Crystallizers are one of the main technologies for achieving a zero discharge in an industrial effluent treatment process. This means that the treatment or purification process does not produce any liquid discharge, and typically results in good quality water that can be reused in factory processes, as well as a solid waste that is usually recoverable for internal/external marketing or as fuel. When it cannot be reused due to lack of value, it can be sent to controlled landfills.
Some of the processes that particularly contribute to achieving zero discharge are crystallization, thermal drying, and liquid stabilization.
To achieve these results, a prior concentration stage is usually required using highly energy-efficient vacuum evaporation equipment, to obtain concentrated effluents, which will then be minimized using one of the aforementioned techniques.
Crystallizers
Crystallization is a separation operation in which the transfer of a solute from the liquid phase to a crystalline solid phase occurs, by varying the temperature or the composition of the solution.
The industrial crystallization process is fundamentally based on temporarily obtaining a supersaturated solution in relation to equilibrium; this is the true driving force of the process. Supersaturation can be achieved by reconcentrating the solute through solvent evaporation, cooling the solution, or the action of another chemical product added to the solution to decrease the solubility of the original solute, or even a combination of the three processes.
In the crystallization process, there are a series of factors, different from supersaturation, that determine the kinetics of crystal formation and, therefore, their size. These factors are temperature, agitation, and time; by acting on them, it is possible to obtain very fine or coarse crystals.
Evaporation crystallizers operate under vacuum, allowing water to evaporate at much lower temperatures (35-80ºC). The water condenses and can be used as distilled water.
The special configuration of the evaporation vessel with a jacket heating system, through which the heating fluid (steam, hot water, thermal fluid) circulates, allows for high concentrations in the chamber with the presence of solids without causing any issues for the process.
At the outlet of the crystallizer, a final dehydration system for the salts is generally required, with the most efficient being:
- Centrifuge: This equipment allows for batch dehydration of large quantities of crystals of all types of salts.
- Dehydrating filter: The batch of mother liquor and salts is discharged onto a mat that drains the liquid back to the head of the evaporative crystallizer, while the salts are retained and separated by a scraper at the end of the path, which discharges them into a container.
- Drainage container: Follows the same procedure as the previous one, but its larger dimensions allow for the treatment of greater quantities of crystallized salts.
- Rotary drum: with a cooling jacket on the outer cylinder and a scraper that extracts the crystals deposited on the inner surface. The liquid to be crystallized comes from a concentration stage by evaporation and is therefore hot. The cooling fluid can be water from a cooling circuit with an evaporative tower or refrigerant fluid maintained at very low temperatures with industrial cooling equipment.
- Decanting reactor: a process that uses prior evaporation to concentrate the solute, but in the equilibrium zone, through the dosing of a chemical product specifically studied for each case, which can be another salt, another solvent, a polymer, etc., creates an imbalance in the original solution that leads to the precipitation of crystals that are extracted from the reaction tank by a device designed for that purpose. This process allows for fractional crystallization and the separate obtaining of different crystals of high added value substances.
The vacuum evaporation and crystallization processes must be specifically studied for each case. Condorchem Envitech has extensive experience in the design, manufacture, and installation of this equipment for a wide variety of different industrial processes.
Thermal drying (spray drying)
Thermal drying consists of spraying a solution rich in dissolved solids, never in suspension, into a chamber that is kept hot by the action of combustion gases from a burner or hot air (180 to 400 ºC). Upon contact with the temperature, the solvent evaporates instantly, and the solid precipitates at the bottom of the chamber. A venturi system allows for the extraction of the dried solid, which is separated from the cold water vapor and combustion gases (approx. 100ºC) that are emitted to the outside. A filtering/washing process of these gases ensures compliance with emission limits to the atmosphere.
Since it is a process that consumes a large amount of energy (kW/liter evaporated), it is preferable to use it after an evaporation process to reconcentrate the solute and reduce the volume of water to be evaporated. The obtained solid can be reused when possible or sent to a controlled landfill.
Stabilization / inertization
Liquid stabilization is highly recommended when the management of liquid waste is very costly or impossible and when crystallization or thermal drying cannot be applied for technical or investment reasons.
It consists of mixing the liquid or pasty waste, previously concentrated by evaporation, with a low-cost inert material. Typically, clays, quicklime, slaked lime, cement, etc., are used for this purpose, although some dehydrating polymers such as bentonite, sepiolite, etc., are also commonly used. In some cases, another solid waste (for example: sewage sludge, ashes, slags, etc.) can be used.
The mixing process is done in batches or continuously in a piece of equipment called a BLENDER, which consists of a drum where the liquid or paste feed and the stabilizing solid product arrive separately, are mixed until a homogeneous mass is formed, and are discharged through the front mouth into a container.
The mixture sets in a few hours and over time loses practically all moisture, becoming solidified and inert. This product can be taken to a landfill without further issues as it will never dissolve again.
The amount of stabilizing cementing product per liter of liquid or paste will depend on the type of waste but is usually between 0.8 and 2 liters of cementing agent per liter of liquid or paste waste. In the treatment of industrial liquid discharges, the goal is to achieve the so-called zero discharge, which means that the treatment or purification process does not produce any liquid discharge, and typically results in good quality water that can be reused in factory processes, as well as a solid waste that is usually recoverable for internal/external marketing or as fuel. When it cannot be reused due to lack of value, it can be sent to controlled landfills.
