Note: Descriptions are shown in the official language in which they were submitted.
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HYDROMETALLURGICAL PROCESS AND SYSTEM
FOR DECONTAMINATING SLUDGES AND/OR SOILS
BACKGROUND OF THE INVENTION
1. Field of the invention:
The present invention relates to a process and system
for decontaminating sludges andlor soils, in particular but not exclusively
the fine fraction of these contaminated sludges andlor soils.
2. Brief description of the prior art:
Sludges and sediments contaminated with heavy metals
andlor organic materials pose a serious environmental threat world-wide.
This is mainly due to the contaminants' toxic nature as well as the non-
biodegradability and the risk of bio-accumulation of these contaminants.
There is increasing international concern considering the enormous
volumes of contaminated material. Therefore, it becomes urgent to
develop economical treatmentldecontamination technologies.
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The current technologies available for the
decontamination of sludges andlor soils focus mainly on a physical
separation of a fine fraction (particle size smaller than 60 Nm) from a
coarse fraction (particle size larger than 60 Nm). The coarse fraction can
be treated by numerous physical and chemical methods. Physical
treatment tools such as hydrocyclones, density separators and magnetic
separators are economically and technically more attractive and are
therefore currently used to treat the coarse fraction. The remaining fine
fraction, consisting mainly of clay, silt and humic substances remains
untreated. This fine fraction is usually more contaminated than the
coarse fraction due to its large surface area and high
adsorptionlabsorption capacity. Moreover, the fine fraction has very
unfavourable handling and dewatering characteristics.
In the case of river and harbour sediments, the bulk of
the contaminated solids is very fine and forms sludges with very low solid
contents (20% to 35% by weight of solids). As the above mentioned fine
fraction, these sludges cannot be treated physically. Accordingly,
chemical processes have been proposed for cleaning these hazardous
substances. Prior art chemical processes have utilized, for example, a
combination of acid leaching and advanced oxidation for dissolving the
heavy metals and decomposing the toxic organic pollutants, leaving
behind "clean" solids. Unfortunately, the particle size and the ability of the
fine fraction to adsorb/absorb heavy metals and retain water makes
handling and dewatering extremely difficult. Since separation of the
heavy metals from the solidlliquid mixture is inefficient, at least a part of
the heavy metals are retained in the solidlliquid mixture. Also, most of the
prior art chemical processes are uneconomical due to the difficulty of
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obtaining clean, easily washable residues with low water content. Other
problems are related to the high cost of the reagents and equipments.
OBJECTS OF THE INVENTION
An object of the present invention is to overcome the
above described drawbacks of the prior art and to improve
decontamination of sludges andlor soils by means of a hydrometallurgical
treatment giving special attention to the fine fraction.
SUMMARY OF THE INVENTION
More specifically, in accordance with the present
invention, there is provided a process for decontaminating sludges and/or
soils by means of acid leaching, in particular but not exclusively sulfuric
acid leaching and subsequent controlled precipitation and aggregation of,
for example, gypsum on the inert sludge andlor soil particles.
The objects, advantages and other features of the
present invention will become more apparent upon reading of the
following non restrictive description of a preferred embodiment thereof,
given by way of example only with reference to the accompanying
drawings.
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BRIEF DESCRIPTION OF THE DRAWINGS
In the appended drawings:
Figure 1 is a schematic diagram of a system according
to the present invention, for decontaminating sludges andlor soils by
means of a hydrometallurgical treatment;
Figure 2 is a graph of the contents of metal versus time
illustrating a typical leaching behaviour of sludges; and
Figure 3 is a SEM picture of clean residue obtained with
the process and system in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The coarse fraction (particles having a size larger than
60 Nm) of the contaminated sludge and/or soil is first separated from the
fine fraction (particles having a size smaller than 60 Nm). The coarse
fraction is then cleaned by means of conventional physical techniques
such as hydrocycloning, sieving, magnetic separation, density separation,
flotation, other physical sludge and/or soil washing techniques, etc.
