The mining of gypsum or phosphogypsum produces incredible volumes of gypsum and phosphogypsum waste. This waste is typically stacked in very large piles, or gypsum stacks, which sometimes cover up to 400 acres in area to a depth of 100 to 200 feet or more. Thus, the land upon which the gypsum or phosphogypsum waste is stored is severely restricted with respect to any future uses. Therefore, gypsum stacks typically provide economic drain on the landowner or landholder.
Currently, a closed or capped phosphogypsum stack poses several economic challenges to the phosphate industry. In addition to the loss of use of the land upon which the stack is situate, governmental regulations that impose post-closure maintenance requirements necessitate ongoing expenses for decades. Until the current invention, gypsum stacks and other mineral waste sites, provided little or no economic return for gypsum mining companies to offset these ongoing expenses. Thus, for every company in the U.S. and abroad that mines, processes or produces gypsum, a gypsum stack always generates the inevitable: a large nonperforming asset. Numerous gypsum stacks currently exist with many more yet to come.
The present invention provides means and methods for converting presently existing gypsum stacks, or other mineral waste piles such as those found in any type of mining activity, into huge containers for holding refuse like biomass such as sugar, sugar cane, sugar cane waste, seaweed, fish, fish waste, shellfish, shellfish waste, agricultural waste, waste from forestry operations, other solid wastes, industrial waste or phosphate waste. The present invention is similarly applicable to other mineral waste piles such as those found in many types of mining activities, strip mining for example. The present invention also provides methods of doing business that will turn existing and future phosphogypsum stacks or mineral waste piles into income-producing waste containment and processing facilities.
Gypsum and waste phosphogypsum are capable of being formed into container structures which will withstand high compressive loads. Gypsum and phosphogypsum waste can therefore be used to form the bottom and walls of large containers. Thus, present gypsum stacks can be excavated to form large concavities which can be used as receptacles for solid waste. Current and future gypsum stacks can be formed into containers having one or more concavities as they are being created or enlarged. These concavities can be lined with geosynthetic liners or other membranes in order to prevent or limit leakage from the concavity. Examples of suitable geosynthetic fabrics, membranes, geocomposites, liners, liner combinations and liner-drain combinations and systems can be found in Designing With Geosynthetics, 2nd Edition, by Robert M. Koerner (1990, Prentice-Hall, ISBN 0-13-202300-8) which is incorporated herein by reference. Space in a concavity according to the present invention can be used to store waste materials, thereby providing an income-producing use of the gypsum stack or other mineral waste pile, and the land upon which they are located.
Thus, by providing hitherto unavailable waste storage facilities, the present gypsum stack and mineral waste pile conversion invention will significantly decrease the amount of new land that is used for landfills, while simultaneously converting gypsum stacks or waste piles from nonperforming assets into income-generating landfill facilities. The present means and methods for converting gypsum stacks or mineral waste piles into waste containment facilities is both economically and environmentally advantageous.
For instance, in comparison with conventional methods that would clear an additional 400 or more acres of forested or otherwise productive land to create a new MSW or C&D landfill, the present invention affords, among other advantages, at least one additional use for the presently existing tract of 400 or more acres of land by converting the space above, within or partially within an existing gypsum stack or mineral waste pile into waste containment or processing facilities. According to the present invention, instead of creating a new landfill or waste containment facility on a heretofore productive or uncleared parcel of land, the new landfill is constructed on top of, or wholly or partly within, a gypsum stack or other mineral waste pile, thereby sparing the productive land. Indeed, with the installation of one or more liner systems, and some minor modifications to the conversion site, the conversion site is very quickly able to receive waste such as biomass or other refuse.
The economic advantages of the present invention are significant. For instance, the governmental permits for establishing a 400-acre landfill are as much as $800,000.00. The present invention eliminates or substantially reduces such expenses because phosphogypsum stacks are already permitted as waste disposal sites. Thus, the present invention provides for an income-producing dual use of land upon which gypsum stacks or other mineral waste piles already exist.
In conventional use in the United States, when gypsum stacks reach maximum capacity at 100 to 200 feet of elevation, they are capped with an HDP membrane or liner in accordance with EPA regulations. In some embodiments of the present invention, the already existing gypsum stack cap membrane or liner can be used as a bottom liner for a waste containment concavity. Therefore, the cost to install a secondary liner in a landfill bottom liner system is reduced or eliminated in some embodiments of a phosphogypsum stack conversion according to the invention because the secondary liner is already in place in the form of the gypsum stack cap membrane or liner.
An additional economic advantage of the present invention pertains to subgrade costs. The costs relating to the preparation of the subgrade of a waste-containing site, which often requires the importation of clean subgrade material from offsite sources, can be as much as $2,000,000 U.S. for a 400-acre site. This expense is substantially reduced or eliminated with some embodiments of the present gypsum stack conversions since the phosphogypsum already available at the gypsum stack conversion site can be used as the subgrade material.
One significant aspect of the present invention is based upon the engineering parameters of mineral waste having high compressive strengths, such as waste gypsum, phosphogypsum or other mineral mining waste or byproducts. With the present invention, such waste or byproducts can be formed into one or more large concavities. Formation of a concavity according to the invention, can be effected, for instance, by excavation into an existing gypsum stack and then adding side walls of waste gypsum on top of the stack to extend the depth of the concavity to a desired dimension, by forming a concavity directly from phosphogypsum waste as the waste is being added to a particular geographic site, or by any other method or means that result in one or more concavities of desired dimensions. The concurrent delivery of waste and formation of the concavity is particularly useful when the gypsum waste is being transported to the site in the form of aqueous slurry, which may then be provided with strengthening binders or additives such as Portland cement as the concavity is being formed on top of an existing stack or as the concavity is being formed in place on the site. Thus, the cost of excavating a gypsum stack will be diminished.
The present invention can be used also with conventional means, devices and elements of the waste containment arts to thereby arrive at a facility of desired capabilities and capacities. For example, depending on the use to which a particular concavity is to be placed, liners or covers can also be provided to seal the bottom, walls or top of the concavity. Also in accordance with the invention, binders such as cement, for example Portland cement, can be combined with the phosphogypsum waste as it is being transported to the containment site. Addition of binders, such as cement, may be used to increase the compressive strength of the waste gypsum material and thereby assist in stabilizing the shape of the resulting concavity.
There is therefore a need for methods and means to provide additional and alternate uses for mineral waste piles, such as gypsum stacks. There is also a need for methods of forming phosphogypsum stacks into facilities useful for such purposes, and for business methods for producing revenue streams from such facilities.