Grinding circuit design sag mill

The dry ore from crushers is stored in a stockpile . The stockpile then feeds the milling circuit . It is claimed that grinding accounts for 60% of the power consumption of a mineral process plant. Elliott (1991) indicates that for a typical copper or zinc concentrator, grinding consumes 12 kWh/t, crushing 2–3 kWh/t, and the rest of the plant 2–3 kWh/t. Obviously, the finer the grinding, the higher the energy consumption. There are two main forms of grinding:

  • Dry grinding when the water content is <1% by volume
  • Wet grinding with the addition of >34% water by volume

Between 1% and 34%, the slurry is very difficult to handle and grinding is inefficient. In some plants, an initial grinding process may be followed by some form of classification such as flotation or magnetic separation, which in turn is followed by a second grinding process. This approach tends to eliminate at an early stage a good portion of the gangue.

It is not possible to achieve the particle size needed through a single grinding phase unless coarse output is required. When a coarse product is required, crushed materials are transported to a rod mill via a conveyor belt and the output is delivered from the rod mill. This is essentially an open circuit.

Closed circuits may include SAG and ball mills, hydrocyclones, and centrifuges. Grinding mills are designed with different approaches to feed and discharge. The energy required to reduce the size of a particle is usually a function of its diameter raised to an exponent. Holmes (1957) indicated that this exponent s not a constant but a variable. His method of iteration is fairly complex and would require a computer program.

For wet grinding, which is where the slurry circuit starts, the resistance to comminution is measured by a grindability work index. It is established by test work.

Equation 7-1 is based on reduction of the rock size in a 2.44 m (96 in) ball mill. This equation applies in the case of wet grinding, which is often the first step in a slurry circuit. Typical examples of the grindability work index are presented in Table 7-1.

Material Grindability work index Reference
Barite 5 Elliott (1991)
Bauxite 9 Elliott (1991)
Clav 7 Elliott (1991)
Coal 11 Elliott (1991)
Dolomite 11 Elliott (1991)
Feldspar 12 Elliott (1991)
Fluorspar 9 Elliott (1991)
Granite 15 Elliott (1991)
Limestone 12 Elliott (1991)
Magnetite 10 Elliott (1991)
Quartz 13 Elliott (1991)
Quartzile 10 Elliott (1991)
Sandstone 7 Elliott (1991)
Shale 16 Elliott (1991)
Taconile 23 Elliott (1991)

The feed, its shape, and mechanical properties ultimately influence the performance of the grinding circuit and the degree of efficiency of ore extraction. The performance of the grinding process is dependent on a successful grinding operation.

In an autogenous mill, the feed itself is used as a grinding medium. The larger the particles, the more energy they release on impact with each other. A coarse feed (larger than 150 mm or 6 in) is important for a fully autogenous mill. Typically, the feed has an 80% passing size of 200 mm (8 in).

In a semiautogenous (SAG) mill, steel or high chrome white iron balls are added to the circuit as a grinding medium. As they rotate and are carried away by centrifugal forces, they fall by gravity and impact against the feed or crushed rocks. Due to the difference in density between the steel balls (typically 7610 kg/m3 or a specific gravity of 7.61) and rocks (with a range of specific gravity of 1.3 to 4.0), smaller steel balls in a SAG mill have the effect of large rocks in fully autogenous mills. The d80 of the feed, called F80 in SAG mills, is typically 110 mm (4.5 in).

View of a closed circuit grinding copper ore. In the back of the photo is the large 12.2 m (40 ft) diameter SAG mill that receives the ore from the stockpile. In the front, the ball mill grinds the underflow from the hydrocyclone.

View of the hydrocyclones set at a height of 30 m above the base of the SAG mill. The overflow is diverted to centrifuges to separate the gold ore from the lighter copper ore. The copper ore is then diverted to the ball mill (on the left-hand side of the photo) for secondary grinding.

Components Of Slurry Plants

(a) Overflow mills (wet grinding only)

  • Used for rod mills in open circuits and ball mills in closed circuit
  • Grinding with maximum specific area and suitable for very fine output
  • Simple and robust

(b) Diaphragm or grate mills

  • Not suitable for rod mills, and mostly used for closed circuit
  • Used for Autogeneous and Semi-Autogeneous Grinding for very fine output
  • Coarser output than overflow mills

(c) peripheral central port discharge

(d) peripheral discharge at the end

Peripheral discharge mills are essentially reserved for rod mill grinding, wet or dry Used for coarse grind where close control of final feed size is required, either coarse or fine suitable for open or closed circuits

Typical Examples of Grindability Work Indices (For Wet Grinding in a Ball Mill)

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