major customer is the Penrice Soda Products plant at Osborne, S.A., where 508,000 tonnes of high grade marble are used per annum. The marble is burnt in vertical shaft kilns (calcining) as part of the Solvay Process for soda ash (sodium carbonate) manufacture. Since early 1993 the kilns operations have been plagued by elevated kiln pressures coupled with the production of undesirable wastes called 'grits' (i.e. fine grained, partly calcined marble). These undesirable operating parameters and end products create limitations within the kilns plant and ultimately affect the down stream production of soda ash. It is known that the Penrice marble, although chemically pure, does not always behave ideally during calcining as it often has a tendency to disintegrate in the kilns. This disintegration, termed 'decrepitation', generates excessive amounts of granular material that interferes with the airflow updraft in the kilns. This results in elevated kiln pressures, increases in 'grits' production and difficulties in controlling other operating parameters. Given these criteria, it was apparent that this decrepitation had been the major contributor to these poor kilns performances since early 1993. Consequently, it was decided that a series of new investigations needed to be undertaken to determine: i. The major cause(s) at the quarry of potentially decrepitating marble given that the kilns operations are essentially constant, ii. Why the levels of decrepitation have increased, and iii What changes could be made to current quarry practices to prevent or at least minimise the occurrence of decrepitating marble. These investigations commenced with a break down of the current mining practices at the quarry which indicated that the most significant change to occur was the introduction of new production scheduling procedures (i.e. in-pit blending). Production records showed that since the introduction of these new scheduling procedures a significant proportion of marble for the Osborne plant was sourced from the lower benches in the northern end of the pit. Historically this marble was considered to be of unsuitable grade for the Osborne plant and as such almost all Osborne marble was sourced from the southern end of the pit. Further investigations indicated that the major difference between the southern and northern ends of the pit was that much of the mining in the northern end of the pit was below the water table and necessitated the use of waterproof explosives (i.e. Handibullc Wet*). For this reason it was decided to engage ICI Explosives to undertake some trial blasts and subsequent photogrammetric analyses to determine the differences between wet and dry blasts and the influence they had on the production of potentially decrepitating marble. The trials showed that both wet and dry hole blasts produced structurally weak marble around the explosive column in the blastholes, which was subsequently termed 'blast affected marble'. The main finding of the blasting trials was that wet hole blasting (Handibulk Wet) produced 6.6 per cent blast affected marble while dry hole blasting (ANFO) produced only 1.5 per cent. These results suggested that the increase in decrepitation and therefore poor kiln performances were linked to an increase in the utilisation of low strength (i.e. blast affected) marble as a result of increased wet hole blasting. Petrographic analyses were undertaken by the author in an effort to understand the nature and intensity of the fracture systems produced via blasting and how those findings could be related to blasting practices. Some point load strength tests were also conducted in order to gain an appreciation of the degree of strength loss produced between heavily blast affected marble and marble unaffected by blasting. Blend changes incorporating less marble from the northern end of the pit have helped to alleviate the problem while further work is continuing into alternative explosive types and dewatering methods within the lower benches of the pit to enable ANFO type explosives to be used.