PART I PROJECT DEVELOPMENT
3. CRUSHING AND ORE STORAGE
The key drivers in this area of the process plant are:
Crushing plant throughput Operating schedule
Ore competence
Ore material handling properties ROM and product size required
Requirements for blending and surge capacity Environment.
These are discussed in more detail below.
3.1. Crushing plant throughput
The throughput has a major impact on the type of crushing plant selected for the duty. The cost of crushing plants rises in a non-linear way with ca- pacity and a small mine is often not able to afford an expensive, automated facility. As a result, a smaller plant (o1 Mtpa) may require a single-stage primary-crushing facility operating on line with the milling circuit with min- imal control. A large mine (5 Mtpa or larger) is more likely to use a large gyratory crusher with surge bin, sacrificial conveyors and coarse-ore stockpile.
As a general rule, a jaw crusher is used for plant capacityo2 Mtpa and a gyratory crusher is used for plant capacity 44 Mtpa. The area between is somewhat grey and requires individual evaluation.
Typical plant configurations for large, medium and small gold plants that demonstrates this and other points discussed later are shown inFig. 1.
Some examples will demonstrate this point:
A 600,000 tpa gold mine was constructed in South-East Asia. The location had low-cost labour but was in a jungle area requiring a wood-picking station. The design included a small dump hopper with retaining wall, pri- mary jaw-crusher, wood-picking station and in-line feed to the primary mill.
A 4 Mtpa gold mine was constructed in Africa. The design included a gyratory crusher, concrete surge hopper, sacrificial belt with tramp magnet, wood picker and stockpile feed conveyor to a coarse-ore stockpile.
3.2. Operating schedule
The key issue here is whether the crushing plant will operate 24 h per day or only part time. This influences the size of the equipment, need for standby equipment and the requirements for surge capacity between crushing and milling.
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(a)
(b)
(c)
Fig. 1. Typical layouts for (a) large, (b) medium and (c) small gold plants.
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Generally speaking, smaller mines operate on a 24-h schedule or as close to it as can be achieved. This avoids the overcapitalization of the crushing plant.
In addition, in the event of a breakdown, a smaller plant can be kept op- erating with a small loader and emergency hopper.
A larger plant often operates on a dayshift-only crushing schedule with a coarse-ore stockpile in-between. The cost of downtime on a large plant is significant and this will often justify the use of a stockpile.
3.3. Ore competence
In the 1980s, many gold ores were oxide, friable and conducive to single- stage semi-autogenous grinding (SAG) mills. This situation has changed somewhat in recent years and more competent granodiorite and greywacke rock types have surfaced. These very competent rock types may require the more efficient impact breakage that can be developed in a multi-stage crushing circuit rather than the more probabilistic approach that occurs in a SAG mill.
It is impossible to generalize on the selection criteria between multi-stage crushing/ball mill and primary crush/SAG/ball mill circuit selection. A de- tailed comparison needs to be conducted for each circuit. Recent compar- isons have indicated that the capital and operating costs for three-stage crushing can be favourable. However, the reduced circuit availability is often a deciding factor.
Once the decision has been made, some general guidelines can apply for multi-stage crushing:
Height is the worst enemy as it adds length to conveyors, so try to keep equipment located at the lowest point on a conveyor.
Always allow sufficient room in the feasibility study design for possible changes in equipment vendor.
Locating the secondary and tertiary crusher on the same line as the primary crusher discharge conveyor avoids the need for transfer conveyors. This is relatively easy with small equipment but is difficult with a large plant.
A multi-deck screen performs the scalping and final sizing role in the crushing circuit and is therefore an efficient use of space. The screen is often located at the discharge of the primary crusher conveyor. This allows the return conveyors to run parallel with the discharge conveyor back to the secondary and tertiary conveyors. This approach is fine for a small plant but is difficult to achieve in a large plant. A separate transfer station is normally required.
