Quarrying and Open Pit Mining

Finding the Best Combination

Large quantities of raw material are produced in various types of surface operations. Where the product is rock, the operations are known as quarries. Where metallic ore or non-metallic minerals are involved, they are called open pit mines. There are many common parameters in design and choice of equipment, and in the process of finding the best combination of drilling and blasting methods. Atlas Copco has the advantage of long experience in all types of surface drilling operations, with a product range to match. With its history of innovative engineering, the company tends to think forward, and is able to advise the user on improving design elements of the operation that will result in overall cost savings.

Quarries

A quarry is a factory that converts solid bedrock into crushed stone. The excavated rock is crushed, screened, washed and separated into different sizes, for subsequent sale and use. The amount of fines should be kept to a minimum. The final products are used as raw material for chemical plants, such as limestone for cement manufacturing, paper and steel industries, and clay shales for building materials, or as raw material for concrete aggregates, highway construction, or other civil engineering projects.

Not all types of rock are suitable as raw material for crushed stone. The material must have certain strength and hardness, and the pieces acquire a defined shape with a rough surface. Consequently, soft sedimentary rock, and material which breaks into flat, flaky pieces, are unacceptable. Igneous rock such as granite and basalt, as well as old Precambrian highly metamorphic rock such as gneiss, are well suited.

Quarries are normally run by operators who sell their products to nearby contractors and road administrators. This means that in areas where construction activities are high, several competing quarries may be established. Apart from environmental aspects, such operations can disturb the neigh bouring area in the form of noise from drilling, vibrations from blasting, and dust from crushing and screening. Quarries, wherever possible, are therefore discreetly located, as close as feasible to the con­struction area, because the products are transport cost sensitive.

Quarries can be either of the common pit type or, in mountainous terrain, the hillside type. Pit type quarries are opened up below the level of surrounding terrain, and accessed by means of ramps (Figure 1). The excavation is often split into several benches, depending on the depth of the operations. In its first stage, when the terrain is rough and bulldozers cannot provide a flat floor, a top-hammer construction type drillrig can be used to establish the first bench. Once the first bench is prepared, production drilling is preferably carried out using DTH- or COPROD techniques.

Shallower Benches

Whereas high benches, above 30 m, were previously quite common, nowadays they are shallower, for improved safety and economy. The width of the bench should be sufficient both to accommodate the spread of rock blast, and to provide space for the equipment.

The height of the working face in hill-type quarries is determined mainly by the topography.

A typical work cycle in a quarry consists of a number of work elements. Drilling of blastholes is undertaken in a predetermined pattern, followed by plugging the drill holes with wooden or plastic plugs to prevent debris from falling down into the holes.

When an adequate number of holes have been drilled, preparations for blasting will start. The holes are blown clean with compressed air to remove water and rock fragments, and are then charged with a booster bottom charge, detonators and explosives. Stemming is inserted into the top of each hole, and the detonator leads are connected. Where electric detonators are used, thе circuit resistance is checked with an ohmmeter. The area is evacuated, equipment is moved away, and the round is fired.

Before loading, the lower quarry floor is cleaned of fly rock with a wheel loader. Blasted rock is then loaded into trucks, and transported to the crusher station. Large boulders are pushed aside, and stockpiled for sub-sequent secondary breaking. Rock is discharged directly, or via a grizzley for size control, into the primary crusher. Thereafter, it is transported by conveyor belts for secondary, and possibly tertiary, crushing.

The different products, comprising rock fragments of certain size ranges, are recovered from the process by vibrating screens, and transported to storage silos or bunker piles on the ground.

Optimum Efficiency

There are various factors to be considered when trying to achieve optimum efficiency and overall economy from quarrying operations, the most important of which is cost reduction. Figure 2 illustrates a typical cost distribution between drilling and blasting, loading and transport, and crushing, screening and storage. It should be noted that crushing, screening and storage represent about half of the costs, whereas drilling represents less than 10%. More often than not, the crushing operation is the bottleneck in the total workcycle. It is sometimes the case that extra expenditure in drilling and blasting might be the only way to assure free flow through the crusher, in order to utilize full capacity in the plant, and improve overall economy.

From the economic viewpoint, however, it is obvious that large-size crushers are primarily designed to handle large amounts of rock material, rather than large-size material. Therefore, it is worth assessing the rock fragmentation derived from the drilling and blasting cycle, in case some additional investment in drilling is the answer (Figure 3).

The whole chain of activities leading to the final product of crushed material must be considered when optimizing total costs.

Drilling costs depend on the hole size and drilling density, in which accuracy of drilling is a factor; blasting costs depend on the amount and type of explosives, and number of detonators; and loading, transport and crushing costs depend on quarry floor roughness, and fragmentation.

For most drilling applications, the optimum ratio between bench height and burden seems to be in the range of 3 to 4, indicating that hole diameters of 125-165 mm (5-6 in) are best for a bench height in the order of 18 m.

Figure 4 shows the trends in choice of drilling method. There is a clear movement towards more hydraulic tophammer, COPROD-drilling and DTH, and away from pneumatic and rotary drilling.

Open Pit Mining

A major difference between open pit mining and quarries is the geological conditions and the demand characteristics on the blasted material. Whereas quarries deliver the majority of rock via the crushing and screening plant in various size fractions, the open pit mine attempts to deliver the ore as pure as possible via crushers to the dressing plant, consisting of mills, separators, and/or flotation, and/or biochemical systems, and finally to smelters, in order to convert minerals to metals.

As no orebodies have the perfect conical shape which is tailor-made for the geometry of a pit, vast quantities of waste usually have to be removed from both the hangingwall and the footwall in order to mine the ore.

Figure 5 shows a sectional layout of a typical pit. The waste-to-ore ratio increases as the pit gets deeper. Eventually, for economic reasons, the open pit will be abandoned, or underground mining will take over. Without jeopardizing slope stability, it is of prime importance to keep the pit slope angle as steep as possible, maintaining excavated waste at a minimum.

The demands on fragmentation of the waste, as it will not pass through the crushing/dressing system, are simple. It should merely suit the loading and trucking equipment used for subsequent removal to the waste dump. On the other hand, good fragmentation of the blasted ore will make great savings in the total costs of the mineral dressing process.

Date: 2015-12-24; view: 1008