(2) Translation: Effluents from metal finishing plants have the potential to discharge unwanted quantities of metals, such as cadmium, nickel, lead, manganese, and chromium, in forms that are deleterious to water and air quality. A local metal finishing plant has identified a wastewater stream containing 5.15% chromium (Cr) by mass and envisions the following approach to reduce risk and recover the valuable metal. The wastewater stream feeds a treatment unit that removes 95% of the chromium in the feed and recycles it back to the plant. The residual liquid stream exiting the treatment unit is sent to a waste pond. The treatment unit has a maximum capacity of 4500 kg of waste per hour. If the waste exceeds the treatment unit's capacity, the excess (anything above 4500 kg/h) is diverted from the unit and combined with the residual liquid exiting it; the combined stream is then sent to the pond. (a) Without assuming a basis for calculation, draw and label the process flowchart. (b) Waste exits the plant with a flow rate m˙1=6000 kg/h. Calculate the flow rate of the liquid forwarded to the pond, m˙6 (kg/h), and the mass fraction of Cr in this liquid, x6 (kg Cr/kg). Answer: 5779.8 kg/h; 0.0154 kg Cr/kg.
(2) Translation:
Effluents from metal finishing plants have the potential to discharge unwanted quantities of metals, such as cadmium, nickel, lead, manganese, and chromium, in forms that are deleterious to water and air quality. A local metal finishing plant has identified a wastewater stream containing 5.15% chromium (Cr) by mass and envisions the following approach to reduce risk and recover the valuable metal. The wastewater stream feeds a treatment unit that removes 95% of the chromium in the feed and recycles it back to the plant. The residual liquid stream exiting the treatment unit is sent to a waste pond. The treatment unit has a maximum capacity of 4500 kg of waste per hour. If the waste exceeds the treatment unit's capacity, the excess (anything above 4500 kg/h) is diverted from the unit and combined with the residual liquid exiting it; the combined stream is then sent to the pond.
(a) Without assuming a basis for calculation, draw and label the process flowchart.
(b) Waste exits the plant with a flow rate m˙1=6000 kg/h. Calculate the flow rate of the liquid forwarded to the pond, m˙6 (kg/h), and the mass fraction of Cr in this liquid, x6 (kg Cr/kg). Answer: 5779.8 kg/h; 0.0154 kg Cr/kg.
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