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3.4. Ore material handling properties
Ore material properties affect the equipment selection, bin and transfer chute design. For studies, the following guidelines are used:
For high clay-bearing or sticky ores, steep sided bins, apron feeders and steep chutes are used. Avoid the use of screens and vibrating grizzlies.
Impact crushers are also favoured.
For competent, abrasive ores belt or vibrating feeders can be used. Bins and chutes should have replaceable linings. Double-toggle jaw crushers are of- ten favoured over single-toggle crushers for abrasive ores as the compress- ive action is more normal to the rock and does not lead to as high an abrasion rate.
For ore types with a very high crushing work index, the high-pressure grinding roll can be an option. It provides a high reduction ratio and a relatively fine product. It is best suited to ores where there is a peak in ore competence in the 5–50 mm range. For gold ores, in particular, it generates a series of micro-cracks, which can enhance the ability of cyanide to per- meate the ore.
3.5. ROM and product size required
The mining group normally dictates the run-of mine (ROM) size. For small facilities, this size is used to specify the minimum dimension of the crusher. A typical rule of thumb is that the ROM dimension will be no more than 80%
of the minimum dimension of the crusher to avoid crusher blockages. This dimensional specification often results in a crusher that is significantly over- sized in terms of capacity. In order to manage this, a static grizzly can be incorporated to divert the occasional oversize rock. This is pushed aside with the loader. For larger facilities, a gyratory crusher is usually preferred and a rock breaker is used to break down oversized rocks.
The product size is often a compromise between what is demanded by the milling circuit and what can be achieved in the crushing circuit. Typical reduction ratios and product sizes are given in Table 1.
3.6. Requirements for blending and surge capacity
Coarse or fine ore storage can be required for a number of reasons:
A surge pile between crusher and milling circuit isolates the milling circuit from the generally lower availability achieved in crushing circuits.
Feed to the milling circuit needs to be steady and crusher discharge can often fluctuate somewhat, particularly with smaller crushing plants.
A stockpile can be used to blend ore from different sources. This is use- ful for flotation circuits where fluctuations in grade can change the mass
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balance and circulating loads around the plant. Blending can also be done on the ROM pad.
Once the decision has been made, a number of options exist for surge capacity:
The lowest cost alternative is to have no surge at all, but rather to have a crushing plant on line. This is workable for small-scale plant with single- stage jaw crushers as the availability of these simple plant is very high provided control over ROM size is maintained.
The second alternative is to use a small live surge bin after the primary crusher with a secondary reclaim feeder. Crushed ore feeds this bin contin- uously and the bin overflows to a small conveyor feeding a dead stockpile. In the event of a primary crusher failure, the crusher loader is used to reclaim the stockpile via the surge bin, which doubles as an emergency hopper.
For coarse ore, the next alternative is a coarse ore stockpile. Stockpiles of this type are generally 15–25% live and require a tunnel (concrete or Armco) and a number of reclaim feeders to feed the milling circuit.
Multi-stage crushing circuits usually require surge capacity as the availability of each unit process is cumulative. A fine-ore bin is usually required. Smaller bins are usually fabricated from steel as this is cheaper. Live capacity of bins is higher than stockpiles but they also require a reclaim tunnel and feeders.
3.7. Environment
Climate can affect the design of the crushing and ore storage circuit:
In heavy rainfall areas, open stockpiles can become quagmires and a cov- ered system must be used. Covers are also needed over bins and chutes.
This is a material-handling issue for oxide ores. However, it becomes an oxidation and acid mine drainage issue for sulfide ores.
In dry, windy climates, dust control and prevailing wind must be con- sidered. Dust control can range from dust suppression sprays through to full wet dust scrubbers.
Table 1
Typical reduction ratios and product sizes
Crushing Stage Reduction Ratio Product Size (mm)
Primary jaw 4–6 100–200
Primary gyratory 4–6 150–300
Secondary cone 3–5 30–60
Tertiary cone 2–4 10–25
Tertiary HPGR 6–10 3–10
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Try to avoid locating the office downwind of the ROM or coarse-ore stockpile.