Gerry Spoors, development manager,Omex Environmental, says that using magnesium hydroxide, as a stable suspension, rather than caustic soda or lime, to treat acidic effluents, has a number of technical and cost advantages.  

Many metal treatment processes produce acidic waste streams, which have to be neutralised before discharge, to meet pH and metal consents.  Traditionally caustic soda and lime have been used, but these materials are difficult and dangerous to handle, and often create other environmental problems.

The viability of using magnesium hydroxide as a safe, cost-effective alternative alkali has now been recognised, and Omex have introduced a stable, easy-to-use suspension, Magmex 706, which is now being used for pH correction and metal precipitation in a wide range of effluent treatment applications.

What are the benefits of magnesium hydroxide that are making it so popular? When used in aqueous suspension, magnesium hydroxide reacts in a controlled manner to raise the pH to the desired level within the discharge consent range.  In so doing, it has the following advantages over caustic soda and lime:

• Greater alkaline efficiency

• ‘Buffers’around pH 9.5

•  Easy to control

•  Produces lower metal hydroxide sludge volumes

•  Very safe to handle

• Delivered as a stable suspension that requires no agitation

• Cost effective

Additionally, it does not produce sulphate sludge (compared to lime), nor does it freeze at up to 100C, as NaOH solution does. Let us examine some of these points in more detail.

Greater alkaline efficiency

One tonne of Mg(OH)2 will do the same job as 1.37 tonnes of NaOH or 1.27 tonnes of Ca(OH)2 as shown, for example, by the neutralisation of sulphuric acid expressed in gram moles:

Mg(OH)2 + H2SO4 =  MgSO4 + 2H2O

58.3           98           120.3        36

2NaOH    + H2SO4 =  Na2SO4 + 2H2O

     80           98             142         36

Ca(OH)2   + H2SO4  =  CaSO4.2H2O(ppt)

 74            98              172

In the case of lime, it can be seen that the product is solid calcium sulphate, compared to the soluble magnesium sulphate.  Caustic also produces a soluble sulphate, but this can crystallise out – for instance in pipelines - if conditions are right.


Buffering and control

With caustic soda or lime, accidental overdosing can cause the pH of the discharged effluent to rise above consent levels, resulting in significant environmental discharge. This cannot happen with magnesium hydroxide, which reacts in a two-stage manner.  This is due to its relatively low solubility which leads to a limited number of hydroxyl ions being initially present in solution.  Only when these have taken part in acid neutralisation can further hydroxyl ions become available.

Provision of hydroxyl ions takes place according to the reaction:

Mg(OH)2  =  Mg2+  =  2OH


Rapid reaction with the acid at low pH gives way to a much slower reaction as the pH gradually rises and the acid is neutralised.  This prevents the ‘spiking’ that occurs when using the other alkalies and enables the plant operator to achieve a very high degree of control.  Furthermore, even a massive overdose is unlikely to take the pH beyond 9.5.


Low sludge volumes

When used for removing metals from solution, the slightly slower reaction rate when using Mg(OH)2 results in the formation of  a more granular metal hydroxide precipitate which settles faster and traps much less water.  Hence there is less water to filter off and less sludge to dispose of, than when using other alkalies.  Fig 1 (below) shows the comparative sludge volumes achieved with Mg(OH)2, NaOH and Ca(OH)2 when precipitating copper from solution.

With sulphuric acid based effluents, soluble magnesium sulphate is formed, removing the problem of sulphate disposal.  Lime produces insoluble calcium sulphate.


Health, safety and handling

Magnesium hydroxide in aqueous suspension is a gentle alkali that does not burn the skin or eyes, unlike lime and caustic soda which are hazardous materials.  In fact, a pharmaceutical grade of magnesium hydroxide – milk of magnesia – has been used as an ant-acid medicine for many years.

Magmex 706 magnesium hydroxide suspension is easily pumped, and stored, and has a freezing point of 0˚C, whereas 50% NaOH will freeze at 10˚C and so often has to be trace heated for effective handling.

Overall, magnesium hydroxide is a safe and effective alkali for the neutralisation of acidic wastes, and for the precipitation of metals.


Case Study – Metal Cleaning

A company using sulphuric acid to clean the surface of steel rods was using 47% sodium hydroxide for effluent treatment.  Significant problems were encountered:

• Difficulty in achieving discharge consent levels, particularly for iron and suspended solids.

• Occasional overdosing leading to high pH discharges.

• Storage and handling problems, including freezing of caustic solution during cold weather

A laboratory evaluation showed that treatment with Magmex 706 would allow consent limits to be achieved (Table 1).  Furthermore, considerable reductions in sludge volumes were possible (Fig 2).  Plant trials were successful, with residual iron concentrations consistently below 1ppm from iron levels of up to 12g/l in the raw effluent.   Other metal concentrations were reduced to <0.1ppm.