Crushing and Grinding in Mineral Processing: A Comprehensive Overview

Posted on Oct 13, 2024 in Physical Education

Crushing and Grinding

Crushing Stage

The crushing stage is the first process in reducing the size of rocks and other materials. The goal is to achieve a more uniform fragment size, typically around half an inch.

Crushing Stages

  1. Primary Crushing: Reduces fragment size to approximately eight inches in diameter.
  2. Secondary Crushing: Further reduces the material size to about three inches in diameter.
  3. Tertiary Crushing: Finalizes the crushing process, achieving the target size of around half an inch.

Crusher Types

Primary crushers apply force at low speeds between two surfaces. Two main types exist:

  1. Jaw Crushers: These crushers utilize two plates or jaws, one mobile and one fixed. The mobile jaw presses against the fixed jaw, fracturing the material between them.
  • Blake Type: Features a top pivot and a fixed entry point.
  • Dodge Type: Has a bottom pivot, resulting in a variable inlet area but a fixed discharge area.
Gyratory Crushers: Primarily used in primary crushing, these crushers consist of a fixed and a mobile surface with an inverted cone shape. The moving surface’s eccentric motion crushes the ore against the fixed surface.

Secondary and Tertiary Crushers

These crushers utilize kinetic energy. The impact force increases with finer fragments due to their increased homogeneity. Common equipment includes cone crushers, roller crushers, and hammer mills.

Cone Crusher

A modified gyratory crusher with a flattened crushing chamber for higher capacity and reduction ratio. This design retains the material longer, leading to greater size reduction.

Milling

Purpose of Milling

Milling further reduces particle size to a maximum of 180 microns, liberating copper ores as individual particles.

Milling Process

Milling occurs in large rotating cylindrical mills, using two methods: conventional milling and SAG milling. Water and reagents are added to create a milky fluid (pulp) for the subsequent flotation process.

Conventional Milling

Traditionally a two-stage process using rod and ball mills, though modern plants often only use ball mills. Ore is mixed with water for efficient grinding. The resulting pulp proceeds to flotation.

  • Rod Milling: Employs 3.5-inch diameter steel rods as grinding elements. Material from the tertiary crusher is continuously fed via a conveyor belt. Grinding occurs through the movement and impact of the rods.
  • Ball Milling: These mills (e.g., 16 x 24 feet) are filled to 35% capacity with 3.5-inch steel balls. In about 20 minutes, 80% of the ore is reduced to a maximum size of 180 microns.

SAG Milling

A recent innovation, SAG mills (e.g., 36 x 15 feet) are larger and more efficient. They shorten the crushing and grinding process.

SAG Milling Process

Ore from the primary crusher (approximately 8 inches) is mixed with water and lime. Size reduction occurs through the impact of various-sized mineral particles (hence SAG – Semi-Autogenous Grinding) and 5-inch steel balls (12% of mill capacity). The mill’s size and rotation create a free-fall effect for the balls, enhancing grinding.

Crushing and Grinding Efficiency

SAG milling is more energy-efficient, eliminating the need for secondary and tertiary crushing stages. Most SAG-ground material proceeds directly to flotation, with only a small portion requiring further ball milling.

Flotation

Purpose of Flotation

A physico-chemical process separating copper sulfide minerals and other elements (e.g., molybdenum) from the remaining rock. Reagents are added to the pulp before flotation.

Flotation Process

Pulp from the mill enters flotation cells (pool-like receptacles). Air is bubbled from the bottom, and the mixture is agitated for intensive processing.

Pyrometallurgy and Hydrometallurgy

Pyrometallurgy: Involves metal collection and recovery using heat (e.g., smelting).

Hydrometallurgy: Uses liquid solutions (aqueous and organic) for metal extraction and recovery.

Screening

Screening Operation

A dimensional classification of grains based on size and shape using a perforated surface. Smaller grains pass through, while larger grains are rejected.

Aims of Screening

  1. Separate coarser fragments for removal or further crushing.
  2. Separate smaller particles as the final product or for removal.
  3. Classify the fragmented product into commercial sizes.
  4. Classify products for further mechanical, physical, or chemical processing.
  5. Extract material meeting product specifications from a crusher feed to optimize machine capacity and efficiency.

Screen Surface Selection Criteria

  1. Robust and wear-resistant.
  2. Regular openings.
  3. High open area percentage.
  4. Low blinding tendency.
  5. Low tendency for obstruction by interlocking grains.

Factors Affecting Screening Capacity

  1. Percentage of fines.
  2. Percentage of hard grains.
  3. Moisture content.
  4. Grain shape.

Screen Types

  • Stationary Screens (Grizzlies): Used for coarse particles in crushing circuits.
  • Vibrating Screens: Particles are repeatedly presented to a perforated surface with uniform openings. These are categorized into:
    1. Dependent vibrating, medium and normal straight surface.
    2. Dependent vibrating, medium and circular.
    3. Horizontally or slightly inclined vibrating.

Hydrocyclone

Pulp enters tangentially under pressure, creating a swirling motion. Larger particles are forced towards the wall and discharged as thick pulp. Finer particles and liquid exit through a central tube. Separation size (10-500 microns) is adjustable by controlling the swirl and hydrocyclone geometry.

Glossary of Terms

  • Pivot: Part of a jaw crusher that allows grinding.
  • Screen Surface: Must be robust.
  • Shell: Composite combining the best characteristics of metal alloys.
  • Spider: The upper part of a cone crusher.
  • Mantle: The bottom part of a cone crusher.