Evaporation and evaporators

Examples of application

Zero discharge in aluminium poly-chloride production

Motivated by environmental authorities and by higher disposal costs, a Spanish company, located in an industrial area not far from Barcelona, developed in 2001 a plant (see Fig. 1) designed to treat the acid eluates of the resins used to process the wastewater of their production line.
The purpose of the plant was to recover the aluminium dissolved in the wastewater coming from offset plates printing plates.

The evaporator supplied was a heat pump vacuum evaporator with intermediate fluid: the material in contact with the concentrated waste and condensate is PTFE-coated steel and heat exchange is achieved in two PVDF and silicon carbide exchangers.

A year after start-up, analytical results confirmed the initial expectations of the end product - a constant density solution as demanded by the market for chemicals for water treatment. The distillate recycling for the production of the regeneration baths allowed to have local permission for the expansion of production since a zero discharge was obtained.

In the past, the company disposed of waste consisting of a solution of HCl and AlCl₃ with a high annual cost and significant storage problems.
Currently, this cost item has been eliminated and, together with the limited plant management costs, it is able to sell the concentrated product on a stable basis, guaranteeing an annual income.

Treatment through vacuum evaporation of wastewater from engraving process

In the rotogravure process, the printing elements are engraved in a groove-shape on a cylinder. Then, the cylinder is immerged into the ink, which fills the engraved drawing, to transfer later the drawing directly to the paper.
The cylinder usually has an iron core and, before the engraving, is nickel plated, electrolitically copper plated and rectified. Then the cylinder is subject to the engraving and to the chromium plating before being placed on the rotary press. The engraving occurs by exposing the cylinder surface, on which a photosensitive gelatine has been spread, to a laser beam. The laser burns the gelatine and creates a groove, which will be then subjected to the corrosive action of a ferric perchloride solution.
It is the most important phase of the process. As soon as it is over, the cylinder is washed, in order to remove the traces of ferric perchloride and of the exceeding gelatine, dried and chromium plated.

The wastewaters produced during this process are split in four streams: rinsing waters from the nickel plating, from the copper plating, from the engraving and from the chromium plating. All these wastewaters are aqueous solutions with an acidic pH, a COD higher than 8000 ppm and a high content of heavy metals (nickel, chromium, copper and of other chemical species like chlorides and sulphates).
In particular, the engraving waters have a COD higher than 1500 ppm, an acidic pH lower than 1.5, a high contents of iron and chlorides and a conductivity higher than 80000 µS/cm, index of a high salinity. The iron and chlorides concentration, coming from the ferric perchloride, used in the engraving is critical for this kind of wastewaters.
The high concentration of heavy metals and specially the high contents of chlorides have made the traditional anti-pollution techniques not successful.

In the treatment process, the four streams are stored in a tank in order to homogenize them.
Then the wastewaters are subjected to a pH adjustment, by adding caustic soda 30%, in order to neutralize part of the acidity and to reach a 5.5 pH.
The evaporation occurs by means of two heat pump vacuum evaporators.
These evaporators produce two streams: a distillate, continuously produced, and a concentrate, automatically discharged in a discontinuous way.
The distillation yield is about 90%. The distillate is recycled in the process.
The concentrate, which is stored in a proper tank, is disposed of, in accordance with the regulations in force.

The plant is able to treat 4200 m³/year of waste waters.