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Tin plating industrial wastewater treatment

By 15 September 2016acf, case, mechanical

The Client
The Client is a steel industry based in the Middle East producing tinplates for food cans.
Tinplates provide excellent corrosion resistance and are produced starting from cast iron by several steps. Cast iron is first rolled to get thickness in the range of 0.2 – 0.3 mm than it is electroplated from aqueous acid solutions at a temperature around 60°C. The plating bath contains iron sulphate, phenol sulfonic acid and tin salts, as in the table. At the end of the process plating thickness is around 20 micron.

The Challenge
The customer has the need of keeping constant the concentration of tin in the working bath, by constantly reducing the amount of water or filling with fresh chemicals. Such a need produces over 9.000 tons per year of wastewater to dispose or to treat.

The Solution
EVALED® AC EW 30000: a unit based on the principle of reducing wastewater volumes by vacuum evaporation. The savings on disposal cost and fresh water and chemicals provided by this project allowed a return on investment of less than two years.

The evaporator, heart of the wastewater treatment system chosen by the Steel Industry in object, is an EVALED® EW 30000, classified as a forced circulation, external shell and tube, vacuum evaporator and is able to produce up to 30,000 litres of distillate per day.
The wastewater is sucked via a filter into the boiling chamber by the vacuum. The inflow volume is controlled automatically by level sensors. The circulation circuit pumps the waste through the primary heat exchanger where it is supplied with the heat necessary for boiling. Once heated, the wastewater goes back to the boiling chamber, where it immediately boils (flash evaporation). The vapour rises through the demister in order to damp any droplets and is condensed against the U tube heat exchanger. From the top of the boiler, the distillate is sent to the storage tank where it is discharged automatically according to settings determined at the commissioning stage. The machine is powered by a hot and cold water that provide evaporation and condensation respectively.

Results & Benefits
The wastewater coming from tin plating production is a water-based solution having an acidic pH, high COD value, and a high concentration of metals such as tin and iron. Sulphates, chlorides and phenol were also high in concentration. This particular kind of wastewater, because of the combination of metals, organics compounds and chlorides, reduces the effectiveness of traditional physical-chemical treatments and provides an excellent feed for the evaporation technique, especially when carried out under vacuum conditions.

The EVALED® evaporator provided a thermal separation of the wastewater into a distillate and a concentrate. The distillate, representing around 85% of the inlet volume is re-cycled back to the process for generating new rinsing baths; the concentrate, around 15% of the initial volume, is re-circulated back to the process after an intermediate storage.

In brief:

ZLD: the distillate is reused in the process to produce new rinsing baths
Matter recovery: the concentrate is reused in the production process to keep constant the concentration of tin
ROI < 2 years