In chlor-alkali production plants involving membrane electrolysis, the sodium chloride (or potassium chloride) feed brine must be treated to a very high quality. For example, no suspended solids is tolerated and very low residual hardness such as a few PPB of calcium, magnesium and strontium are allowed. Unidro has a successful and established record of experience in designing and supplying tailor made units for each of the main sections of the brine treatment chain: clarification of the saturated brine, filtration of the clarified brine, super-purification of the filtered brine by means of chelate resin towers.
Membrane chlor-alkali plant produces caustic soda and caustic potash at cellroom concentration of 32% and 29-32% respectively. Unidro can design and supply multi-effect falling film evaporators, that allow for concentrating NaOH and KOH to their commercial grades (50%).
Unidro can propose a conventional circular-shaped clarifier provided with sludge scraper for the clarification of the saturated brine. Alternatively they can offer a rectangular settling tank equipped with lamella internals.
Large circular basins are normally delivered as pre-rolled sheets, rubber lined at our shop and ready for simple assembly at the site. The scraper arms are prefabricated and rubber lined at our fabrication shop.
Rectangular settlers are selected when a very compact system is desired. They are delivered as fully pre-fabricated tanks, rubber lined at Unidro’s shop. The fiberglass lamella plates with titanium spacers are mounted on rubber lined steel frames which are pre-installed inside the settler prior to shipment.
Filtration can be accomplished by two steps in series: the first incorporates anthracite filters to remove the large majority of residual suspended solids; the second uses precoat candle filters to provide a very fine filtration and removal of particles larger than 1 micron.
Unidro can provide high-performance anthracite filters, which incorporate a special backwash concept.
Though the advantage of installing anthracite filters is a substantial savings in pre-coat media consumption, sometimes precoat filters are applied as the sole filtration unit.
Unidro has experience in designing and supplying pre-coat filters based on different types of pre-coatable candles made of either sintered carbon or a fiberglass core, covered by a suitable cloth.
The super-purification of brine is achieved by treating the filtered brine in a chelate resin tower unit, where bivalent ions (calcium, magnesium, strontium) are removed by the selective action of a chelating resin.
This unit typically includes three resin columns connected in series with a “merry-go-round” arrangement; when resin in the first column is exhausted, the column is taken out of service and regenerated through a complex sequence of phases including injection of acid and alkali, and several displacement / washing / rinsing steps. Once regeneration is completed the column is switched back to service. It is now the last one in the series to provide final brine polishing.
Unidro has designed and supplied several chelate resin units around the world capable of a treatment capacity of several hundred cubic meters per hour.