Although in the present disclosure the coarse fraction of
the contaminated sludge andlor soil is defined as including the particles
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having a size larger than 60 Nm and the fine fraction is defined as
including the particles having a size smaller than 60 pm, it is within the
scope of the present invention to depart from the 60 Nm size value to
delimit the coarse and fine fractions.
5 The fine fraction then remains untreated. The particles
of the fine fraction are usually in the form of a sludge containing 20-35%
of solids by weight. Further dewatering through techniques such as
thickening, flocculation, coagulation, filtration, etc. is difficult, and
further
treatment of the particles of the fine fraction has to take into consideration
the large volume of water accompanying the contaminated fine fraction.
Referring to Figure 1 of the appended drawings, the fine
fraction 2 is first leached in dilute sulphuric acid 3 at a slightly elevated
temperature in a leach tank 1.
Acid leach
For leaching purposes, the fine fraction 2 is injected in
the leach tank 1 along with concentrated acid 3, in particular but not
exclusively concentrated sulfuric acid (H2S04). A pH situated between 0
and 1 is maintained in the leach tank 1. In the leach tank 1, the fine
fraction 2 of the sludge andlor soil 2 contacts the concentrated acid 3 at
a temperature between 40°C and 100°C. The level of temperature
in the
leach tank 1 depends on the ease of dissolution of the contaminants by
the concentrated sulfuric acid 3 and is determined experimentally. Steam
4 is injected in the leach tank 1 to heat the contents. Metal contaminants
in the form of oxides andlor hydroxides are directly dissolved to form
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metal sulfate complexes. An oxidant 5 may be added in the leach tank
1 to dissolve metal contaminants such as metallics and sulfides.
Hydrogen peroxide (H202) or a mixture of SO~lair may be used as oxidant
5. When oxidation is required to dissolve all the metal contaminants, the
temperature in the leach reactor (leach tank 1 ) is usually maintained
within the range 60-100°C. Just a word to mention that a plurality of
leach tanks (leach reactors) could be used.
Acid concentrations of 0.1 to 1.0 M are required in the
leach reactor (leach tank 1) for effective leaching kinetics and for
generating a sufficient quantity of gypsum crystals during the subsequent
neutralization (precipitation and aggregation) step. Waste sulfuric acid or
low grade acid can be used as raw material. Most metals such as Cu, Zn,
Co, Ni, AI, As, Cd, Mn, etc. can readily be dissolved into a solution;
metals such as Pb, Ba, Sr and Hg however form insoluble sulfate
compounds. Extraction of the latter metals (such as Pb, Ba, Sr and Hg)
is only partially possible but on the other hand they are effectively
immobilized and no longer form a threat to the environment due to the
very low solubility of the metal sulfate compounds.
Organic contaminants such as PolyAromatic
Hydrocarbons, PCB's (polychlorinated biphenyl), volatile organics and
other carcinogenicltoxic hydrocarbons are partially or totally decomposed
under the strongly acidic and oxidizing conditions prevailing into the leach
tank 1. If volatile organics exist, a waste gas scrubber (not shown) is
employed to capture these contaminants and to allow the vents from the
leach reactor (leach tank 1 ) to be released to the atmosphere.
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Due to the formation of C02 gas in the leach reactor
(leach tank 1 ) the formation of froth is sometimes a problem. De-frothing
agents (not shown) can be added to ensure trouble free operation.
The acidic conditions in the leach tank 1 ensures
dissolution andlor decomposition of the metal contaminants while leaving
the bulk (clay, silt and sand) unreacted. More specifically, after all the
contaminants have been dissolved andlor decomposed, the slurry
consists of clean clay, silt, sand andlor other fine solid particles in an
acidic metal sulfate solution matrix. This acidic slurry is pumped to a
specialized neutralization circuit to render the sludge filterable and to
remove the bulk from the remaining free acid.
