Treating metal-contaminated waste streams with’.
magnesium hydroxide, the main ingredient of “Milk-of
Magnesia”. . .
By J O H N T E R I N G O III
Project Leader
Specialities Chemical Department
Dow Chemical U.S.A.
Midland, Michigan
eutralizing acidic industrial
waste streams is generally accomplished by adding caustic soda or lime. In many cases, these
two chemicals are the most practical
choice. But another alternativemagnesium hydroxide-has definite
advantages, especially when the effluent contains metals from plating
operations.
Magnesium hydroxide is less hazardous than either caustic or lime. It
does not cause the corrosion problems of caustic or the deposition
problems of slaked lime. Magnesium
hydroxide offers several other benefits in acid neutralization, but it really
begins to make economic sense in
metal-removal applications.
At first glance, magnesium hydroxide may appear to be less attractive, than either lime or caustic be-48
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1. SCANNING ELECTRON MICROSCOPE
shows copper hydroxide particles,
enlarged 20,000 times, precipitated by
caustic soda (sodium hydroxide). These
small particles, combined in a web-like
structure, entrain a great deal of water
and are difficult to filter.
cause of its generally higher cost. A
closer examination of all the costs
related to treatment, however, has
lead a number of companies to
choose magnesium hydroxide.
All three treatment chemicals are
capable of effectively removing metals from effluent by converting them
to metal hydroxides. The difference
lies in how they convert metals.
AUGUST, 1987
2. COPPER HYDROXIDE PARTICLES in
this photograph, also enlarged 20,000
times by the scanning electron
microscope, were precipitated by lime
(calcium hydroxide). These particles,
although somewhat larger than those
precipitated by caustic soda, are still
quite small, making them difficult to
filter.. They, too, entrain large quantities
of water.
Magnesium Hydroxide Vs.
the Others
Both lime and caustic are more
soluble than magnesium hydroxide;
they dissociate rapidly when added to
wastewater, to provide hydroxyl ions;
The hydroxyl ions quickly combine
with metal ions, but the reaction ocAUGUST, 1987
3. TWENTY THOUSAND times
magnification of copper hydroxide
precipitated by magnesium hydroxide.
Because of their large size and density,
these particles entrain little water and
can be easily filtered.
curs so quickly that there is little or
no time for crystal growth or
agglomeration.
The result is a precipitate of very
small particles combined in a weblike structure that actually traps water and forms a gel-like sludge (Figs.
1 and 2). The small particles are hard
to filter, and after filtration the
sludge will still contain. a large
amount of water.
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4. RELATIVE VOLUME and density of the
same metal hydroxide sludge can be
compared in this photograph- No
flocculant has been added to any of
these samples. Precipitation was by
sodium hydroxide at left, calcium
hydroxide in the middle, and by
magnesium hydroxide at right. Note
amount of light shining through the
bottom tip of each sample, indicating
the relative density of each sludge.
In contrast, the low solubility of
magnesium hydroxide slows the de50
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velopment of metal hydroxides
enough to allow crystal growth. The
large crystals created produce a more
dense sludge that is easier to dewater
(Fig. 3). Because of the high density
and low water content, the volume of
dewatered sludge produced is considerably less than that produced by
an equivalent system using lime or
caustic.
The difference in sludge volume
depends on the composition of the effluent, but based on extensive experience in chromium plating operations, a 50 to 60 pct reduction in
sludge volume is not uncommon.
The effects of reduced sludge volume on handling can be substantial.
Table I shows the percent solids obtained, filter press cycle times and
densities of dewatered sludge from a
chromium plating operation, based
on treating an equivalent volume o
waste with caustic, lime, and magnesium hydroxide respectively.
Estimating Treatment and
Disposal Costs
How the lower sludge volume of
magnesium hydroxide system affect
AUGUST, 1
operating costs can be shown in a
typical example for a chromium plating operation. The basic assumptions
used in the example are as follows:
l 500 tons per year lime consumption for hydroxide requirements.
l
One ton CaO = one ton
Mg(OH)2 = 1.4 tons NaOH on an
equivalent alkali basis.
l Lime
p r i c e d a t $60/ton
delivered.
l
Mg(OH)2 priced at $250/ton
delivered.
l
NaOH priced at $150/ton
delivered.
l
Sludge produced-18 cubic
*yards/day with lime, six .cubic
yards/day with Mg(OH)2, 21 cubic
yards/day with NaOH.
l
$70/cubit yard sludge disposal
cost.
l 250 days/year operating rate.
