When treating industrial liquid waste, the hope is to achieve zero liquid discharge. This signifies that the treatment or purification process produces no liquid discharge. Good quality water is normally obtained that can be reused in factory processes, in addition to a solid residue which is usually recoverable for internal / external marketing or fuel. When it cannot be reused due to lack of value it can be transferred to a landfill.
Some of the processes that directly affect obtaining zero liquid discharge are crystallization, thermal drying and liquid stabilization.
Normally, in order to achieve such results, a concentrated pre-trial stage is required. It consists of using high-energy efficient vacuum evaporators to obtain a concentrated effluent (brine), which will be subsequently minimized by one of the fore mentioned technologies.
Crystallization generates solid crystals that are separated from a solvent (normally water). Industrial crystallization essentially consists of temporarily obtaining super saturation of a solute above equilibrium, which is the driving force of the process. This can be achieved by re-concentrating the solute by evaporation of a solvent, cooling the solution or by action of the other chemical product that is added to the solution to decrease the solubility of the original solute, or even a combination of all three processes.
Crystallization also verifies different substantial stages that are distinguished from super saturation in addition to marking the kinetics of crystal formation and their size. Acting on temperature, agitation and time, it is possible to obtain very thin or thick crystals using this pattern.
Evaporative crystallizers work by use of a vacuum, evaporating water at a reduced temperature (35-80 ° C). Water is condensed and reused as distilled water. The evaporation vessel is configured with a heating jacket system, where the heating fluid (steam, hot water, thermal fluid) circulates. This special configuration achieves high concentrations in the chamber with the presence of solids without representing a problem for the process.
The outlet of the crystallizer normally requires the help of some final system for salt dehydration.
- Centrifuge: This unit facilities batch dehydration of large quantities of crystals of all types of salts.
- Drying Filter: the batch of mother liquor and salts is poured over a filter that drains the liquid that returns to the evapo-crystallizer header. Meanwhile, the salts are retained and separated by a traveling scraper that deposits them into a container.
- Drainage Container: Follows the same procedure as above but its larger dimensions can treat greater quantities of crystallized salts.
- Rotating Drum: Outer cylinder has a cooling jacket and a scraper that removes crystals that are deposited on the inner surface. The liquid to be crystalized comes from a concentrated phase of evaporation and is therefore hot. The cooling fluid can be water from a refrigeration circuit with an evaporation tower or refrigerated fluid that is kept at a very low temperature by means of industrial cooling equipment.
- Decanting Reactor: This process utilizes previous evaporation to concentrate the solute but in the equilibration zone. Then, a dosage of a chemical specifically studied for each use, it may be another salt, a solvent, a polymer, etc. An imbalance occurs in original solution leading to the precipitation of crystals that are extracted from the reaction tanks by a specifically designed device. This process allows fractional crystallization and obtains different crystals separated from substances of high added value.
Spray drying consists of spraying a solution rich in dissolved solids, not in suspension, in a chamber that is kept warm by the action of flue gases from a burner or hot air (180 to 400 °C). Upon contact with the temperature, the solvent evaporates instantaneously and the solid precipitates in the bottom of the chamber. A venturi system permits the extraction of the dried solid and it’s separated from water vapor and cold combustion gases (approx. 100 °C) that are emitted to the outside. A filtering / washing process of the gases controls emissions into the atmosphere.
Because spray drying is a process that consumes a large amount of energy (kwt / liter evaporated), it is preferably used after an evaporation process to re-concentrate the solute and decrease the volume of water to evaporate. The solid obtained can be reused when possible or disposed in a controlled landfill.
Stabilization / Inerting
The stabilization of liquids is highly recommended when liquid waste management is very costly or impossible and when crystallization or spray drying cannot be utilized for technical or investment reasons.
Stabilization consists of mixing liquid waste or pasty residue, previously concentrated by an evaporator, with an inert, low cost material. Normally used for this purpose are clays, quicklime, slaked lime, cement, etc. Some dehydrating polymers such as bentonite and sepiolite are commonly used. In some cases other solid waste can be utilized (e.g., sewage sludge, ash, slag, etc.).
The mixing process is done in batches or in continuous operation in a unit named BLENDER, which consists of a drum where the feeding liquid or slurry and the solid product stabilizer arrive separately. They are mixed to form a homogeneous mass and are discharged through the front opening to a container.
After a few hours the mix cements. As time passes it loses practically all its humidity, becoming solidified and inert. This product can be taken to a landfill without problem because it will never again dissolve.
The quantity of stabilized cementitious product per liter of liquid or paste depends on the type of residue but is normally between 0.8 and 2 liters of binder per liter of liquid waste or paste.