Large Tonnage Fine Crushing Plants – Trade-Off Study

Common practice for size reduction of run-of-mine or run-of-pit ore in preparation for subsequent treatment utilizes a three-stage system. First stage, or primary breaking is generally carried out with jaw or gyratory crushers. The usual output from this stage provides a feed for fine crushing in the size range of 6 to 10 inches.

Factors Affecting Design

Generally speaking, two crushing circuit options are available for preparation of grinding mill ore feeds – open or closed circuit. Generally speaking, open circuits are used to provide a minus ¾” product, while closed circuits are used to provide a minus ½ or minus 3/8″ product. A careful overall analysis must be made of feed and product sizes, crusher capacities at various settings, required screen areas and ore characteristics before a decision on open or closed circuit can be made.

Ore characteristics play an important part in establishing design criteria and equipment specifications. For example, a plant designed specifically for a hard dry ore may be at a serious disadvantage when presented with wet sticky material. And ores which, when crushed yield either an abnormally high percentage of coarse particles or fines, may require special effort in design to avoid unbalanced circuits and overloaded equipment.

Crushing plant hourly capacity is determined from overall project considerations related to mine and concentrator schedules, stockpile capacities, labor, future expansion requirements, etc.

Once the number of daily crushing shifts has been determined, plant design capacity can be computed as follows:

TPH = TPY/Dy x K/H

Where:

TPH = Tons per hour
TPY = Tons per year
Dy = Operating days per year
K = Availability factor, usually 0.75 – 0.85
H = Hours/Day

Overhead bridge cranes and hoists are provided to meet service and maintenance requirements. Hoist and travel speeds, end approach limits, and method of crane control have a direct effect on ease and safety of equipment service and maintenance. Cab controlled bridge cranes generally require special operators, whereas pendant controlled units may usually be operated by any plant personnel.

Typical Layout Illustrations

The following figures show typical layouts of large tonnage fine crushing plants. No attempt is made to cover all possible – layout schemes, however the majority are similar to, or variations of, these layouts.

Stacked Arrangement

This is a vertical cascade layout with straight-through flow and without intermediate ore surge bin. The arrangement is limited, obviously, to open circuit crushing. With ore of fairly constant character, the plant is simple to operate, and there is no inter-stage material handling equipment.

Advantages:

1. Simple to operate.
2. Control and instrumentation device requirements are low.
3. Finished fines are removed as soon as produced.

Disadvantages:

1. Screens and crushers are high above grade. Requires massive concrete foundations and sophisticated structural support design.
2. Layout does not lend itself to future expansion.
3. Service crane requires high lift capacity and long span.
4. Utilization of building volume is poor,
5. Many service platforms are required, and a man elevator is desirable to simplify access to the higher levels.

Low Profile – Open Circuit 

This arrangement is characterized by crushers set near ground level, with screens ahead of crushers.

Advantages:

1. Crusher location near grade allows for minimum foundation concrete.
2. Screens relatively close to grade reduces problems of support and isolation of vibrating forces from building frame.
3. Crane access to major equipment items is good.
4. Expansion requires relatively little revision provided suitable provision is made in the original design.
5. Intermediate ore bin provides a short time lag for either manual or automated adjustment of crusher feed, and results in high productivity.

Disadvantages:

1. Crusher lube systems must be located below grade.
2. All fines from the secondary crusher and feed screen are feed to the intermediate bin. This is particularly undesirable with wet ore.
3. This layout does not lend itself to easy conversion to closed circuit crushing.