Using the above information to determine the total annual cost of treatment chemicals and sludge disposal
yields the data shown in Table II.
These estimates do not include labor savings resulting from decreased
use of the filter press, the capital savAUGUST, 1987
ings that could be obtained by installing a smaller filter press or not having to install additional filter-press
capacity, nor the reduced maintenance cost of operating the equipment for shorter periods of time. It
is estimated that in the example,
switching from caustic to magnesium
hydroxide would allow a reduction
from one hundred 1.2-square-meter
plates to thirty l-square-meter plates.
How it’s Done
Magnesium hydroxide does not react in the same way as caustic and
lime. Whereas lime or caustic are added to the ‘waste stream until the
desired pH is achieved, adding magnesium hydroxide in this same way
will result in a gross overuse and
waste of the chemical.
When magnesium hydroxide is
used to neutralize an acidic metal
waste stream, a necessary residence
time must be allowed before the effluent will achieve the desired pH. A
number of factors affect this residence time, including the type and
concentration of metal(s) to be
removed.
PRODUCTS FINISHING 51
pH vs Time in Neutralization/Precipitation
of 1000 ppm Cr+ 3
0
40
20
Time (min)
60
5. MAGNESlUM HYDROXIDE is added,
the pH rises rapidly, and then levels off
between five and six. As the metal
hydroxide precipitates, after about 20
min, the pH again begins to rise.
Magnesium hydroxide, however, buffers
at a pH of 9.0, so even over-treatment
will riot increase pH above this level.
Fig. 5 shows what actually happens
to pH when magnesium hydroxide is
added to a waste stream containing
1000 ppm of chromium. When the
Mg(OH), is first added, the pH rises
rapidly, indicating that the acid is being neutralized. The pH then levels
off between five and six while the
Cr(OH)2 sludge is being produced.
The pH remains within this range until all the soluble chromium has been
precipitated, and then rises again.
The reason for this and other pH
plateaus is that at a particular pH, the
magnesium hydroxide is dissolving
(i.e., providing hydroxyl ions) at the
same rate as the soluble metal ions
are reacting with the hydroxyl ions to
form the metal hydroxide sludge.
Then as the metal hydroxide precipitates, a gradual buildup of hydroxyl
ions occurs and the pH rises.
Because of the slow pH response
of magnesium hydroxide, another
method is needed to determine the
proper amount to add to the waste
stream. The recommended method
requires some initial experimentation
and calculation, after which the process becomes quite simple.
For batch-treatment systems, a
100-ml sample of waste is taken and
titrated with 1N NaOH to the desired
pH to determine the amount of alkali
required to neutralize the acid and remove metals. The volume of 1N
NaOH used in the titration is multiplied by an alkali equivalency factor
to determine the quantity of magnesium hydroxide required to neutralize that batch.
The amount of magnesium hydroxide is calculated as shown in Equation 1. Add this equivalent amount
E q u a t i o n
ml IN NaOH
ml 1N NaOH
52
x
x
1
Pounds of dry powder
Gallons of
water to x 0.000254 = magnesium hydroxide
(Dow MHT-100) to use.
be treated
Gallons of 55-60%
Gallons of
water to x 0.000373 = magnesium hydroxide slurry
(Dow MHT-50S) to use.
be treated
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AUGUST, 1987
of magnesium hydroxide and WAIT
until the pH reaches the desired level.
Experience with Mg(OH)2 neutralization has shown that the majority of
metal is removed during the first 230
minutes. If sufficient resident time is
not available for the full reaction, the
batch may be “topped off” with lime
or caustic (typically less than 10 pct)
to remove the remaining metal. This
will produce a somewhat greater volume of sludge than when using magnesium hydroxide by itself, but the
sludge volume- will still be considerably less than if conventional alkalies
were used alone.
Incorporating the proper amount
of magnesium hydroxide into a continuous-treatment system can be handled easily once the correct information is determined ahead of time. As
in batch treatment, a 100-ml waste
sample is titrated with 1N NaOH to
the desired pH. An equivalent
‘amount of magnesium hydroxide is
calculated using Equation 2.
