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A PROCESS FOR RENDERING INERT AND/OR IMMOBILIZING ~I~RMFUL
SUBSTANC~ IN AQUEOUS DiSPERSlONS, SUSPENSIONS, EMULSIONS
AND/OR SOLUTIONS OF WASTE MATERIAI S
The purpose of the process is to use certain bindin~ agents in order ~o
5 treat environmentally harmful substances contained in aqueous dispersions,
suspensions, emulsions and/or solutions of waste materials, and/or to treat
organic compounds, in suGh a way as to bind them, render them inert and/or
immobilize them.
The Filter cakes produced in filter presses or similar dewatering devices
10 from the waste water of flue gas scrubbing facilities or from sludges and
similar aqueous dispersions, suspensions, emulsions or solutions, are by their
nature mixtures of extremely fine-grained materials and this both favours and
enhances the binding of harmful substances in the filter cakes. After they leavethe filter press, these fil~er cakes often still have a water content of 50 to 7~%
15 or higher. Nevertheless, it has proved extremely difficult to mix these fine-particled, often plastic filter cakes with cement or other binders in such a waythat the binder is homogenously distributed throughout the material of which
the filter cakes are made up. ~ut, the particles of waste material which have
not been broken down constitute weak points in the system because they
20 impair the binding of the harmful substances, and also as a rule they negatively
- influence the hardening and the stability of the moulded or compacted productsmade from the material consolidated by the binding agent. In addition, the
miscibillty of the waste materials with the binders is impaired by certain
textures which form during the pressing process and which are similar to those
25 which come about during the deformation of plastic clays in the ceramics
industry. These textures increase the need for water and negatively affect the
miscibility of ~he materials with the binders. The only way in which to achiev0
a reasonably acceptable distribution of the binder in the material of which the
filter cake is made up is to add large additional amounts of water while the
30 binder/filter cake mixture is being prepared. This reduces the strength and frost
resistance of the T~oulded product formed, impairs its permeability and lowers
its ability to bind the harmful materials.
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The use of inor0anic binders to treat was-te wa-ters is known, -for
example, from the following pubications:
EP-A1 17741 - which stric-tly speaking has a very narrow field of
application - d(3scribes a method o-f converting phosphoric acid extraction
5 residues into a dumpable product by treating them with ordinary hydrated lime
or ordinary burnt lime. A neutraliza-tion reaction takes place in the filter cake
which forms. The Ca-oxide or hydroxide used is not a hydraulic binder.
A similar problem and its solution is the subject of EP-A1 50370
according to which dilute acids are treated and likewise nelJtrallzed in practice
10 by means of brown coal ashes, which exhibit an alkaline reaction and possess
only partially a latent hydraulic binding capacity, and even -that only in the
presence of activating agents.
The consolidation of "dilute" liquid wastes described in LU-A 42343
requires the addition s)f quite specific natural or synthetic, organic
15 agglomerating agents. In orderto complement their effect, "high-gravity solids"
are added to the sludge which is formed, but the primary aim of doing this is
not to bind the harmful substances by causing the mixture to set, instead the
harmful substances are caused to adhere to particles with large surface areas
and higher density, as is clearly shown by the use of inert substances such as
20 finely ground limestone and "tailings", as provided for in the patent. Cementand gypsum are suggested as equally good high-gravity solids, but their binding
capability is not examined any greater detail. The process according to this LU-A depends on the use of expensive organic agglomerating agents whose
anticipated long-term harmful effects after being dumped are not discussed in
25 that document.
The purpose of the present invention is to treat thin, aqueous waste
products of widely varying origin, which as is known accumulate in very large
amounts and often discontinuously in batches, and to convert them in an
economically viable manner into fully dumpable solid products containing
30 wi~hin themselves the immobilized harmful substances which cannot leach outat a later point in time; and the large amounts of water which are separated
out during the process must be practically free of harmful substances and
suitabie for discharging into a receiving body of water.
