Petroleum coke, or “petcoke,” is a waste by-product of the oil refining industry. The majority of petcoke consumption is in energy applications; catalyst coke is used as refinery fuel, anode coke for electricity conduction, and marketable coke for heating cement kilns. Roskill has predicted that long-term growth in petroleum coke production will be maintained, and may continue to increase slightly through 2012. Petcoke must first be calcined to drive off any undesirable petroleum by-products that would shorten the coke product-life cycle. As an example, the calcining process can take place in large, rotary kilns heated to maximum temperatures as high as approximately 1400–1540°C. The kilns and combustion/settling chambers, as well as some cooler units, are insulated with refractory bricks and other, interstitial materials, e.g., castable refractory materials, to improve the efficiency of the calcining process. The bricks are typically made of 70–85-percent bauxite, and are slowly worn away by the calcining process; bricks used to line the combustion chambers wear away, as well, but at a slower rate. It has been recognized that the refractory materials contain slight amounts of naturally occurring radioactive materials (NORM) from the uranium- and thorium-decay series. Similarly, low levels of NORM could be present in the petcoke feed stock given the nature of its origin. Neither the petcoke nor the refractory bricks represent appreciable sources of radiation or radioactive waste. However, some of the demolished bricks that have been removed from service because of the aforementioned wearing process have caused portal alarms to activate at municipal disposal facilities. This has lead to the current investigation into whether there is a NORM concentrating mechanism facilitated by the presence of the slightly radioactive feed stock in the presence of the slightly radioactive refractory materials, at calcining-zone temperatures. Research conducted to date has been used to determine the speciation and concentration of nuclides in both the feed stock and the various refractory materials, as well as the slag that forms at the interface of the two materials, as a function of temperature. Further investigation into any potential for generation of a NORM hazard as a result of refractory demolition has been conducted. Aerosol generation (mass loading), particle size distribution, and pulmonary solubility class have been investigated as a function of demolition-task description. In addition, external radiation levels in the kilns, chambers and waste piles, as a function of temperature profile and brick/operating history have been investigated.
The History and Future Challenges of Calcined Petroleum Coke Production and Use in Aluminum Smelting
