Oil Shale Beneficiation for Processing

Abstract

Physical beneficiation has frequently been proposed as a method for upgrading oil shale feedstocks in order to improve the economic feasibility of exploiting these resources to produce synthetic fuels. Oil shales typically contain four to six times as much ash-forming minerals as oil-forming kerogen; hence large amounts of inorganic material need to be processed along with the kerogen during retorting which increases the subsequent problems of waste disposal and management. The reduction of inorganic material prior to retorting offers the attractive alternative of reducing the capital and operating costs.

The simplest conceptual method for obtaining higher grade feedstocks is the selective mining of higher grade zones within an ore body. Although this seems feasible, in practice it would be quite difficult, given the large areal extent of the deposits. Beneficiation offers a route to provide consistent, high grade material for retorting.

The beneficiation technologies that can be applied are similar to those employed for concentrating low-grade, finely disseminated metallic ores. The extent to which beneficiation is carried out is a function of the physical and chemical properties of the ore. Oil shale is a special case since the kerogen is finely disseminated in an inorganic mineral matrix and there are essentially no distinct grain boundaries between desirable and undesirable mineral components in the shale.

In order to effectively beneficiate any ore, it is first necessary to liberate the desired mineral components from the gangue minerals in a series of comminution stages. Coarse crushing is carried out in cone, roll or gyratory crushers. Although some liberation of coarse minerals may occur during this stage, there is little, if any liberation of finely disseminated kerogen. Finer grinding is accomplished with rod mills, ball mills, semi-autogeneous (SAG) mills or stirred ball mills until the desired level of liberation is achieved.

Once liberated, various separation processes can be used to isolate the kerogen in a more concentrated form. If a sufficient differences in the specific gravities of mineral (and organic) components exist, gravity-based processing methods such as jigging or dense medium separation can be utilized. For oil shales, the presence of significant amounts of ear-gravity material precludes the use of jigs in most cases. Near-gravity material is defined as the weight percent of material that is within ±0.10 specific gravity units of the desired separation gravity. An attractive alternative to gravity-based separation is froth flotation which exploits the surface chemical differences between mineral components, and, most particularly, wetting behavior.

Flotation is extensively practiced for the recovery of fine mineral ores, and utilizes two types of flotation devices; conventional agitated cells and column cells. Columns have been shown to be particularly advantageous for very fine particles (<25 µm).

The use of each of these beneficiation processes and their applicability to oil shale will be described. The description will include principles of operation, equipment types, and results that have been obtained on a variety of oil shales. While no single unit operation may be capable of producing the high grade material desired from a given shale, there are specific advantages to each operation which can enable the optimum design of a circuit to maximise upgrading at minimum cost.