Water Treatment & Solid/Liquid Separation
WATER TREATMENT AND HEAVY METALS separation for mining
Why precipitate heavy metals?
High concentrations of heavy metals are usually toxic to fauna and flora. Accordingly, environmental regulations are put in place to limit their concentration in wastewater. Precipitation processes are characterized by the capability to remove metals from solution metals that are in excess concentration down to their solubility limit for the present solution conditions.
What heavy metals are commonly targeted for precipitation?
- Arsenic (As)
- Cadmium (Cd)
- Copper (Cu)
- Lead (Pb)
- Mercury (Hg)
- Nickel (Ni)
- Silver (Ag)
- Tin (Sn)
- Zinc (Zn)
How to precipitate heavy metals chemically?
Depending on the targeted heavy metals, inorganic coagulants such as aluminium sulphate (alum), ferric salts (sulphate or chloride), calcium hydroxide (lime), as well as organic coagulants such as PolyDadmac (diallyl dimethyl ammonium chloride), PolyAmine (epichlorohydrin dimethylamine) and cationic flocculants will work by reducing the zeta potential of the colloidal system to a value sufficiently low for the colloidal particles to be destabilised and coalesce.
What if this is not enough?
While most of the heavy metals will be captured in larger aggregated particles, the heavy metal concentration present in solution in the wastewater may still be above the authorized limit.
In these cases, the addition of TMT 15® (trimercapto-s-triazine) supplied by EVONIK can precipitate heavy metals by creating a heavy metals complex particle bounding which can then be easily separated by using a clarifier or filter press. The chemical solubility characteristics of the pollutants to be removed will impact the optimum process pH.
What are the benefits of using TMT 15®?
- Effective over a wide pH range
- Ready to use solution
- Odorless
- Nonhazardous substances
- Easy to integrate into existing wastewater treatment plants
- Avoids expensive secondary treatments
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Coagulants & Flocculants are used to improve the rate of solid / liquid separation in an aqueous slurry i.e. to destabilise the slurry so that the solids settle or can be filtered more easily. Bringing together small particles or aggregates into much larger aggregations using long chain molecules which bridge between particles and bind them into a macro-structure (floc).
COAGULANTS
Why use coagulants?
To promote the natural aggregation of fine particles during inter-particle collisions by enhancing the attractive forces between particles.
What types of coagulants are available?
There are a great number of coagulants available in the market.
Typically, the coagulants are divided into two families. In water treatment, the most common family are “Inorganic”. The Inorganic coagulants include aluminum salts, iron salts, bentonite, lime, etc.
The other type of coagulants are the organic coagulants. These include Poly; DADMACs, Polyamines, Polyacrylate ester, formaldehyde-amine resins. Blended Organic-Inorganic coagulants are also available in the market. The functionality depends on the process needs.
It is important to understand that the selection of a coagulant is based on which substance needs to be removed from the liquid. Also, what the overall effect of adding a coagulant may have on the rest of the process and the environment
How does aggregation of solids work?
Aggregation of the solids through flocculation leads to increased permeability of the solids fraction in relation to the water phase.
Sedimentation – increase settlement rate
Filtration – increase rate of water release
Applications: clarification, centrifuges, thickeners, organic binders, filter presses, drilling mud, run off streams, geo bags, vacuum drum, tailings, tailings dust suppressant, mineral recovery, arsenic treatment.
Flocculation
Why Flocculate?
To bring together small particles or aggregates into much larger aggregations using long chain molecules which bridge between particles and bind them into a macro-structure (floc). According to “Stokes” law, flocculants increase the rate at which water can be separated from solids.
Dry Flocculants versus Liquid Emulsions?
The decision of whether to use a dry flocculant versus a liquid goes back to the 1970s. Dry flocculants as the name implies are made in dry form and are shipped in dry form. They have between 5 – 10 % moisture and are considered the most active form of flocculant per kilo received.
Liquid flocculants were developed in order to address feeding challenges that the dry flocculants were known for in the same era. Liquid flocculants are shipped in liquid form and are generally in the range of 25%-35% active.