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Getting on stream: Gerry Spoors, development manager, Omex environmental looks at the neutralisation and treatment of waste water streams.

Nearly all metal processing, fabrication and finishing processes generate an effluent of some kind. Processes involving metal treatment and processing invariably produce acidic or metal bearing streams which have to be neutralised before discharge, to meet pH and other consents.

For the neutralisation of acidic waste streams, there is really only a choice of three alkalies, and two of these--magnesium hydroxide and calcium hydroxide - are derived from natural minerals. The third, sodium hydroxide, is a by-product of chlorine production and as well as being subject to massive price fluctuations due to the ups and downs of the construction industry (chlorine is used to produce PVC used in pipes and other components), it is a very dangerous chemical if not handled properly.

Investment in new product development and processing technology means that Omex is now the sole UK supplier of magnesium hydroxide suspensions and has also recently started supplying calcium hydroxide suspensions for waste-water treatment applications, as well as magnesium oxide for a wide range of industrial applications.

The company has also recently invested in a new plant that produces calcium nitrate solution for odour control applications.

Magnesium hydroxide neutralisers

Magnesium hydroxide has become recognised as a safe, cost-effective, alternative alkali for the neutralisation of acids.

To meet different customer needs, storage requirements and applications, Omex supplies three grades as concentrated aqueous suspensions all produced at the Bardney plant.

All three Magmex grades use naturally derived magnesium carbonate as the starting material, which is converted to magnesium oxide and shipped to the Omex plant for further treatment and processing. At Bardney, mineral processing involves size classification using a sophisticated cyclone system, followed by hydration under carefully controlled conditions.

For large scale applications requiring accurate pH neutralisation and low volumes of precipitated sludge, Magmex 740 is the standard formulation. For smaller scale operations where longer term storage in IBCs is required without the need for agitation, Omex produces a stable easy-to-use suspension, Magmex 706. Here, hydration is followed by treatment in a roller mill to produce a smaller particle size, and this grade is now being used for pH correction and metal precipitation in a wide range of effluent treatment applications.

For those applications where higher pH levels are required to enhance precipitation using magnesium hydroxide, a new product, Magmex 1060, has been developed. This unique product is made under carefully controlled conditions, and is the subject of a patent application.

Calcium hydroxide suspensions

Omex has also recently introduced a new calcium hydroxide lime suspension. The starting point for the Calmex 40 product is calcium carbonate which, after conversion to calcium oxide, is hydrated and then made into a suspension of 40% solids.

When the customer needs a higher pH or needs to remove sulphate from his system then Calmex 40 calcium hydroxide suspension is the recommended product.

Neutralisation comparisons

What are the relative benefits of magnesium and calcium hydroxide? 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 (sodium hydroxide):

* 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

Let us examine 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 as shown, for example, by the neutralisation of sulphuric acid expressed in gram moles:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

This is displayed graphically in Figure 1.

[FIGURE 1 OMITTED]

Buffering and control

With caustic soda, 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+ = 20H-

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 caustic soda and enables the plant operator to achieve a very high degree of control.

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 NaOH.

Fig 2 shows the comparative sludge volumes achieved with Mg(OH)2, NaOH and Ca(OH)2 when precipitating metals from acidic waste-water.

[FIGURE 2 OMITTED]

Health, safety and handling

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

Magmex magnesium hydroxide suspension is easily pumped, and stored, and has a freezing point of 0[degrees]C, whereas 47/50% NaOH will freeze at 8-10[degrees]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 magnesium hydroxide suspension would allow consent limits to be achieved (Table 1). Furthermore, considerable reductions in sludge volumes were possible.
Table 1 Laboratory treatment of acidic metal wastewater
with Mg(OH)2.

Dissolved
Metal
ConceNtration
(mg/l)

                 pH     Fe     Zn      Cu      Pb    Cr

Original        1.14   4120    43.0    2.20    0.37  17.0

Treated         8.63    0.4       <       <       <     <
                                0.1     0.1     0.1   0.1

Dissolved
Metal
ConceNtration
(mg/l)

                 Ni

Original         7.25

Treated             <
                  0.1


Plant trials were successful, with residual iron concentrations consistently below 1 ppm from iron levels of up to 12g/1 in the raw effluent. Other metal concentrations were reduced to < 0.1 ppm.
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Title Annotation:WATER TREATMENT
Publication:Finishing
Date:Jul 1, 2012
Words:1120
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