Mine dewatering

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This article outlines the important parameters to consider when designing an efficient underground mine dewatering system. An optimal system involves high operational efficiency, low maintenance and low overall costs. Excess water entry into an underground mine can cause costly issues for the operating company. A mineral deposit may not be feasible to extract if the inflowing water exceeds economic pumping capacity. It is important for the mine planning department to have an accurate estimate of groundwater flow that will be encountered in the underground mine and to have a method of dewatering the mine if deemed necessary. When designing an optimal underground sump and pumping system it is of utmost importance to know the water inflow quantities and their sources, the total head and the characteristics of the mine water.

Contents

Sources of Water

For water to cause problems in underground mine sites there must be a water source nearby and there must be a mode of entry for the water to reach the mine. Inflow rates of water depend on the type of water sources present, and the route traversed to reach the mine.

Sources of water near mine sites depend on the regional climate and hydrogeology.[1] The presence of any of the following near the excavation can be a source of water:

  • Heavy rain fall
  • Aquifers, springs, artesian wells
  • Surface accumulations (lakes, rivers, seas, oceans)

Hydrologists have the responsibility of locating sources of water for the mine operation and should note and plot surface water flow, springs, and seeps on maps as a part of the geologic mapping process [2]. Water that is encountered during exploration drilling should be noted for its location, and then measured for flow rate, pH, various dissolved solids, and temperature. To determine the origin and flow pattern of mine water inflow hydrochemical tracers can be used [3].

In order for the water to reach the underground mine, there must be a mode of entry for the water to travel through. Many mine sites have water sources present nearby that pose no problem as the country rock that the ore is hosted in is impermeable. Water may reach excavations through the following pathways:

  • Permeable country rock
  • Major faults/fracture zones
  • Direct contact between excavation and water source [3]

Water may also be introduced to the mine through backfill and service water leakage.

Issues that Arise from Water in a Mine

Wet working conditions for underground mines are undesirable for various reasons and the problems that occur depend on the resistance to erosion of the rock type encountered, the pH of the water, and the inflow rate of the water. Problems that occur from excess water in an underground mine include:
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WaterIssues.jpg
[4]
  • Dangerous working conditions
  • Difficulty in ore handling
  • Increased price of explosives
  • Reduced operating life and efficiency of machinery
  • Possible floor heave
  • Water seepage can carry backfill into the sump which can lead to a solid blockade
  • Flooding of mine
  • Instability in mine

Methods of Dewatering Underground Mines

Water in underground mines is usually removed in a two-step process. Sumps are used to collect the water in specific areas of the mine, where energy is created by pumps to transport the water out of the mining operation.


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Sumps

In order to remove water from an underground mine the water must first be collected. The most common and effective way to do this is through the construction of sumps. A sump is a pit or designated area in which water is directed, either by gravity or by pumping. The water collected in sumps is then pumped to the surface for further treatment.


Sump Design

There are many different ways that a sump dewatering system can be designed in an underground mine. Usually, there is a main sump that is located at the bottom of the shaft. Additionally, there can be a series of smaller, intermediate sumps constructed on various levels throughout the mine. These sumps are connected through a system of boreholes (often called drain holes) to ensure proper water balance[5]. The drifts of the levels are designed with a small slope (usually a 2-3% grade) to allow for water to drain into a sump or down a borehole to a lower level.

Sump design is based on a pre-analyzed water balance. In order to prevent floods, the inflow of water from all sources (ground water, service water, backfill leakage, etc.) must be estimated, and an appropriate dewatering system put into place. Design considerations include:

Number of Sumps

Mines with production on many levels simultaneously may have a sump on each level. Alternatively, there may be a sump on every nth level, connected by a series of boreholes. Some mines with very little water inflow may only have one main sump at the bottom of the shaft to which all water is directed.

Size of Sumps

Sump size is dependent on total water inflow and pumping capabilities.

Clean/Dirty Water Sumps

Water that is collected in a sump will pick up coarse and fine material on its way. This material can cause large amounts of slime to accumulate in the sumps which makes pumping and/or draining difficult. Sources of slime include, but not are not limited to [5]:

  • Drilling
  • Crushing
  • Excess explosive material
  • Back fill
  • Spillage in haulageways

A common practice is to have two sumps side-by-side. The first sump (dirty water sump) is where the dirty water with solids and slimes is collected. This water is then decanted into a secondary (clean water) sump for further pumping[6]. Alternatively, some mines with large inflows cannot attain the settling sump capacity and must pump the dirty water using specially designed dirty water pumps. These pumps have wear surfaces made of hard metal to prevent damage from the solid particles of rock[5].

