Fuller's earth is a general term for industrially used smectite or polygorskite-sepiolite clays with a large surface area and strong absorptive, binding, gelling, thickening, and decolorizing ability. Fuller's earths are composed of several combinations of absorptive clay minerals, often with varying amounts of impurities such as silica, kaolinite, zeolites, and carbonate minerals. Mineralogy, crystalline property and shape, particle size and crushability, and specific gravity all are critical to the absorptive and cation exchange capacity and decolorizing capability. Therefore it is important to obtain only refined products. The adsorptive properties of the crystals are determined by the removal of bound water by thermal treatment.

The American Geological Institute's definition of fuller's earth is that it is a fine-grained, earthy substance with high water absorptive capacity and the ability to refine and decolorize edible, synthetic, and mineral oils.

Industrial Uses of Fuller's Earth

Attapulgite (AT) or polygorskite is a family of fibrous, hydrous magnesium silicates. AT clay has numerous other components besides AT such as quartz and carbonate. These chemicals exist in the form of relatively large particles, which will deteriorate the crystal matrix unless removed by purification.

In the areas of environmental technology, fuller's earth is used mainly to treat lubrication and transformer oils, waxes, resins, polymers, diesel fuel, jet fuel, etc. AT crystals have an extremely large surface area; one pound of AT crystals has an approximate surface area of 13 acres.

The capability of AT to adsorb contaminants is used in two ways:

Contact Method: The oil to be treated is pumped through cartridges filled with fuller's earth at a flow rate designed to provide sufficient contact time between the oil and the AT crystals. The surface loading of the cartridges depends on the particle size of AT used.

Percolation Method: Here the oil is spread over the surface of a column of AT. Again the surface loading of the column must take into account the required contact time between oil and AT.

Adsorbent Handling and Regeneration

The purpose of the thermal pre-treatment of AT is to drive off the bound moisture adsorbed during shipping and storage. Optimum activity is achieved by driving off a pre-determined amount of residual moisture from the composition. For AT, the "tempering" temperatures are 500 °F to 800 °F.

The regeneration has as its objective the restoration of the adsorptive efficiency of the adsorbent to as close as possible to its initial level. Even under the best of conditions, the adsorbent loses some of its adsorptive capacity with each repetition of the regenerative process.

This degradation of efficiency is due mainly to increasing losses of bound water, which results in the alteration of the crystal structure, pore volume, and surface area. It is also due to the fusible materials precipitated and thermo-fused on the adsorbent by the oil being refined. Bound water losses increase rapidly at temperatures above 1,100 °F. Therefore, the regeneration temperature must be carefully controlled between 1,000 °F and 1,100 °F with the lower temperature preferable.

Degradation of Efficiency

The degradation of adsorbent efficiency can be expressed as a fraction of that of the newly tempered adsorbent. The rate of efficiency degradation conforms to the equation:

EW = a / (n + k)b

Where EW equals the weight efficiency fraction of the adsorbent which has been through n number of cycles of adsorption and regeneration.

EW is further defined as Yn / Y1 in which Yn is the yield from a weight unit of adsorbent at the nth cycle and Y1 is the yield from the same weight unit of adsorbent on the first cycle.

a & b are constants for a particular system. n equals the last of a sequence of adsorption and regeneration cycles.

k equals a constant which appears related to the control of the vessel in which the adsorbent is regenerated. This is always a number between 0 and 5.

This equation can be plotted as is shown in Figure 2. The various curves illustrate clearly the effect of regeneration vessel temperature control on the rate of adsorption degradation. These curves also indicate the point at which the adsorbent should be discarded. The usual criterion for discard is economic and depends on many factors such as the cost of power required to regenerate, the cost of new AT, the amount of air pollution released during regeneration, cost of manpower, etc. The economic conditions usually dictate three to six cycles prior to discard.

Figure 2: Efficiency Decline with Repeated Regeneration (Static Bed Percolation)

Figure 3: Mixture of Activated Alumina and AT (Static Bed Percolation)

Adsorption Characteristics of a Mixture of Adsorbents

The rate of adsorptive degeneration for a mixture of activated aluminum and AT is less than that of only AT. The equilibrium efficiency method with make-up and no discard is normally used. The make-up normally amounts to 1% to 2% by weight per regeneration cycle. Degradation rates and equilibrium efficiency curves are shown in Figure 3.

It is evident that the equilibrium efficiency rises with increased rates of make-up, making this method ultimately economically disadvantageous. Another interesting point shown in Figure 3 is that the efficiency for producing a light-colored product decreases faster that that for a darker color product.

The Environmental Effects of Adsorbent Regeneration

When the filtrate has reached its specification level, the feed to the adsorption column is discontinued. This leaves the head space above the adsorbent and the voids and pores of the adsorbent itself full of oil. This amount is about 70% of the bulk volume of the adsorbent. This oil is recovered by washing with a solvent such as naphtha. The washing step is followed by steaming the column to displace the final portion of the wash. The steam functions to vaporize the remaining naphtha. The effect on the environment and attending personnel consist mainly of release of a considerable amount of toxic hydrocarbon vapors into the atmosphere. This must be weighed against the unfavorable economics of constructing an elaborate system and controls to regenerate an adsorbent that is available throughout the world at relatively low cost.

The conclusion is invariably that it does not make economic and environmental sense to regenerate a universally available and cheap adsorbent.