Orica - MHL

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 can 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.