Kinds of treatment

Methods of Chemical Treatment of Waste MRF

There are three basic methods:

Chemical splitting with polyvalent metastable salts

Chemical de-emulsification with polymers

Various combinations of #1 and #2 above

Treatment by polyvalent metastable salts

The typical metastable salts are:

Compound	Type
Sodium Chloride	monovalent cation
Calcium Chloride	divalent cation
Magnesium Chloride	divalent cation
Magnesium Sulfate	divalent cation
Ferrous Sulfate	divalent cation
Ferric Chloride	trivalent cation
Aluminum Sulfate	trivalent cation

The most common methods are:

Acid – alum – caustic split

Acid – calcium chloride – caustic split

Both of these methods use a salt, either alum (aluminum sulfate) or calcium chloride, to provide the necessary positive charges (aluminum with three positive charges per ion, as Al⁺⁺⁺, or calcium with two positive charges per ion, as Ca⁺⁺).

The concurrent addition of acid to a pH of 2.5 to 3.0 and one of the above salts helps to destabilize the emulsion. The most common and least expensive acid (per pound) to lower the pH to this level is sulfuric acid. This is due to the fact that most surfactant chemistry works best in the alkaline pH ranges of 8.0 to 13.0. The net amount of aluminum or calcium varies between 300 mg/L to 3,000 mg/L depending on the emulsion stability of the solution being treated.

After about 15 minutes of contact time with the acid and aluminum or calcium, the pH is raised (typically with sodium or calcium hydroxide) to somewhere between 5.5 and 8.5 depending on the surfactant chemistry present. If there is a sufficient amount of oil present, the resulting destabilized oil phase will gradually float, and a water-like phase will separate to the bottom.

MRF Effluent Characteristics After Sulfuric Acid, Aluminum Sulfate, Sodium Hydroxide
Chemical Method

	BOD₅	COD	O&G	pH
Fluid A	500	850	80	5.0
Fluid B	1,100	2,500	200	.5.2
Fluid C	2,000	6,500	500	5.5
Fluid D	1,200	20,000	250	5.0
Fluid E	1,500	22,000	240	5.3
Fluid F	120	28,000	110	6.0

All readings are in mg/L except pH, which is in standard units.

Treatment by Polymers

Polymers can contain metastable salts and/or complex proprietary organic chemistries that have a unique affinity for oil. The polymers draw the oil phases into their organic molecular structure, thus causing a separation. Careful polymer selection can result in very good oil separation, with very high oil/water density ratios. These oil water concentrations after polymer treatment can be as high as 80% (volume/volume).

With very stable emulsions, the addition of an acid, metastable salt, positively charged (cationic) polymer, sodium hydroxide, and a negatively charged polymer (anionic) in a series reaction may be required to produce any effective separation.

Some solutions with strong amounts of chelating compounds, such as sodium EDTA can be virtually impossible to treat by any chemical method.

Overall, these treatment methods can be done in a batch process or in a series reaction with cascading tanks, with each treatment stage in separate tanks. Separation can be enhanced with the use of fine micro-bubble air injected near the bottom of the final process tank. Dissolved air flotation is one common method to provide enhanced micro-bubble separation.

The chemical cost to treat by this method varies by the concentration of oil in the spent metal removal solution and / or the strength of the emulsifiers that are present. The chemical costs, for a 5% volume/volume spent metalworking solution can be between 0.6 and 1.2 cents per gallon. These costs can further vary by the type and amount of chelating chemistries present since they interfere with the precipitation and flocculation process.