The Advantage of Mixed Salt Crystallizers in Zero Liquid Discharge (ZLD) Wastewater Treatment Systems | Alaqua Inc

True ZLD wastewater treatment is required by environmental requirements in a variety of industries, including electricity, manufacturing, refining, mining, pulp and paper, and chemical processing. This means that all industrial effluent at a location is converted to dry solids before being recycled or disposed of. The factory recycles any usable water collected during the waste treatment process. A falling film evaporator, a membrane technique like reverse osmosis, or both are frequently used to preconcentrate large volumes of wastewater. Concentrated wastewater contains 100,000-300,000 mg/l total solids and flows at a rate of 3 to 100 gpm (0.01 to 0.38 m3/minute). A forced circulation crystallizer is then used to decrease the volume to dry solids.

Crystallization has long been employed in the production of common compounds like sodium chloride and sodium sulfate. Unlike commodity manufacturing, when just one salt crystallizes, industrial waste is often reduced to dryness by crystallizing numerous salts. To prevent difficulties like extreme foaming and quick scaling, this sort of mixed salt crystallizer necessitates drastically different design settings. Furthermore, when sizing vapour compressors for mechanical vapour recompression (MVR) cycle, mixed salt solutions have considerably large boiling point increases, necessitating careful consideration to design factors. 

Finally, to lower the cost of smaller crystallizer systems, new filtering techniques have been devised. Typical functioning crystallizer systems will be examined, including steam and MVR cycles, various solids separation devices, and low-flow-rate systems. Even with synthetic waters, the crucial use of testing will be examined.

Mixed Salt Crystallizers

A two-pass horizontal or one-pass vertical external heater, in which the solution is heated by steam in the shell, is typical of crystallizers. The heated solution is then sprayed into a huge container known as a vapour body. Boiling water in the heater tubes is suppressed by the liquid elevation in the vapour body, thus the tubes are inundated (submergence). Crystals are removed from a slipstream that is pulled into a solids separation equipment (usually a centrifuge or automated pressure filter). Various energy sources are employed.

• Steam Driven: The amount of evaporation per pound of steam used is around one pound, which might result in high running expenses. The vapour created must also be condensed, which necessitates the use of cooling water.
• Thermocompressor Driven: A thermocompressor can be used to reduce the amount of steam required if greater pressure steam is available. The thermocompressor's suction recycles a part of the vapour that has evaporated in the vapour body. The motive steam is typically between 150 and 200 psig. Steam use is often reduced by 20% to 30% while using this arrangement. The rest of the vapour that isn't returned to the thermocompressor must be condensed, therefore a condenser is still necessary.
• Vapour Compression Cycle: A system in which the energy for the crystallizer is supplied by a vapour compressor powered by electricity or a gas or steam turbine. The evaporated water is squeezed to elevate its condensation temperature slightly over the boiling point of the recirculating brine inside the tubes. The discharge steam from the compressor condenses on the tubes' outside. For bigger flows, a single-stage centrifugal compressor can be utilised instead of a positive displacement rotary blower.
• Calandria Crystallizer with Salt Basket: A calandria crystallizer can be utilised for minor flow applications. The heater is housed inside the vapour body, resulting in a compact design. Low-pressure steam is used to power the calandria crystallizer. The vapour is either condensed or released into the environment. The condensing steam (shell side) gives up its latent heat to the rising liquid via a propeller pump placed in the bottom half of the crystallizer. The heated liquid (brine) loses its vapour and returns to the propeller suction when it reaches the surface. During crystallizer operation, the unique salt basket is a vertical tank used to collect solids. The following procedures are done to dewater and discharge the salts from the basket:

1. The vapour body and the basket are separated (calandria).
2. The salt basket's brine is emptied.
3. The solids in the basket are dewatered using steam.
4. The salts are discharged by opening the flanged cover. The salt basket is ideal for mixed salts with big crystal salts like sodium sulphate or sodium chloride that are quickly dewatered. Instead of a salt basket, the calandria crystallizer can be equipped with a pressure filter or centrifuge for more challenging filtering applications.

Waste savings using mixed salt crystallisation has been proved. A successful system, on the other hand, necessitates design characteristics and characteristics that differ significantly from those found in traditional single salt commodity crystallizers.

Alaqua is a crystallizer supplier in the USA along with other processing equipment such as the evaporator, solvent recovery system, distillation equipment and heat exchangers for various industrial purposes. We also provide the system with the necessary size tanks, pumps, and pipework to ensure proper operation. This will allow the customer to have a fully functional system when it has been installed. PFD, PID with pump specs, pipe size, control loops, instruments and controls, and GAD with loads will all be included in Alaqua's scope.