JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III MINERAL PROCESSING Previous Question Paper (August - 2022 - R18)

Ore dressing, also known as mineral processing, is the initial stage in the extractive metallurgy process. It involves the physical and physico-chemical separation of valuable minerals from gangue minerals based on differences in their physical or surface chemical properties, without altering the chemical identity of the minerals.

Scope and Objectives:

1. Upgrading (Beneficiation): The primary objective is to increase the concentration of the valuable mineral (grade) by removing unwanted gangue materials, thereby reducing the volume of material to be transported and processed in subsequent metallurgical stages.
2. Economic Viability: It renders low-grade or complex ores economically exploitable by producing a marketable concentrate.
3. Removal of Deleterious Impurities: It aims to eliminate or reduce elements that would cause technical difficulties or excessive costs during smelting or refining (e.g., removing sulfur or arsenic).
4. Preparation for Extraction: It produces a feed material with consistent physical and chemical characteristics, which is essential for the efficient operation of downstream processes like roasting, leaching, or smelting.

Theory of Liberation:

Liberation is the process of breaking the ore into smaller particles so that each particle consists of either the valuable mineral or the gangue mineral, but not both. It is the fundamental prerequisite for effective separation.

The degree of liberation depends on the texture and grain size of the mineral association. If the valuable mineral is finely disseminated within the gangue, a finer grind is required to achieve liberation. The process involves:

1. Comminution: Crushing and grinding are used to reduce the particle size.
2. Mineral Association: Particles that contain both valuable and gangue minerals are known as middlings or composite particles.
3. Liberation Efficiency: The goal is to achieve maximum liberation at the coarsest possible particle size to minimize energy consumption in grinding. If the particle size is too coarse, liberation is incomplete (high middlings); if it is too fine, it leads to excessive energy usage and potential losses of fine valuable minerals (sliming).

A ball mill is a cylindrical grinding device used to grind materials into extremely fine powder. It consists of a hollow cylindrical shell rotating about its horizontal axis, partially filled with grinding media such as steel balls, ceramic spheres, or flint pebbles.

Construction: The mill comprises a cylindrical shell made of steel or rubber-lined material, supported on trunnions at both ends. It is mounted on bearings and driven by a motor via a gear or chain mechanism. Inside the shell, the grinding media occupies 30-50% of the volume. A feed inlet is provided at one end and a discharge outlet at the other, often equipped with a screen or grate to retain the grinding media.

Working Principle: The mill operates on the principle of impact and attrition. As the shell rotates, the grinding media is carried up the side of the shell by friction until it reaches a point where the gravitational force overcomes the centrifugal force. The balls then tumble or cascade down, striking the material particles trapped between them. This impact force causes breakage, while the sliding motion between the balls and the shell walls creates attrition, further reducing particle size.

Applications: It is widely used in the mineral processing industry for fine grinding of ores, preparation of coal, cement manufacturing, and in the production of pigments, ceramics, and chemical powders.

Limitations: The ball mill has low energy efficiency, with a significant portion of energy dissipated as heat and noise. It requires substantial maintenance due to the wear of the grinding media and the liner. Additionally, it is not suitable for grinding materials that are heat-sensitive or sticky, as they may clog the grinding media or adhere to the shell walls.