Contents
Salt is a vital part of both daily human life and industrial processes. It is extracted from underground deposits using salt mining techniques. This article outlines the diverse methods by which salt is procured, focusing on two dominant underground mining techniques – shaft mining and solution mining – and the crucial role of subsequent evaporation in refining the final salt crystals.
What is salt mining?
Salt mining is the process of extracting salt from underground salt deposits. Through underground mining, most commonly shaft mining and solution mining, salt deposits are drilled to extract, process, and produce salt crystals. Mined salt is usually rock or industrial salt.
Different types and uses of salt
Rock salt
Mined salt that is used for the de-icing of roads during winter.
Industrial salt
Mined salt that the chemical industry depends on has a variety of applications, such as textile dyeing, manufacturing detergents, and producing resin products.
Culinary salt
Mined or solar evaporated salt that is meant for human consumption. There are strict health and safety standards for mining and producing culinary salt.
Mining and harvesting salt
Salt crystals are produced in two main ways, either through underground mining or evaporation methods. Evaporation is also an important step during solution salt mining. Each category of salt production can be classified into two main techniques of processing the salt crystals, which are:
- Deep shaft mining (underground mining)
- Solution mining (underground mining)
- Solar evaporation
- Vacuum evaporation
Also read: What is mineral exploration?
Underground salt mining
Underground mining is one of two dominant mining techniques that are used for mineral exploration. It involves creating tunnels and shafts underground and extracting ore deposits from here. Unlike open-pit mining, which removes ore by excavating large surface craters, underground methods are chosen when the deposit is located too deep for surface mining to be economical – or when surface disturbance must be minimized.
Salt exists in deposits in underground seabeds, which is why the underground mining technique is needed in order to extract salt. Room and pillar is a common method when executing underground mining in practice, which leaves sections of ore to support the roof.
Underground salt mining methods
- Deep shaft mining
- Solution mining
Deep shaft mining process
Shaft mining is underground mining where the room and pillar system of mining is employed. Salt mines have shafts that are sunk down to the floor of the mine. When accessing the salt deposits, large chambers of salt are created through the room and pillar method. This includes the drilling and blasting between shafts. Pillars of salt are left to support the mine’s structure.
Extraction
Drillers bore holes into the salt walls. Using explosives or cutting technologies, the salt is broken into larger pieces. These pieces of salt are then loaded and hauled to crushers for further processing.
Processing
Salt is crushed and sized before being conveyed to a central shaft for hoisting to the surface.
Typical salt produced
- Rock salt
Solution mining process
Solution mining is underground mining that uses the room and pillar technique in combination with a water solution. Wells are drilled into underground salt beds, and water is injected into the beds to dissolve the salt. The solution, also known as brine, is pumped out and taken to the plant for evaporation.
Extraction
Water is injected under pressure, which dissolves the salt to create a saturated brine.
Processing
The brine is pumped to the surface and into vacuum pans where it is boiled, purified, and evaporated until only the salt crystals are left behind. The salt crystals are dried and refined.
Evaporation can be done using solar evaporation or vacuum evaporation, which we’ll explain in detail later in the article.
Typical salt produced
- Table salt
- Industrial salt*
*When salt solution mines are located near chemical plants, the salt crystals are used for chemical industrial purposes.
How is evaporation important in salt mining?
Evaporation is an essential part of salt mining as it separates the salt from the water. In essence, water is vaporized and salt crystals are left behind. It can either be achieved by solar evaporation, where sun and wind dry out the brine, or vacuum evaporation, where steam evaporates water.
Also read: What are critical minerals?
Harvesting salt by solar evaporation
While solar evaporation is a method for evaporation during the final processing stage of solution mining, it is also a salt production method in its own right. Solar evaporation is the process of harvesting salt through seawater or salt lakes, where wind and sun evaporate the water from shallow pools, leaving the salt crystals behind. After harvest, salt is washed, drained and cleaned, and then refined. Finally, it is screened for industrial purposes or human consumption.
Solar evaporation is the oldest and purest way of extracting salt, and the salt produced is table salt. However, only areas with low annual rainfall and high evaporation rates have successful solar evaporation plants. Such examples are Australia and countries near the Mediterranean.
Vacuum evaporation
Just like solar evaporation, vacuum evaporation is also a method for salt production. This method involves evaporating the brine using steam heat within vacuum pans.
From mining to evaporation
The salt is dissolved from salt beds during the solution mining. Once the water has been pumped down one well, the salt has been dissolved, and the brine has come to the surface, it is piped into large tanks for storage. From there, the brine is pumped into the vacuum pans. These are huge closed vessels that are arranged in a series of three, four, or five.
The process explained
The vacuum pan process operates from one principle: Whenever the pressure is lowered, the temperature at which water will boil is also lowered. This means that the location of the vessel (ref. the air pressure) impacts the boiling point.
The first vacuum pan in the series is fed by steam, which causes the brine to boil. The steam from the boiling brine is used to heat the brine in the second pan. For each succeeding pan, the pressure (and therefore the boiling point) is reduced. Several pans in a row produce more salt per pound of steam, which enables greater energy efficiency.
How is salt cleaned after mining?
After mining, salt must be cleaned for it to be consumed by humans or used in chemical and industrial processes. Salt is cleaned after a step-by-step process known as the salt refining process, which is the industrial process used to purify raw salt by washing it.
Salt refining process
Washing and milling the salt
The first step is to wash the salt, which removes dirt and impurities from the crystals. They are also broken down into smaller pieces.
Chemical treatment
This step is a continuation of the washing process, where chemicals are added to remove magnesium, calcium, and other impurities through precipitation. These impurities are separated from the salt brine.
Thickening and centrifuging
The brine is thickened before the salt is separated from the solution using a centrifuge.
Drying the salt crystals
Next, heated air is used to remove moisture from the salt crystals. During vacuum evaporation, boiling vacuum pans evaporate the brine, leaving the salt behind, rendering this step obsolete.
Screening
Dried salt crystals are screened to sort them by size, ensuring a consistent product which is ready for human consumption or chemical use. Additional processing can be relevant if there are specific industrial needs.
Summary
Salt mining covers a wide range of methods, from traditional underground drilling to advanced evaporation techniques, all aimed at extracting a mineral vital to human consumption and the global chemical industry. The application of these techniques allow major producers like China and the U.S. to meet the world’s demand for this mineral resource.
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