Neutralization
The subsequent controlled neutralization (precipitation
and aggregation) step is performed under such conditions that the
cleaned solids have improved settling and dewatering characteristics. An
inert mineral (for example gypsum) precipitates on the fine particles of the
fine fraction and causes aggregation of these fine particles. This results
in an increase of their apparent diameter, and a reduction of their surface
area.
Referring to Figure 1, the specialized neutralization
circuit comprises a series of two tanks 6 and 7, including a primary
neutralization tank 6 and a secondary neutralization tank 7. Although a
series of two neutralization tanks 6 and 7 is illustrated in Figure 1, it is
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within the scope of the present invention to use a single neutralization
tank or a series of more than two neutralization tanks.
Acidic slurry from the leach tank 1 is pumped to the
primary neutralization tank 6. In the same manner, acidic slurry from the
primary neutralization tank 6 is pumped to the secondary neutralization
tank 7. A base 8 such as lime (Ca0), calcium carbonate (CaC03) or
(CaMg)C03 is added to the acidic slurry in both tanks 6 and 7. The same
temperature as in the leach tank 1 is maintained in the tanks 6 and 7. In
the tanks 6 and 7, the concentration of H2S04 varies between 0.1 and
0.0001 M. Gypsum precipitates upon addition of calcium ions to the slurry
and acid is neutralised at the same time according to one of the following
reactions:
Ca0 + H SO + H O -1 Ca SO ~ 2H O
2 4 2 9 2
Ca C03 + HZ SOq + HZ O ~ Ca S O9 ~ 2 Hz O + COZ
The rate of addition of the base 8 in tanks 6 and 7 is controlled such that
the gypsum crystals form long needles and aggregate with the fine
particles of the fine fraction to form large crystalline aggregates. The rate
of addition of the base 8 is also controlled so that the pH in each tank 6,
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7 is constant and situated between 1 and 4, but the pH (pH2) in tank 7 is
higher than the pH (pH,) in the previous tank 6. Seed crystals can be
added to the primary neutralization tank 6 to further enhance the
precipitation/aggregation conditions by avoiding homogeneous nucleation
of gypsum crystals to occur. Under such supersaturation controlled
precipitation conditions very large aggregates of gypsum and clay can be
obtained. However, for the effective treatment of certain contaminated
sludges andlor soils, addition of seed crystals may not be necessary.
The final pH (pH2 in the secondary neutralization tank 7 in the illustrated
example) is chosen such that no metal precipitates through hydrolytic
reactions. This usually means that the pH can be raised to 3-4, without
any metal co-precipitation. The final pH is determined experimentally for
each individual slurry stream and is mainly dependent on the types of
metals to be removed and their concentrations.
Solidlliquid separation
The above described aggregation of the fine particles to
increase the diameter of these particles and reduce their surface area
tremendously improves solidlliquid separation.
The slurry from the secondary neutralization tank 7 is
pumped to a filter 9 where the fineslgypsum aggregates are filtered from
the slurry. Due to the high degree of crystallization and the relatively
large particle size, filtration is fast and can be performed by continuous
vacuum filters rather than pressure filters.
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The filter cake is formed by a clean residue 10
containing between 60% to 80% of solids, and no longer constitutes a
threat to the environment. It can be safely returned to its original site, or
transported elsewhere for any purpose. In certain cases the clean
residue 10 can be used as construction material, filling material, cement
5 aggregate, etc.
Recycled or fresh water 11 is used to wash the filter
cake on the same filter 9. The filtrate and wash water 12 are combined
and sent to a filtrate neutralization tank 13. The filtrate and wash water
10 12 are a dilute sulphuric acid solution containing all heavy metals.
Filtrate treatment
In the filtrate neutralization tank 13, the dilute sulphuric
acid solution containing all heavy metals is neutralized with a base 14, for
example lime (Ca0), to precipitate the dissolved metals as metal
hydroxides.
