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ABSTRACT Industry is slowly moving beyond the concept of zero liquid discharge toward the ideal of zero waste discharge. While zero liquid discharge means no liquids are discharged off site, the tons of dry solids removed from treated wastewater are often hauled to landfills off site if they cannot be stored at the plant. In recent years, some plants have opted to recover valuable salts and chemicals from wastewater to reduce the cost of hauling away useless mixed salts. Some plants even recover some of the cost of wastewater treatment by selling recovered salt.
This paper will discuss three industrial sites where all wastewater is treated and recycled and most salts removed from the wastewater are turned into saleable products. The case studies will be a coal mine in Poland, where distilled water and sodium chloride are recovered from mine drainage; a uranium mine in the Czech Republic, where distilled water and ammonium alum are recovered from acid waste; and a power plant in New York, where distilled water and calcium chloride are recovered from scrubber blowdown.
INTRODUCTION In all three case studies, a falling film evaporator is used to concentrate wastewater and recover distillate. At the two mines, concentrated wastewater is then sent to a crystallizer, where mixed salts are separated into saleable products. The crystallizer design is quite different at each mine and will be discussed separately in each case study. The evaporator design is similar at each site, however. A general description follows. Evaporator description Figure 1 shows the main components of a typical falling film evaporator system,
[figure shown in full paper]
In a feed tank (not shown) the pH is adjusted with sulfuric acid to achieve a value of 5.5. Feed then passes through a plate heat exchanger, which raises the temperature to near boiling. It then passes through a deaerator, where carbon dioxide and oxygen are removed by steam stripping. The hot deacrated feed then goes to the evaporator sump, where it combines with the concentrated brine slurry. The brine is pumped from the sump to the top of a bundle of 2-inch (50 mm) heat transfer tubes, where it falls in a thin film down the inside of the tubes. A portion evaporates; the rest falls back into the sump for recirculation.
The vapor from the inside of the tubes flows down the tubes with the brine, then is drawn through mist eliminators into the vapor compressor. The vapor compressor rakes the temperature of the steam above the boiling point of the recirculating brine. Compressed vapor is sent to the outside of the heat transfer tubes, simultaneously heating the brine Falling inside and condensing as distilled water on the outside of the tubes. Distillate is fed back through the heat exchanger, giving up its heat to the incoming feed. Distillate typically contains less than 10 mg/l total dissolved solids (TDS) and may be used in a boiler with minimal polishing or reused in other areas of the plant requiring high purity water.
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