Sump Maintenance

Sump maintenance is important to prevent the life cycle of the pump from decreasing. Dirty water with coarse material, solids, and slimes will wear on the pump, while a sump with good maintenance will provide cleaner water for the pump. This decreases the mean time to repair, and helps to reduce and control costs [6].

Sump cleaning is usually performed by a loader. The loader operator will use a bucket to scoop up the slime that has collected in the sump and dump it in an area of the mine where there’s no further development or production planned. Although it is good practice to maintain a clean sump, many operating mines do not consider sump maintenance a high priority. They would prefer to have their loaders working on development and production. Additionally, loaders get damaged while cleaning out sumps; management would rather pay for the cost of repairing the pump than having to pay for the cost of a damaged loader through repair and lost production [6].

Pumps

Pumps are often used in mining operations for mine dewatering deep excavations, active pits and to pre-drain areas close to the pit excavation. The most common pump used in the industry is a centrifugal pump (Bise, 1986) as seen in the SME mining handbook. Water enters the pump through an inlet located at the middle of the pump. The pump then using an impeller pushes the water out the discharge upwards. This can be seen in Figure 1:


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Pump Design

The essential information required to properly size a pump includes the required flow rate and the total dynamic pump head. The required flow rate is based on the expected flow rate of water into the mine. To calculate the total dynamic pump head three elements must be found: the static head, the pipe friction and the velocity head. The static head is the difference in elevation between the centerline of the pump station and the surface where the water will be discharged. The pipe friction is the distance lost to friction between the pipe and the water. As pipe friction increases, the pump head will also increase, requiring more power to pump out the liquid. The velocity head is the velocity at the end of the pipe representing the extra power to get the water out of the pipe. The manufacturer of the pump will give the buyer a pump curve using total dynamic pump head for one axis and flow rate as the other. An example of this is shown in Figure 2:


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PumpCurve.jpg

Therefore, once the total dynamic pump head is calculated, the curve tells the operator what flow rate is needed to obtain the required pump head. The pump curve isn’t only based on the specific model of the pump. The pipeline system also has an effect on the flow rate required. If multiple pumps are placed in parallel or in series, the total pump curve will shift.[2]

Pump Calculations

Once the pump head is calculated, the power input to the pump is needed to figure out the cost of pumping. First, the pump output power is found using:

Po = gamma * Q * ha

Where ϒ is the specific weight of the liquid, Q is the flow rate of the liquid and ha is the total pump head. Then, the power input to the motor of the pump, often given in kilowatts, is measured by:

Pmotor = Po / (em * emotor)

Where e_m is the mechanical efficiency of the pump and e_motor is the efficiency of the motor. Once the power input to the pump is found, the cost of pumping can be calculated based on the cost of pumping per kilowatt. The cost would be based on the electricity costs of the given area.[7]

Pump Improvements

All components of pumps are made of high chrome for better wear resistance and an increased longevity. New pump models are designed with the option of an adjustable impeller in order to regain pressure after a bit of wear. Pumps are improved with proper motor insulation which causes the pumps to remain dry for longer. New models also involve a design that ensures fine material will stay away from the face of the pump seal.

Some Canadian northern mines have the common issue of high acidity in their water, with pH levels as low as 2.3. This high acidity can corrode pumps and pipes causing additional maintenance costs. In these cases stainless steel impellers and cast iron bodies and discharge connections are used.

== See Also ==
  1. . (1993). Mine Water and the Environment.
  2. 2.0 2.1 . (1993). Society for Mining, Metallurgy, and Exploration.
  3. 3.0 3.1 . (1993). Mine Water and the Environment.
  4. Robertson Geoconsultants. Quecreek Mine Flooding Disaster. Retrieved January 2013, from: http://www.robertsongeoconsultants.com/index.php?page=page&id=69
  5. 5.0 5.1 5.2 De La Vergne, J. Centre for Excellence in Mining Innovations. Retrieved January 2013, from: https://www.minewiki.org/index.php/HRMH_-_Mine_Dewatering
  6. 6.0 6.1 6.2 . (2008). Sumps and Pumps.
  7. . (2012). Hydraulics for Mining Applications.
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