pa1778data.pdf

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