Orica - MHL

MHL, Physical Properties

MHL is a highly concentrated liquid alkali containing up to 60 % by weight of magnesium hydroxide. The unique physical properties of MHL, that is its low viscosity, stability to settling, high concentration and low freezing point (0°C), make it very easy to handle. MHL can be stored and pumped in much the same way as liquid caustic soda, eliminating solids handling issues associated with lime and soda ash. Magnesium hydroxide, the active agent in MHL is the major ingredient in many antacid formulations (milk of magnesia) and as such poses little health risk. Accordingly MHL is classified as a non-dangerous and non hazardous alkali reagent.

Product Specification:

Total Solids Content	58 - 62 % w/w
Viscosity	18-30 sec, ASTM 1200 Flow Cup. (corresponds to <100 cP @ 139 sec-1)
Typical Chemical and Physical Properties
Appearance	Off-white low viscosity liquid
Chemical Composition
Magnesium Hydroxide	(Mg(OH)₂) 55 % w/w (Min.)
Calcium Hydroxide	(Ca(OH)₂) 1.0-2.0 % w/w
Silica (SiO₂)	0.5 % w/w
Water	42 % w/w (Max)

Note:

This product is derived from a natural Mineral Ore.
Minor variations in Chemical Composition and Physical Properties may be apparent.
Bulk Density: 1.5 tonne per cubic metre (T/m3) @ 60% w/w solids.
Stability: Non Settling. No agitation required. Viscosity is stable for at least 3 months if stored in sealed container.
Hazard class: Not classified. (Non Hazardous)
Toxicity: Non- Toxic.
(Refer to Materials Safety Data Sheet)
Packaging: 1000 litre returnable semi-bulk container.
Bulk (via road tanker).

Comparative Dose Rates:
Comparison of theoretical alkali equivalents based on stoichiometry allows the calculation of comparative dose rates. The calculation is based on infinite residence time and 100 % utilisation.

Theoretical Alkali Equivalents.
1 T CaO (95 %)	1.6 T MHL
1 T Ca(OH)₂ (20 % slurry)	0.8 T MHL
1 T Na₂CO₃ (100 %)	0.9 T MHL
1 T NaOH (50 %)	0.6 T MHL

Benefits of MHL for pH Control

Dosing Control
Irrespective of how much MHL is added, the maximum pH achievable is within the range of pH 9 - 9.5. This is in contrast to traditional alkalis which if added in gross excess can result in pH values between 12 and 14. Notably, pH 9 is the upper pH limit specified in many trade waste discharge agreements and is approaching the upper pH limit tolerable by many aerobic and anaerobic digester bacteria. This upper pH limit of 9.5, or buffer effect, achievable with MHL essentially eliminates the risk of high pH excursion allowing simplification of dosing control systems.

Scaling
Unlike calcium sulphate, magnesium sulphate is highly soluble. Therefore neutralisation of sulphate containing effluent with MHL will not result in voluminous sulphate sludges or pipe blockages due to sulphate scale formation often associated with lime dosing.

Salinity
By replacing sodium alkali for pH correction with MHL, the levels of sodium in treated effluent is minimised. Further the increased magnesium levels result in a reduction in the Sodium Absorption Ratio (SAR) of the treated effluent. If the effluent is destined for irrigation or tertiary treatment using wetland systems, the reduction in SAR can reduce the risk of soil degradation and subsequent loss in soil fertility. In addition MHL provides beneficial nutrients to plant and animal life.

Operator Safety
Magnesium hydroxide, the active agent in MHL is the major ingredient in many antacid formulations (milk of magnesia) and as such poses little health risk. Accordingly MHL is classified as a non dangerous and non hazardous alkali reagent. MHL can be stored and pumped in much the same way as liquid caustic soda eliminating solids handling issues associated with lime and soda ash.

Benefits of MHL for Treatment of Metal Finishing Effluent

Dosing Control
Irrespective of how much MHL is added, the maximum pH achievable is within the range of pH 9 - 9.5. This is in contrast to traditional alkalis which if added in gross excess can result in pH values between 12 and 14. Notably, pH 9 is the upper pH limit specified in many trade waste discharge agreements and coincides with the minimum solubility of many metal species. The upper pH limit of 9.5 achievable with MHL eliminates the risk of high pH excursion preventing the re-solubilisation of precipitated metals.

Sludge Volume Reduction
The inherent safety of MHL and the observed buffering within the pH range of 9-9.5 when added to solution are both the result of the relatively low solubility of magnesium hydroxide. An additional benefit stemming from this low solubility is apparent upon the neutralisation and precipitation of acidic metal containing effluent. Traditional alkalis raise pH and precipitate metals very quickly upon treatment. This often results in a very fine metal precipitate which can form large volumes of slow settling, gelatinous sludge. Sludge of this nature is often very difficult to de-water. In contrast metal precipitation with MHL occurs gradually. The result of this is a relatively coarse, particulate metal precipitate which settles rapidly to form a dense, porous and compact sludge with excellent filterability and de-watering characteristics.

