Mining, or the process of extracting or separating valuable minerals from sources of raw material, is one of the oldest industries in the world. A wide variety of different materials are obtained through mining, including a multitude of metals, gemstones, natural gas, and coal. These valuable minerals are processed and extracted in many different ways, but the common denominator is that as these minerals are processed, vast amounts of unused byproduct or waste collects at the site, often called mine tailings. These tailings will need to be continually displaced as to not disrupt the mining operations production rate.
What are Mine Tailings?
Mine tailings are the finely ground rock and mineral waste products from mineral processing operations. Naturally, not everything the mines produce will be immediately usable, as they must first be processed and refined which is what creates the tailings material. Therefore the tailings are often further crushed, slurried and pumped away from the mining site where they are eventually collected into dry pools or plastic-lined ponds called tailing ponds. These ponds act as a holding area for the tailings and are often designed to include poly or plastic liners to protect the chemicals from leaching into the groundwater.
However, as tailings ponds begin to fill to capacity, the mines will need to figure out how to displace the tailings to make more room for production and not interrupt the operation. This is often a challenging process but is often accomplished with sensitive dredging equipment designed for dredging with a liner present. This is needed to ensure the process goes smoothly without harming the liner material.
Since mineral extraction can involve many things, including chemicals, the tailings are often low or high in pH and very viscous. These compounded issues create havoc on conventional pumps, causing them to have a lot of trouble moving the material without constant maintenance problems. Finally, the tailings often still hold high value, depending on the resource. This allows the tailings to be extracted again using a hydraulic dredging system and pumped to a separation plant for an additional round of extraction. This is seen in many gold and iron ore plants that are keen to extract as much of the value material as possible.
Methods of Transporting Tailings
The methods of displacing or transporting mine tailings generally fall into two categories. The first method involves wetting the tailings into a slurry mixture that can be pumped using a slurry pump or gravity to move it down a pipeline system into a tailings pond. Pumping using centrifugal style slurry pumps are still the most common type of pumping system used in tailings slurry transportation. Often, when a tailings trade-off study is performed for a proposed storage facility, the limits of centrifugal pumping are almost always more cost-effective than paste transportation (particularly for high tonnage operations). However, in cases where there are limited land and water availability, paste is the only viable option.
Paste tailings can be pumped by centrifugal pumps, up to a limit. Beyond the limits of centrifugal pumps, positive displacement pumps are then required for paste tailings. This is typically done using piston diaphragm or hydraulic piston pumps. Peristaltic pumps are also used for smaller operations.
The other method involves the use of conveyor belts to continuously move debris away from the mining site and into a designated area for stacking and processing. This is accomplished by creating filtered tailings, or wet cake tailings. Wet cake tailings are tailings with around 18% moisture content, allowing them to be stacked like cakes, and transported on conveyor belts. This method is generally preferred when water conservation is a priority, which is becoming increasingly more common in the mining industry. In recent years, there has been much improvement in the field of dewatering mine tailings using pressure and vacuum filtration methods. Wet cake transportation adheres to two methods: plow and radial stacker.
The Plough method is where a movable plow is employed to direct the wet cake tailings from the main conveyor onto a delivery conveyor or discharged to a storage allocate.
The Radial stacker method is meant for low tonnage operations, where the use of a truck and shovel is required. The radial stacker can rotate, up to around 120 degrees, which feeds to separate conveyors.
Things to Consider
Mine tailings storage management is an increasing challenge to maintain. Customers need to consider the increasing volume of tailings and the increasing environmental regulations associated with tailings management. An increasing amount of tailings is associated with an increased use of water to transport these tailings, which leads to the aforementioned waters costs associated with tailings.
The specifications of the dredge pump used for mine tailings is very dependent on the composition of what is being transported, as well as the distance at which the operator wants it to be transported. Therefore, most slurry pump transport systems have to operate with variation to support different viscosity and density material. These factors are used to calculate the specific input and output pressures that will allow the tailings to travel within the pipe. Specific examples would be if the output pressure of the dredge pipe is too high, the pipe will be blown out. If the output pressure is too low, the material will settle in the pipeline and get clogged, causing downtime.