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Th~3 subject of the present invention is thus a process for rendering inert
and/or immobilizing inorganic ancl/or organic environmentally harmful
substances contained in aqueous dispersions, suspensions, emulsions and/or
solutions of waste materials by adding hydraulic binders be~ore separating a
solids phase and an aqueous ph~se, and an agglomerating agent is also addecl;
the process is characterized by the fact that - without adding natural or
synthetic organic agglomerating agents - at least one inorganic hydraulic
binder is added to the aforementioned dispersions, suspensions, emulsions
and/or solutions following treatment to precipitate and/or convert any heavy
10 nletals, sulphur compounds, or other harmful substances and/or organic
compounds contained in them; the binder is based on Portland cement clinker,
alumina cement clinker and/or hydraulic lime, and a homogeneous rnixture of
the harmful substance(s) and the binder(s) is brought about in the dispersion,
suspension, emulsion and/or solution preferably by stirring; and, possibly after15 gravity separation, the water content of the homogeneous mixture of harmful
substance and binder thus obtained is reduced by compressing the mixture in
a press. The dispersion, suspension, emulsion and/or solution of waste
materials has a low pH value, preferably less than 5, or the pH is adjusted to
this low value, and it is mixed, preferably at a high temperature, with an
20 inorganic material containing magnesium compounds, selected from the group
of unburnt, semi-burnt or fully burnt dolomite or dolomitic lirnestone,
magnesium chloride and magnesium sulphate, preferably containing more than
5%, in particular more than 10% and especially more than 25% magnesium
compounds, calculated in each case as MgO, relative to the dry state of the
25 waste material~s) free of ignition losses, and/or it is also mixed with an
inorganic material containing aluminium compounds, selec~ed from the group
of blast furnace slags, pozzolanic materials, fly ashes, trasses, Portland cement
clinker, alumina cement clinker and/or hydraulic (Al-Gontainin~) burnt lime,
containing more than 5%, in particular more than 10% and especially more
30 than 2~% Al203 equivalent rela~ive to the dry s~ate of the waste material(s), free of ignition losses.
By adding the stated binders, after treating the waste waters, not only
can one avoid the use of costly agglomerating agents, whose possible long-
term harmful effects have not been investigated, but also the conditioned
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harmful substances are encased in SUCII a way that, together with the long-
term consolidation of the binders and -the simultaneous lelimination of]
secondary reactions, the harmful subs-tances are particularly well rendered iner-t
and immobilized, while -the dumpable product-thus obtained has a hiyh degree
5 of mechanical strength.
The attainment of these effects can be additionally simplified by waiting
with the dewatering, and in particular the fiiter press stage, until a certain
amount of time has elapsed, depending on the harmful substances and the
binder involved, but before the binder sets. In this way, an optimally
10 homogeneous mixture of harmful substances and binder is obtained and as a
result the harmful substances are uniformly, durably and reliably bound,
rendered inert and immobilized throughout the mixture without any weak
points occurring. Given the fact that the dispersion, suspension, emulsion
and/or solution of waste materials is highly diluted with water, the harrnful
1~ substances are very rapidly bound in particularly large amounts by chemical or
adsorptive processes to the calciun-l ions of the binder or its hydration products
with their large surface areas, and this process is further promoted by the hightemperatures which are often present.
A subsequent compressing stage, which is carried out for e~ample in a
20 filter press, vacuum press, ram press, roller press, briquetting press or another
similar water-reducing device, is greatly simplified and accelerated by the factthat the binder has already been mixed with the dispersion, suspension,
emulsion and/or solution of waste material; the formation of structures is
prevented and the dry matter con~ent of the filter cake thus formed is
25 increased. Another possible advantage resulting frorn the greater reduction of
the water content is the associated reduction in the chloride content of the
filter cake, thereby improving the strength of the moulded product,
consolidated with cement, obtained from the press. The large surface area of
the hardening products and also the denser structure resulting from the low
30 water content both considerably reduce the diffusion of ions in the product
thus obtained and further improve the immobilization of the environmentally
harm~ul compounds.
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The unexpectedly high degr~e to which the harmful subs~ances are
bound can be achieved in the process according -to -the invention by adding the
inorganic materials to precipitate and convert the harmful susbtances.
Typical harmful substances which can be particularly ef-fectively
5 rendered inert by the process according to the invention are lead, zinc,
cadmiurn, thal!ium, arsenic, and chromium compounds, as well as fluorine
compounds, organic acids such as tartaric acid, oxalic acid, etc. It is
appropriate to wait, usually for a few rninutes to a few hours, depending on
the type of harmful substance involved, on the binder used, and also on the
10 prevailing temperature, in order to achieve the maximum possible degree of
binding. As regards the amount of binder needed, binding eFfects can be
expected starting from amounts of binder in the ~ to 5% range relative to the
dry mass of the waste material and the binder.
Typically, the amount of binder added coincides with the experience
15 gained in soil stabilization practice. A significant binding effect is normally
obtained from 7% binder onwards. The quantitative ratios mentioned in Claim
2 are especially preferred.
In order to convert into harmless ettringite any sulphur compounds
contained in the aqueous dispersion, suspension, emulsion and/or solution of
20 waste material, it has proved particularly efficient to choose a method as
described in Claim 3. ~y forcing ~he formation of ettringite in this way before
the water content is subsequently reduced, e.g. in a filter press, the binder isprevented from expanding at a later point in time, and also it improves the
filtering capability and dry substance content of the filter cake. Finally, by
25 precipitating out the sulphur compounds, a very low sulphate content is
achieved in the water expressed during the subsequent filtering step. This is
especially important if the expressed water has ~o be discharged into a public
sewer or a receiving body of water which are normally governed by strict limits
on the amounts of sulphate they may contain, and if these limits could not
30 otherwise be met.
