Sanjeeva Latchireddi

Abstract:

Comminution is the most energy intensive process in mining industry and it consumes up to 3% of the total electrical energy generated in the world. More than 80% of the plant comminution is spent in the area of milling. There are three fundamental process mechanisms -Particle breakage, charge motion and discharge, that control the energy efficiency of grinding mills. Fundamentally, comminution has been considered as a combination of breakage and classification (or removal of product size particles) processes. Since the work done in comminution is proportional to the energy usage (or power draw), a great emphasis has been laid towards understanding and development of relationships between energy and breakage processes in the last few decades of research. This was followed by the development of discrete element modeling (DEM) in 1980s to understand the charge motion and this technique has been successfully used to optimize the design of shell lifters. Although the quest for efficient removal of product particles from tumbling mills has been initiated as early as 1914, a deeper understanding and success has been seen in early 2000s. The most energy efficient breakage system could be the one where the particles leave the energy field as soon as they become product size particles, so that the energy is used to break the new particles. Crushers (with highest energy efficiency) are a good example in this regard, where broken particles smaller than the closed side setting (CSS) fall freely due to gravity and hence the energy is predominantly used to break new and big particles. Contrary to this, in grinding mills, the gravity creates the energy field, by lifting the rocks and balls and a special discharge arrangement is attached to facilitate the removal of product, which consists of discharge grate and a pan or pulp lifters. The efficiency of product removal depends on the design of these discharge arrangement systems. The conventional design – Radial and spiral/curved, has been in use over a century, but its inherent inefficiency has been understood off-late in 1995, thus resulted into an innovative design by the author. Industrial implementation of the new design (Twin Chamber Pulp Lifter) in several ag/sag mills around the world has well proven that energy efficiency and productivity of the milling circuits can be improved when the optimization of discharge mechanism is combined with optimization of particle breakage and charge motion processes. Operational data from several operating plants around the world would be presented to illustrate the concepts where up to 20% of energy have been recorded while increasing the productivity to the similar levels. This paper will discuss how the existing grinding circuits (Brown-filed operations) can effectively use these techniques to improve their energy efficiency and productivity to lower their operating expenditure (OPEX). Optimization of these important process mechanisms at the design stage can also lead to significant savings in capital expenditure (CAPEX) while designing the new grinding circuits for Green-field operations.