The filtrate and wash water 12 have a temperature
situated between 40°C and 100°C and still contains all the
dissolved
metals. The dilute sulphuric acid solution formed by the filtrate and wash
water 12 is neutralized with a base such as lime to a pH situated between
7 and 10. At this pH, all metals precipitate from the solution as metal
hydroxides andlor oxides. The remaining sulfate in the solution is
converted to gypsum crystals. The final overall metal concentration is
lower than 1 mglL.
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The slurry 20 from the filtrate neutralization tank 13 is
thickened in a thickener 19.
The underflow 21 from the thickener 19 is filtered on a
vacuum or pressure filter 22. The solid residue 15 from the filter 22
contains a high concentration of metals and is usually disposed of as
toxic waste. The liquid outgoing stream of the filter 22 is clean water 16
that can be discharged (see arrow 17) or recycled (see arrow 18) to
supply the leach tank 1 andlor as water 11 to wash the cake of the filter
9 as explained in the foregoing description.
When Gust one or two metal contaminants are present
in the slurry 20, it may be economically and technically viable to
selectively precipitate the metals and recycle them as concentrates for
the industry. Generally, less than 5-10% of the original contaminated
solids report to such waste residue.
Waste water treatment
The overflow 23 from the thickener 19 may be combined
with the liquid outgoing flow 16 from the filter 22. This water may still
contain traces of residual dissolved organic substances, and must be
treated prior to discharge into a waterway or sewer. If water (see 18) is
recycled to the process, additional treatment may not be necessary.
When required, water treatment is conducted by conventional aeration
methods, as commonly used in the water treatment industry. Biological
treatment may be used as well, especially if residual sulfate needs to be
removed.
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Therefore, the original contaminated sludge andlor soil
is basically converted into three outgoing streams:
- the clean bulk residue 10 consisting of clay, silt, sand, gypsum, etc.;
- the small amount of residue 15 containing all the heavy metals in the
form of hydroxides; and
- clean water 16.
Table 1 presents a summary of the process conditions:
Table 1:
Stage [HZSO,] pH T(C) Base OxidantRemarks
1 Leach 0.1-1.0 0-4 40-100---- H202 Dissolution
M
SOZ/airof metals:
Decomposition
of organics
Neutralization0.1-0.0001M1-4 40-100CaO, CaC03.--- Precipitation
&
(CaMg)C03 aggregation
of
gypsum
Filtrate ---- 7-1040-100Ca0 ---- Precipitation
of
treatment dissolved
metals
such as Cu,
Zn, Ni,
Cd, Pb, etc.
as
hydroxides
Testwork results
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Heavy metal extraction:
Tests were conducted on laboratory scale on several
polluted sludges and soils. These contaminated samples originated from
different sites and contained different types and levels of pollution. Prior
to testing the samples were separated into coarse (particle size greater
than 63 pm) and fine (particle size smaller than 63 Nm) fractions by
means of wet sieving. The fine fraction was recovered as a sludge
containing approximately 30% by weight of solids, and was as such used
for testing. The sludge was heated to a temperature ranging between
40°C and 90°C on an electrically heated hot plate. Sulfuric acid
was
added to obtain an acid concentration varying between 1.OM and 0.5M.
Hydrogen peroxide was added to obtain a 0.02M H202 solution for those
samples that contained metallics. Samples were taken as a function of
time to evaluate the leaching kinetics. It was found that most of the
metals dissolved within the first 10 minutes of the leach. Longer leach
times did not result in higher metal concentrations in the solution. The
results of a typical leaching experiment are shown in Figure 2. It shows
that both zinc and copper, which were present as very fine metallic
particles, were leached almost instantaneously upon addition of Hz02 and
sulfuric acid. This kind of fast leaching kinetics was observed for all
treated sludge samples, which means that very short leach residence
times can be maintained and thus only small reactor volumes are needed.
After leaching for one hour the sludge was neutralized
with a milk of lime (Ca0 in water) to a final pH ranging between 3 and 4.
