Gypsum: To mine or not to mine?
By Guest Author Dr. David H. M. Alderton
Environmental considerations have increasingly become an important and well publicized factor in influencing the viability of a mining operation. A mine’s effect on the local physical environment can readily be appreciated as a key aspect, but less direct environmental considerations can also be important. One interesting illustration of such an indirect effect relates to the mineral gypsum.
An exposure of the Castille Formation, a massive gypsum bed in the Permian Basin of West Texas
What is gypsum?
Gypsum consists of hydrated calcium sulfate. Most deposits of gypsum have been formed by the intense evaporation of bodies of salt water; as such, large accumulations are both common and widely distributed.
The mineral has a large number of important uses, particularly in the construction industry; it is the major constituent of wallboard and is a key additive (as retardant) in cement. It is also important in agriculture as a soil improver. And there are a large number of additional uses, for instance in brewing, cosmetics, foodstuffs and pharmaceuticals.
Demand for gypsum is primarily linked to building activity and overall, worldwide consumption and production are steadily increasing in response to new commercial and residential developments.
Traditionally gypsum has been produced by mining, both opencast and underground. There has not been much recycling of gypsum-based materials due to the difficulties in separating mixed products. But its ultimate disposal (to landfill) has to be as special/hazardous waste, rendering this as an expensive option.
Gypsum production
Although gypsum has historically been sourced from mining, it is also produced as a byproduct from some industrial, chemical processes (leading to products such as phosphogypsum, citrogypsum, titanogypsum and fluorogypsum). It can also be produced during the extraction of salt from seawater.
However, the drive to clean up pollutant emissions from fossil fuel burning power stations has led to the production of large volumes of a highly important, additional synthetic form of gypsum. The removal of sulfur-bearing gases from coal-fired power plants can be achieved by reacting the waste gases with water and lime or limestone, the resulting gypsum being known as FGD (flue gas desulfurization) gypsum. This type of synthetic gypsum is relatively cheap, readily available and has high purity, and thus is an attractive alternative to mined gypsum. It currently dominates the synthetic gypsum market. A large coal burning power plant can produce tens of thousands of tonnes of FGD gypsum each year and wallboard plants are often sited nearby to take advantage of this output.
The figure above shows the amount of gypsum produced in the US over the last three decades (compiled from USGS Mineral Commodity statistics). It clearly illustrates that gypsum production has risen during that period. However, whilst the amount of mined gypsum (blue) has only slightly increased during this time, the total increase has mostly been due to synthetic gypsum (yellow); in 1990 its contribution was minimal, but it has now increased to a proportion close to 50%.
The ready availability of FGD gypsum has sometimes necessitated a decrease in gypsum production in individual mines and a concomitant decrease in the associated workforce. So, here we have an interesting example of environmental pressures influencing the demand and supply of mined gypsum.
Gypsum crystals exposed on the surface, Powder River Basin, Wyoming
The future?
Environmental pressures are now leading to a reduction in the burning of fossil fuels and a steady decommissioning of coal-based power plants. Therefore, one could predict that the amount of available FGD gypsum will decrease in the near future. Presumably, as this FGD gypsum source is depleted, there will be a need to increase the amount produced by mining to satisfy an adequate supply in a globally expanding market.
Actually, the picture is not quite so straightforward, for several reasons:
Some coal-fired power stations are still being built (mostly in parts of Asia).
There is a large stock of excess FGD gypsum, as not all is currently being used. Some of this excess has been stockpiled in mono-fill landfills, so could easily be reworked if needed.
And there are moves to recycle more gypsum. Whilst it is currently possible to recycle material from the wallboard production phase, it has been more difficult to recycle construction and demolition waste gypsum. But new legislation in some countries is demanding that recycling of this waste is increased, and technological developments are starting to allow this to take place.
Notwithstanding these factors, it is certainly ironic that the environmental concerns which initially led to a supply of an alternative, non-mined source of gypsum could now result in an increase in mining of this commodity.