GCI TECH NOTES ©

Anyone who has been in the hazardous waste business for the last ten years will recognize the term "spent potliner" (SPL).  This is the conductive carbon used to line the pots in the electrolytic process that separates elemental aluminum from the alumina raw material. This Tech Notes is about SPL and its potential use as a fuel and mineralizer in cement kilns.

Brief Regulatory History of SPL Like a number of wastes, SPL is not particularly hazardous if managed properly. It contains a small amount of cyanide and a few heavy metals, primarily lead and chrome, as well as considerable sodium and fluoride. SPL is really made up of two layers, a conductive carbon layer and a non-conductive insulation layer. Most aluminum manufacturers split these layers into the First Cut (the carbon layer) and the Second Cut. In the 1980's a couple of cement kilns successfully utilized the First Cut material as fuel feeding it in with the coal into coal mills.  However, when the EPA classed the First Cut (but not the Second Cut) as a hazardous waste, assigning the waste code number K088, this practice stopped.

One aluminum company developed a process that was purported to render the K088 material suitable to be land-filled.  This process has been found to be inadequate. This has initiated another round of interest in placing SPL as a kiln fuel.

First and Second Cut SPL Chemistry The table below gives ranges for the major constituents of the two SPL cuts. Please note that the Second Cut is not classed as a hazardous waste.

The First Cut has sufficient carbon to achieve the BIF interim standard of 5000 BTU/lb minimum requirement. SPL is dense and the samples should be ground very fine and a small amount of an initiator, such as white mineral oil, must be added to the sample to obtain an accurate value from a standard bomb calorimeter. If this is done properly, the heat content of the SPL will be in the 6000 to 8000 BTU/lb range.

The cyanides are present as complexed inorganic salts. Cyanide concentrations may range from 0.03% to 0.6% in the First Cut.

Use of SPL in Kilns Prior to 1988 two cement kilns utilized First Cut SPL as a fuel. Stack emissions testing conducted at that time indicates that the cyanides were destroyed and very little of the fluoride was emitted. Fluoride concentrations were barely above background levels in the stack gases.  The majority of the fluoride ends up in the clinker. The fate of the sodium and the heavy metals is consistent with their fates documented in numerous tests since 1992.

Unfortunately, the First Cut SPL is a very hard material which the normal coal mill is not designed to handle. That difficulty, coupled with the EPA’s classification in 1988 of the First Cut SPL as a hazardous material, ended this successful use of SPL.

Now, however, with kilns looking to replace dwindling sources of fuel quality wastes, the renewed interest by the aluminum industry in placing SPL at cement kilns is fortuitous to both. Indeed, one successful trial recently occurred at a facility with a longer trial being sought as soon as possible. Other kilns are also interested in scheduling trials - primarily to confirm predicted chemical and operational effects of the use of the First Cut SPL. However, even if all of these tests are successful and the kilns proceed with routine use of the First Cut SPL, this would be insufficient capacity to consume all of the K088 waste produced in North America.

In addition, the Second Cut SPL, which is non hazardous, is also available. And while it does not contain appreciable fuel value, it does contain alumina, silica, calcium, sodium and fluoride a known mineralizer or fluxing agent. A kiln that could tolerate the additional input of sodium could benefit from the reduced heat input the fluoride’s fluxing action would produce.

There are other non hazardous wastes produced by the aluminum industry.  These are small in comparison to the 50,000 tons per year of First Cut SPL. Some of these are high iron, alumina, silica or carbon all needed by cement kilns.

Nearly every cement production facility in the United States is at or approaching a sold out condition. Certainly a different condition than that of a few years ago when clinker was being stockpiled and less efficient kilns were shut down, some permanently. Alternate fuels and raw materials are often thought of as impediments to full production or more trouble than they are worth, particularly in good economic times. The belief that alternate fuel or materials use always negatively impacts production is not true.  There are kilns that have set clinker production records in the same month they have set waste fuel consumption records. And clearly, activities that reduce production costs are appropriate at anytime. Indeed, the benefits may be more pronounced during the good times due to higher prices for the extra tonnage and bigger margins due to lower raw material’s cost.

If you wish to know more about the use of alternate fuels or raw materials, please contact GCI.

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