Waste management and pollution prevention opportunities in the iron and steel industry.

The types of emissions generated by the iron and steel industry and the methods of managing these types of industrial waste are the subjects of this summary article.

The iron and steel industry involves a myriad of operations which generate vast volumes

of air emissions, liquid effluents and solid wastes. This article presents an overview of waste management and pollution prevention opportunities in this industry, with the primary focus on this industry in Ontario. However, these opportunities are equally applicable to the iron and steel industry in Canada as well as in a global context as well as other metallurgical operations. Although this article will be of most interest to the iron and steel industry, it will also be valuable for discussions between various parties involved and interested in the clean up of the Great Lakes, eliminating persistent toxics and achieving economically feasible (and/or zero discharge) technology trains.

To appreciate the magnitude, diversity and complexity of process effluents, an overview of iron and steel production processes and effluents is presented. These operations include the subcategories: cokemaking, ironmaking (blastfurnace, sintering, etc.), steelmaking, casting, hotforming and finishing. Also presented are assessments of the pollution prevention opportunities and the Best Available Treatment Technologies (BATT) for the abatement and treatment of various effluents, air emissions and solid wastes.

Air emissions are generated as both particulate and gaseous emissions in the transportation of raw materials and in processing operations. It is imperative to control dust by water spraying and by adequate covering of conveyer belts and drop points. By far the largest source of air pollution is sulfur dioxide generated in the combustion of coke oven gas (COG) and liquid fuels used in various operations for heating. S[O.sub.2] emissions generated by the combustion of COG can be reduced by desulfurizing the COG using the Stretford and Zimpro processes. Other air emissions include hydrogen sulfide gas generated during the blast furnace slag cooling by water spray, volatile organic compounds emitted from cokemaking by-products, and nitrogen oxides released during hotforming and other reheating operations.

An enormous volume of non-contact cooling water (NCCW) is also used in the various operations. It is important that process streams be segregated at their source (process/operation), whichever possible, and treated separately from the NCCW streams. It is also important that the NCCW be separated and recycled where applicable, to decrease the volume of the effluent to be treated.

Cokemaking contributes the most complex as well as the most toxic pollutants. Many cokemaking batteries are old and need replacement for efficiency and environmental reasons. Reducing coke requirements by using innovative alternative fuels (coal/tar/gas injection) in the blast furnace, or non-recovery cokemaking systems, will significantly reduce cokemaking effluents. In addition, the large amounts of water used in the quenching of coke can be avoided by dry quenching with nitrogen gas, as practised in Kehin Works (Japan).

Ironmaking effluents have high concentrations of total and suspended solids, cynides, sulfides, lead and zinc. Very fine zinc suspensions escape filtration, and contribute significant loadings in ironmaking discharges. These can be minimized by upgrading the filtration of process effluents (such as through use of the DYNA sand filter enhancement) and by employing higher recycling rates of process effluents. Massive consumption of NCCW, and mixing and dilution of the ironmaking effluents, must be avoided. Effluents from pickling, alkaline cleaning, and chrome (passivation) processes contain solids, metals, and oil and grease. Recycling of spent acids and alkalies, ion exchange separation for hexavelant chrome solutions, ion exchange and/or adsorption for the recovery of toxic metals, and treatment for the removal and recovery of oil and grease (including the use of ultrafiltration followed by reverse osmosis), are the more recent environmental technologies that must be considered. Recommended priorities and opportunities for effective pollution prevention and abatement of wastewaters are elaborated in a more extensive published report.(1)

The major solid wastes at cokemaking plants are tar residues, sludges, spent oxide box wastes, residues from materials handling, by-product recovery, wastewater disposal sludges and ashes. Contamination of the soil with the likes of tar, phenolics, polynuclear aromatic hydrocarbons (PAH) and volatile aromatics (BTX) is not unusual. Several alternatives are available for the bioremediation of contaminated soils. Sludges from the waste ammonia still and biological treatment system, along with the tar-type sludge generated during the cokemaking process, can be sprayed on the coal piles for eventual combustion/recycling.

The major ironmaking solid waste requiring consideration is the coarse dust collected by the dust catcher. The dust generation amounts to around 43.5 kg/t of hot metal. This dust should be recycled through sintering operations (if available). Another source of solid waste is the enormous quantity of spent refractories that are generated during periodical blast furnace relining operations. Environmentally suitable means of disposal of this hazardous waste is a priority.

The major solid wastes from steelmaking operations (both BOF and EAF) are slag and the flue dust recovered from pollution control treatment systems. About 140 kg of slag and 19 kg of dust are generated per tonne of steel. The steelmaking slag is usually recycled through the blast furnace. However, the flue dust contains lead and cadmium, and is banned from landfills without first being stabilized or treated for removal of these pollutants. Some recent developments in treatment technologies for flue dust include the inclined rotary reduction system, glassification, and the ISI process.

The major solid wastes from hotforming processes are mill scale from the scale pit and sludge from the wastewater treatment system. It is estimated that the total mill scale generated and collected, including the mill scale from the reheating furnaces and the slag and sludge from the scarfing machine, is about 62 kg/t of steel. The mill scale should be cycled through the blast furnace (preferably via the sinter plant) as is the practice at the Stelco, Hilton Works.

Solid wastes from finishing operations contain significant amounts of oil which, if recovered, can be used as a fuel in the rolling mill reheat furnaces. Dewatered sludge from the wastewater treatment system can be recycled through the sinter plant if one is available. Zinc loadings in effluents can be reduced by removing zinc from the zinc bearing scrap prior to recycling. Tin cans after detinning can be recycled through the steelmaking [TABULAR DATA FOR TABLE 1 OMITTED] operation. The solid waste generated from alkaline cleaning must be reduced as the disposal of this hazardous waste is expensive. A process under development includes an ultrafiltration system that continuously treats the alkaline cleaning solutions and allows higher reuse rates, resulting in significant savings in both chemical and disposal costs.

A summary of pollution prevention technologies (opportunities) in the iron and steel industry is provided in Table 1. These include source reduction, reuse, on-site and off-site recycling, end-of-pipe treatment and waste disposal.

Endnotes

1 This short article presents some highlights abstracted from the author's Major Environmental Report entitled Waste Management and Pollution Prevention Opportunities in Iron and Steel Industry. This major environmental report is aimed at Engineers, Industrial Environmental Managers, Research Consultants and Government decision makers. The report also deals with industrial wastes in the light of strategies available for pollution abatement and management.

Om P. Bhargava, MSc, DIC, FCIC is a member of the Environmental Assessment Board appointed by the Premier of Ontario in January 1995. In 1997, he was appointed by the Natural Science and Engineering Council (NSERC), member of the Strategic Projects Selection Panel. He is President of Omtek Inc., which provides consulting services in the fields of Analytical Chemistry and Pollution Prevention and Treatment Technologies. He has been Senior Research Scientist under contract to the Environment Canada's Wastewater Technology Centre (1991-1994). He has 28 years of service with Stelco Inc. (1963-1991) where he was Supervisor of Corporate Analytical Chemistry. Bhargava was also with Alcan as a Research Chemist (1960-1963). He is a Fellow of ASTM. You may contact him at 28 Turnbull Road, Dundas, ON, L9H 3W3; Tel: 905-627-0305; Fax: 905-627-2311.