Sludge Stability
Again as a result of the low solubility of magnesium hydroxide, the addition of a small excess MHL c an result in a solid waste that exhibits exceptional stability with respect to leaching easily passing the Australian leachate criteria (TCLP) without the need for additional fixation. The un-reacted MHL intimately mixed with the metal precipitate maintains a high pH environment (9 — 9.5) coinciding with the minimum solubility of many proscribed metal hydroxides. This can result in significant reductions in sludge disposal costs.

Operator Safety
Magnesium hydroxide, the active agent in MHL is the major ingredient in many antacid formulations (milk of magnesia) and as such poses little health risk. Accordingly MHL is classified as a non-dangerous and non hazardous alkali reagent. MHL can be stored and pumped in much the same way as liquid caustic soda eliminating solids handling issues associated with lime and soda ash.

Mechanism of Acid production and Concrete Corrosion
The generation of acid resulting in sewer corrosion is understood to be a two step process. In the first step, sulphate reducing bacteria in the sewage reduce sulphates sulphides. At the near neutral pH of sewage, the bulk of the sulphide is in the form of molecular hydrogen sulphide which volatilises at the liquid / gas interface and concentrates the sewer headspace. In the second step, gaseous hydrogen sulphide dissolves in the moisture present on the sewer wall where sulphur oxidising bacteria oxidise H₂S to sulphuric acid.

The sulphuric acid reacts with the calcium hydroxide in the concrete to form calcium sulphate as a corrosion product. Calcium sulphate is a soft, expansive compound with no binding properties. Unchecked corrosion will eventually lead to the destruction and collapse of the sewer.

SulfaLock^TM HiGel Physical Properties
A unique feature of the regular MHL is the combination of low viscosity and high solids content. In addition to this, regular MHL exhibits what is referred to as a gel characteristic. Gelling describes the tendency of the liquid to lose fluidity and form a solid gel structure some hours after the removal of shear whilst upon the application of shear, the gel instantaneously reverts back to its fluid state. This property has been manipulated resulting in a SulfaLock^TM HiGel, a low viscosity liquid alkali under moderate shear that gels instantaneously upon the removal of shear. This property enables the pumping and spraying of a relatively low viscosity liquid and the application of a paste like solid. With SulfaLock^TM HiGel, a 3 mm high pH magnesium hydroxide coating can be applied in a single spray pass with very little run off.

Benefits of SulfaLock^TM HiGel for Sewer Line Corrosion Protection

Extremely Effective Corrosion Protection
Results obtained in an actual acidic sewer application show that SulfaLock^TM HiGel can be sprayed on to sewer walls where it forms a very durable alkaline surface coating. Where applied, a pH of between pH 7 and 9 was maintained on the concrete substrate for a twelve month period effectively precluding further concrete corrosion through acid attack. This contrasts strongly with the pH values of 2 - 3 measured in similar unsprayed areas. One year after application, the applied coating con tinued to provide coverage over the concrete substrate. These results suggest that the spray application of SulfaLock^TM HiGel onto sewer walls constitutes a viable solution to the problem of acid corrosion of sewer lines.

Simple and Reliable Technology Replacing Conventional Chemical Dosing,
The addition of large quantities of chemicals to the raw sewage has historically been employed in an attempt to effect the prevention of sewer corrosion. Fundamentally, chemical addition attempts to provides corrosion protection by limiting the volatile sulphide levels in the raw sewage. The flow-on effect of this is a reduction in the concentration of gaseous hydrogen sulphide in the sewer head space which in turn limits the amount of sulphuric acid formed on the sewer wall.

However chemical dosing has been found to be intrusive, expensive and as a result of a number of variables, not always effective. Where H₂S formation is not completely eliminated the formation of sulphuric acid will still occur.

In contrast, sewer corrosion control by the spray application of magnesium hydroxide onto the sewer wall is achieved by directly curtailing acid generation. The basis of the technology is that the application of an alkaline surface coating firstly neutralises acid present at the concrete surface and further more, ensures that a high pH is maintained on the concrete surface. It is the enduring alkaline surface coating which is believed to constitute the major advantage of corrosion prevention in that under alkaline conditions, further acid formation, and subsequently corrosion, is avoided by significantly retarding the proliferation of the acid forming Thiobacillus concretivourous bacteria. Thus the technology is able to achieve corrosion protection irrespective of the gaseous H₂S concentration and unlike chemical dosing, is not effected by changes in temperature, pH, flow and effluent composition.

Cost Effective
In comparison to high capital options, ie epoxy sewer relining and ventilation, and continuous chemical dosing, corrosion protection using the spray application of SulfaLock^TM HiGel is a cost effective alternative.