pa1778data.pdf
pa1778data.pdf
pa1778data.pdf
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U.S. STEEL DUQUESNE WORKS<br />
HAER No. PA-115<br />
(Page 107)<br />
gas cleaning systems gained increasing importance in the 1970s<br />
because of federally sponsored clean water legislation and the<br />
creation of the Environmental Protection Agency. As a result of<br />
a negotiated agreement between the E.P.A. and the United States<br />
Steel Corporation in the late 1970s, the combustion air<br />
production and delivery system at the Duquesne blast furnace<br />
plant underwent significant changes. The crux of the settlement<br />
centered on controlling the amounts of suspended solids, cyanide,<br />
ammonia, and phenol discharged daily from the corporation's gas<br />
cleaning facilities. In an ill fated effort to accomplish this<br />
objective, a water quality control system was installed at the<br />
Duquesne plant in 1979. The evaporative system, as it came to be<br />
known, consisted of a new clarifier, a hot well, a six unit<br />
cooling tower, and a cold well. There were also a number of<br />
chemical feed stations located throughout the gas cleaning and<br />
water treatment systems.<br />
A total of 14,500 gpm of wastewater from the gas cleaning<br />
processes at blast furnaces number three, four, and six was<br />
recirculated through the system. Water entering the hot well<br />
came from two sources, the clarifier and the relatively clean<br />
water coming from the gas cooling towers. After being pumped<br />
from the hot well, the water passed through an in-line strainer<br />
and a chemical treatment station before it proceeded to the cells<br />
of the water cooling tower. Near the top of the tower, the water<br />
flowed through a low pressure spray distribution system<br />
vertically downwards into a basin while air was induced up<br />
through each of the cylindrical ceramic cells by a large fan<br />
located at the top of each cell. In this manner, the clarified<br />
water was cooled from 114° F to 90° F. As the water entered the<br />
basin at the bottom of the cooling tower, it was treated with<br />
chemicals before being admitted to the cold well through sluice<br />
gates where it was subsequently pumped over to the on-site gas<br />
cleaning systems.<br />
Chemical additions were a key component in the process<br />
because the suspended solids, like iron oxide and lime dust, and<br />
the soluble compounds which were removed from the blast furnace<br />
gas tended to build up in the wastewater on each pass through the<br />
recirculating system. One reason for the increasing<br />
concentration of dissolved compounds in the system was the<br />
evaporation of pure water passing through the cooling tower. The<br />
compounds which had been contained in the water vapor were<br />
consequently left behind in the system. In addition to the<br />
effect of evaporation, more soluble calcium salts were introduced<br />
into the water on each pass, thus increasing the build up<br />
further. If left unchecked the calcium salts built up to such a<br />
point that the water no longer held them in solution. As a<br />
result, they precipitated into hard water mineral scale which