Neutralization was performed in a supersaturation controlled manner,
which practically meant the addition of Ca0 in small dosages over a
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period of 30 minutes. This lead to the formation of large gypsum crystals
and aggregates of the clay and silt particles with these gypsum crystals.
The slurry was subsequently filtered on a vacuum filter and washed with
tap water. The filtration rates obtained were typically of the order of 200-
300 kglm2~h. This filtration rate includes a wash sequence of 2 Ukg dry
solids. The clean filter cake was dried and chemically analysed by
performing a complete dissolution test in boiling aqua-regia. The results
of the treatment tests on four different samples is shown in Table 2. It
shows that a large percentage of all metals is removed.
The percentage of solids recovered after filtration is
consistently over 60%. Scanning Electron Microscope (SEM) pictures
were taken of the clean residue, which clearly showed the presence of
large crystalline aggregated particles (see Figure 3).
Table 2: Decontamination results of four different fines samples
SampleRiver Contaminated Contaminated Contaminated
Sludge Soil Soil Soil
A B C
ElementBeforeAfterRemvlBeforeAfterRemvlBeforeAfterRemvlBeforeAfterRemvl
gramsglt % gJt glt % glt gft % g/t glt
Iton
Cu 82 12 85 21.0 1.1 94 51.0 20.260 97.0 12.2 87
Zn 442 12 97 1175 4.6 99 228 16 92 567 31 94
Ni 44.0 11.6 73 29.0 2.1 92 137.014.189 175.011.2 93
Pb 182 21 88 4465 247 94 112 20 82 345 71 79
Cr nla n/a. - nla nla - 247.02.3 99 189.012.2 93
Fe 285002000 93 35790 212594 n/a nla - nla Na -
solids32 63 - 85 87 - 41 61 - 38 64 -
Dilutions and analyses pertormed in compliance with EPA (Environmental
Pollution and Atmospheric Chemistry) protocol.
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The filtrate and wash water filtrate from each test was
further neutralized using a milk of lime to a final pH of 9. All metals in the
solution precipitated as metal hydroxides due to hydrolysis. The slurry
was left to equilibrate for 30 minutes, after which the precipitate was left
to settle. It settled rapidly to 10-15% solids within 30 minutes. The
5 settled slurry was then filtered on a vacuum filter. Typical filtration
rates
obtained were in the order of 100-130 kg/m2~h. The filter cake generally
contained 50% solids. Since the cake is contaminated, it does not require
washing. The filtrate contained less than 1 mglL total dissolved metals
and may contain traces of dissolved organic materials. After treatment
10 this water can be safely discharged into the environment.
Sequential hydrolysis of metals in solution was not
attempted due to the relatively low metal concentrations and the large
number of different metals present in each solution. However, it is
15 technically possible to separate certain metals by sequential hydrolysis
due to the different pH's at which they precipitate.
Organics treatment:
Laboratory testwork was conducted on the treatment of
organic contaminants from synthetic solutions and synthetic sludges.
Solutions and sludges spiked with naphtalene and BAH (polyaromatic
hydrocarbon) were subjected to the same conditions that exist in the
aforementioned leach reactor. The highly oxidizing acidic environment
and high temperatures caused the decomposition of these toxic and
carcinogenic materials. The results of a typical test are shown in Table
3. Table 3 shows that within 30 minutes both substances are drastically
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reduced in concentration. The PAH is reduced to a lesser extend, but it
must be noted that this is a relatively chemically stable organic compound
found in polluted sludges and soils. The fact that this compound can be
decomposed in the leach reactor means that other less stable organic
pollutants are likely to be partially or completely decomposed.
Table 3: Typical organics decomposition tests
Time (min) [Naphthalene] [PAH]
0 80 mglL 63 mglL
15 0.5 mglL 22 mglL
30 0.05 mglL 15 mg/L
Although the present invention has been described
hereinabove by way of a preferred embodiment thereof, this embodiment
can be modified at will, within the scope of the appended claims, without
departing from the spirit and nature of the subject invention.