As a rule, heavy metals and other harmful substances are well bound in
all the cements. However, a far better than average binding efFect is achieved
by cements or hydraulic limes which form large amounts of ettringite, i.e.
which contain large quantities of aluminate or alumina~e ferrite. Therefore,
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rapid-setting cemen-ts containing 1 2CaO.7AI203 and in particular
11 CaO.7AI203.CaF3 are particularly suitable, and -the amounts of these
substances in the cements should correspond -to those given in Claim ~.
Compared with the customary cemen-ts, these cemen-ts, in particular those
5 c:ontaining quantities of 11 CaO.7AI203.CaF2, are characterized not only by anabove-average ability to bind harmful substances, in particular those which are
difficult to render inert, such as chromium compounds, but also the~/ are less
susceptible to harmful substances such as zinc compounds, phosphate
cornpounds or organic compounds contained in the dispersions, suspensions,
10 emulsions and/or solutions of waste material, which tend to interfere with the
setting process. In addition, because of its acicular structure and its ability to
bind large quantities of water, the et-tringite which forms along with other
hydration products during the setting process provides -for exc~llent filterability
and a correspondingly high content of clry matter in the filter cake.
In light of these facts, ettringite may advantageously be directly added
to the aqueous dispersion, suspension, emulsion and/or solution of waste
material, as provided for in Claim 5, .
In this context, in order to avoid swelling reactions in dispersions,
suspensions, emulsions and/or solutions containing reactive sulphur
20 compounds, it is possible, as provided for according to Claim 6, to add
cements and/or hydraulic limes having the low contents of calcium aluminates
mentioned there.
Also, the formation of ettringite can be prornoted by using Gement
and/or lime with high contents of fly ash or blast furnace slag. In this case, the
25 aluminates or the aluminate ferrites come from these additives. Combinations
of these substances with appropriate cements, such as PSZ 400, have proved
particularly suitable.
In this context, it is also particularly suitable to proceed according to
Claim 7 in order to further boost the abili~y to bind harmful substances,
30 especiallv heavy metals and/or organic compounds and/or disruptive
ammonium compounds. The added amounts mentioned there result from the
fact that the binding capabili~y of these materials depends among other things
on their chemical nature and is, for example, very high in the case of
bentonites and zeolites, but lower for blast furnace slags.
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ln all these cases -the -formation o-f e-ttringite can be controlled, as
provided for accordin~ to Claim 8, by lowering the calcium ion concentration
of the liquid phase by adding alkali compounds, especially alkali carbonates,
preferably in an amount in the range between 0.1 and 5%.
In order to bring about further improvement of the -filtration
characteristics of the aqueous dispersion, suspension, emulsion and/or solution
of waste material, the filtra-tion aids provided for in Claim 9 may
advan~ageously be added in the process according to the invention.
Easily soluble substances, e.g. alkalis, calcium hydroxide or calcium
sulphate, contained in the binder or released at the start o-f the hydration
process can enter into solution while being mixed with the aqueous
dispersions, suspensions, emulsions and/or solu-tions of waste materials and
may be remove(i with the expressed water in the filtering or pressing stage
which follows mixing, and this can have a deleterious effect on the setting
process and on the strength of the moulded or compacted products which are
subsequently produced. Since this is the case, it may be advantageous to add
additional amounts of these easily soluble substances before the binder sets
in order to compensate for the above-mentioned losses and/or to use binders
which contain either small quantities, or none at all, of these easily soluble
substances. For this purpose, particular preference should be given to a variantof the process according to Claim 10. in this context, it is also advantageous
for the filter water or the expressed water, including the easily soluble
componen~s of the binder, and possibly also of the waste materials, which are
contained therein, to be operated in a closed cycle and to be added to new
dispersions, suspensions, emulsions and/or solutions of waste materials who~e
water content has not yet been reduced.
In order to compensate for the losses of easily soluble cornponents, it
is advantageous to proceed according to Claim 11.
~iince the solubility of harmful substances depends very rnuch on the pH
value - at low pH values the solubili~y is frequently several powers of ten
higher - it is a matter of particular interest to ensure the long-term maintenance
of a pH in the mixture of waste material and binder which undergoes water
reduction. ~he long-term maintenance of a high pH value can be achieved by
adding an alkali depot in the manner advantageously provided for in Claim 12.
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This alkali depot is added to the mixture o-f waste material and binder
preferably before the water content is reduced. Dependin~3 on the type and
ef-fect o-F the all<ali depot, it may be necessary to add very clifferent amounts
o-f such material, so tha~ the alkali depot is added advantageously in amounts
5 ranging from 1 to about 75% relative to the combined amount of dry waste
material and binder.