Equation 2
Grams of magnesium hydroxide re=
quired for 100 ml of waste
ml 1N NaOH x 0.03061
%Mg(OH)z
The calculated amount of magnesium hydroxide is then added to
another 100 ml of waste. By monitoring pH vs time for this sample, a neutralization curve can be plotted that
will show a lower initial target pH
that must be achieved in order to
reach the desired final PH. Once the
AUGUST, 1987
A l k a l i
8
pH 6
4
2
0
20
40
Time (min)
60
6. THREE-STAGE continuous
neutralization, with residence time of 20
min in each stage. The pH vs. time
profile, as shown here, provides a
graphic means for adjusting pH in each
stage to obtain the desired pH at the
output. The system illustrated represents
neutralization and precipitation of 1,000
ppm Cr+ 3 .
system is in operation, it will probably be necessary to fine-tune the initial target pH to compensate for actual operating conditions, in order to
achieve the final pH.
Fig. 6 is a hypothetical example of
how magnesium hydroxide is used in
a three-stage continuous operation.
In this treatment system, each of the
three stages has a 20-min residence
time, achieving a final pH of 8.0 in
the third stage.
A pH vs time profile was develPRODUCTS FINISHING 53
oped experimentally by testing a
waste-stream sample. It shows that in
order to achieve a pH of 8.0 for effluent leaving the final stage, a sufficient amount of magnesium hydroxide must be added in the first stage
to achieve a pH of 5.0.
If sufficient time is not available
for the three-stage neutralization,
several options are available:
1. Addition of alkali in upstream
sumps or collection lines.
2. Partitioning of neutralization
tanks or basins to prevent
short-circuiting.
3. Increased agitation.
4. Additional tanks.
5. Topping off with caustic soda.
When an existing system for the
neutralization of acid/metal waste
streams is converted to the use of
magnesium hydroxide, the user must
modify his/her waste-treatment operating procedures and/or the equipment. Based on actual operating experience, however, these changes are
relatively easy to incorporate.
Special Considerations for
Nickel Removal
Nickel is often a component in acid
54
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waste streams generated by plating
operations. Nickel hydroxide,
Ni(OH)2, does not precipitate until
the pH is above 9.2. Although magnesium hydroxide achieves a pH of
only 9.0, this alkali will still remove
the nickel present in a waste stream
through adsorption onto the surface
of magnesium hydroxide particles.
The amount of nickel removed
varies. It depends on how long the
magnesium hydroxide is allowed to
react. If sufficient time is not available, the waste stream should be
topped off with caustic.
Magnesium hydroxide provides the
unique option of nickel recovery. It
works by selectively precipitating unwanted metals, leaving nickel in solution. After solid/liquid separation
of the nickel-rich solution, nickel can
be recovered as a separate Ni(OH)2
precipitate by the use of caustic, or
it can be recovered as metallic nickel
by electrolysis.
As illustrated in Table III, magnesium hydroxide does an excellent
job of removing copper, chromium,
iron and zinc, leaving a purified
nickel-rich solution. On the other
AUGUST, 1987
hand, notice that the sample treated
with caustic, even with very careful
attention to reach a pH of only 7.4,
removed over two-thirds of the
nickel, along with the other metals.
Note also the sludge volume formed:
with magnesium hydroxide treatment, the sludge volume ‘is a mere 11
pct of what it is with caustic.
Summary
While initial chemical costs would
seem to make magnesium hydroxide
unattractive, other cost factors tend
to offset its expense. Mg(OH)2 is
safer to use. It is provided as a slurry,
eliminating the slaker required for a
lime system. It is also less abrasive,
without the scaling effects of
hydrated lime.
Magnesium hydroxide doesn’t require equipment with the level of corrosion resistance necessary for handling caustic. In addition, because
magnesium hydroxide buffers at a
pH of nine, overtreatment should not
cause detrimental effects on downstream biological-treatment systems.
In metal-contaminated waste
streams, magnesium hydroxide offers
the definite advantage of lower
sludge volume and disposal costs. If
nickel is to be removed and possibly
recovered, magnesium hydroxide can
improve recovery efficiency by selectively removing non-nickel metals
from the waste stream prior to nickel
recovery. While treatment procedures
for magnesium hydroxide are different than those for caustic or lime,
they can be easily learned.
PF
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