Prediction of the degree of thermal breakdown of limestone : a case study of the Upper Ordovician Boda Limestone, Siljan district, central Sweden

Abstract: Quicklime (CaO) is an important geological resource with a wide variety of industrial uses. It is a base chemical produced from the burning of limestone or dolomite. This process is referred to as calcination, during which the limestone is heated to temperatures of 900 °C or higher, resulting in the discharge of CO₂, leaving a yield of quicklime. The formula can be written as follows: Limestone: CaCO₃ + heat → CaO + CO₂ Dolomite: CaMg(CO₃) ₂+ heat → CaO + MgO + 2CO₂ During calcination, a pure limestone looses about 44% of its weight, while a pure dolomite looses 48%. The amount of silica, alumina and other impurities of the original rocks is therefore near doubled. Calcination may also result in a notable loss of strength leading to fragmentation of the quicklime and formation of very fine-grained powdery material, so called ‘fines’. In most areas of use, high amounts of impurities and fines are undesirable. In the present case study, three drillcores (KBH 1, 2 and 3), retrieved by Svenska Mineral from the Boda Limestone in the Siljan district, Central Sweden, have been studied with regard to facies and structure to ascertain the suitability of this limestone for the production of quicklime. The Boda limestone consists of large dome-shaped carbonate mud-mounds. The mounds consist of two members; a core member and a flank member. The core member can be subdivided in two overall facies; a red mudstone facies, which is present in all three cores, and a brown core facies only found in KBH 2. The flank member, which consists of pelmatozoan wackestone and packstone, can be found in KBH 1 and 2, while KBH 3 almost entirely consists of the red mudstone. In order to correlate limestone composition with percentage of fines after calcination, the amount of fractures, the presence of stromatactis, and the over-all core-recovery were described for each core. Following this analysis, nearly 200 samples were selected from KBH 1 and 2 and subjected to calcination and mechanical force in order to simulate the industrial handling and production of CaO. The produced fines where then weighted for each sample and plotted against the physical properties of each core. The results show a clear correlation between facies and the amount of fines. The red mudstone facies gave the lowest overall values, predominantly less than 10% fines. The brown core facies has more varying values, ranging from 10-60%. The flank member ranges between 20-80%. Surprisingly, the fracture frequency seems to have little impact on the amount of fines after calcination. The presence stromatactis did not have an ad-verse effect on fines either. It can be concluded that the planning for an optimal use of an economically important limestone occurrence is greatly facilitated by detailed knowledge regarding the facies of the limestone and other properties such as chemistry, mineralogy of impurities and micro-textures.