When the alkali depot is made available in -the form of a rnixture of at
least two substances acting as "neutrali~ation barriers", the second substance
with the low pH reactivity represents an "inert neutralization depot" as long as10 a high pH exists. This depot is activated only if the pl I value drops below 7
and its reaction prevents a further decline in the p~l value and thus an increase
in the solubility of the harmfut substances. In general, -these limits have proved
to be suitable for completely rendering inert or irnmobilizing harm-ful substances
by means of the process according to the invention. In special cases, other pH
15 neutralization barriers can be selected by carefully choosing the materials
which make up the alkaline depot.
By applying the process according to the invention, it is possible to
reduce the water content of the waste material/binder mixture to such an
extent that this mixture can be converted into moulded products after the
20 compressing stage. It is therefore possible, immediately after the pressing
stage, to process the filter cakes into moulded products in another press or
extruder, or similar, by applying pressure and/or heat. Mechanical pressure of
more than 3 or, in more demanding applications, of more than 15 or more than
40 bar, and temperatures of about 5() to 70C have proved appropriate. The
25 higher temperature considerably accelerates the setting of the binder. It mayalso be possible to produce the moulded products simply in the course of the
compressing stage, without the need for any separate, addi~ional press.
It has furthermore proved advantageous to neutralize the alkaline-
reacting compounds, especially calcium hydroxide, contained in the surface
30 layer of these moulded products by means of liquids, preferably carbonic acid,
organic acids, solutions of carbona~es or bicarbonates and similar, and/or
gases, e.g. carbon dioxide, C02-containing exhaust gases, etc.; and/or ~o coa
the rnoulded products with an iner~ protective layer consisting of mineral
materials, e.g. water glass solution or organic su~stances, e.g. based on
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acrylate, styrene bu-tadiene, vinyl copolymer or mixtures, for example of
plastic-modified hydraulic masses.
The mixtures produced by -the process accorcling to the invention may
also advantageously formed into finished parts, building bricks, granula-tes,
5 aggregates, concrete aggregates, and similar, possibly with the ~urther addition
of binders, additives or water.
The fil~er cakes formed in the press may also, however, be allowed to
harden without any further processing and -they may then be dumped ~ ithou-t
any harm to the environment.
The advanta0eous proper-ties of rnoulded products obtained from the
process according to the invention, in particular the strength values which can
be achieved, are explained in more detail in the following example, which is notin any way intended to restrict the scope of the invention:
Example:
A waste water from a flue gas scrubber fitted on a garbage incinerating plant
was treated in the usual manner by adding hydrated lime, iron chloride solution
and reagents to precipitate the heavy metal; the sludge formed had a dry
substance content of 3.9% and was pressed on a filter press. The resulting
filter cake had a dry substance content of 35% and was then mixed in a ratio
of 1:2, relative to the dry state, with Portland cement PZ ~5 ~H) in a positive-drive mixer. In order to achieve an adequate distribution of the cement and the
filter cake, it was necessary to add a further 50% water. Using state-of-the-artmethods, concrete cubes were produced from the filter cake/cement/water
mixture obtained in this way.
In a second series of tests, the same amount of Portland cement was
added to the sludge in the manner according to the invention before the filter
press stage was reached. Because of the high water content of the sludge at
this stage in the process, it was possible to rnix the waste material and cementsimply by stirrin!g the sludge for 3 minutes. The mixture was then fed to ~he
filter press. The Filter cake thus obtained had a dry matter content of 55/0 and
it was tamped into concrete cube moulds without any further addition of water
or cement.
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The strength values obtained in the concrete cubes manufactured in -this
way are listed in Table 1. It can be seen that the addition of cement according
to the invention be-fore the water content is reduced on the filter press
decisively improves the development o-f the strength properties. It was also
5 remarkable that the filtering time in the filter press dropped to a quarter of the
usual value when the binder was added prior to the filtering stage, and also
that, as mentioned, the dry matter con-ter-t of the filter cake increased frorn 35
to 55%
In further tests, the mixture ratio of the "filter cake waste material" to
10 cement was varied. It was found that the amount of additive needed to bring
about hardening could be signifiantly lowered by ad~ling the binder, in the
manner according to the inven-tion, ahead of the filter press.
Even better results can be obtained when alumina cement is used as the
binder.
1 5 T~BLE 1
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Mixture ratio Compressive strength
BiDder added binder:waste (relative (N/mm2) after D days
. _to dry mass in each case) _ I -ll
a*er filter press bef. filter press 1 7 28 ¦ 120
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x 2: 1 n.e 0.8 1.5 2.9
x 2: 1 2.2 3.8 6.3 10.
x . ._ 09 1 _ 1.0 2.1 3.3 4.5
x 0.2: 1 0.2 1.0 1.6 2.7
.....
x 0.4: 1 (Alurni~la) 3.4 3.9 5.0
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20 n.e. = cannot be removed from the moid
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