Method of chemically fixing liquid aqueous sludge by means of a pozzolanic reaction

A liquid aqueous sludge is transferred into a cylindrical-conical tank and gitated by blowing in air through a downwardly extending tube. A closed circulation of sludge is established between this main tank and a reagent tank by means of a recycling pump. The reagent tank overflows into the main tank. Chalk is added to the reagent tank and a pozzolanic material is added to the main tank. When the mixture of dilute sludge, chalk and possolanic material is homogeneous, it is transferred to a filter press by means of a further pump. The filter cake is collected for final storage or discharged to set slowly.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention concerns a method of fixing aqueous sludge by means of a 
pozzolanic reaction. 
2. Description of the Prior Art 
Waste sludge containing toxic products in the solid phase can be harmful to 
the environment into which it is discharged because the solid phase is 
very finely dispersed and therefore has a very large surface area 
interface with external surroundings. This inevitably favors to a very 
considerable degree the passing into solution of toxic pollutants as a 
result of physical dissolving, hydrolysis, biodegradation and other 
phenomena. Additionally, such sludge includes an interstitial aqueous 
liquid phase which is itself contaminated to a greater or lesser degree 
and which constitutes a more mobile fraction. 
The environment pollution problem is solved by fixing such sludge using 
various methods including methods utilizing pozzolanic reactions. 
The pozzolanic capacity of a mineral is its capacity for fixing chalk at 
ordinary temperature to produce a hydraulic binder, by which is meant a 
substance able to set and therefore to harden and solidify in the presence 
of water. 
This effect is extremely slow and it begins some two to three days after 
the mixing is done. The material changes very slowly with time and several 
years may be required to achieve the maximum mechanical strength. 
The most commonly used pozzolanic material is fly ash with a particle size 
in the order of 1/30 micron. A distinction is drawn between silica-alumina 
ash resulting from the combustion of coal and sulfo-calcic ash produced by 
burning lignite. 
In practice the pozzolanic effect of fly ash can be employed by 
simultaneously adding such ash and chalk to the sludge to be treated. 
The chalk may already be present (in the form of Ca(OH).sub.2) in the 
sludge (neutralization or precipitation sludge containing excess chalk). 
Commercial processes using pozzolanic reactions are already known from such 
general works as R. B. POJASEK "Toxic and hazardous waste disposal" volume 
1. Ann Arbor Science 1979 and U.S. E.P.A. "Survey of 
solidification/stabilization technology for hazardous industrial wastes" 
E.P.A. - 600/2-79-056 July 1979. 
The POZ-O-TEC process developed by the American company I.U.C.S. uses fly 
ash and other additives. Two reactions take place simultaneously: a fast 
reaction between the soluble salts and the chalk and alumina present in 
the fly ash, and a much slower pozzolanic reaction between the silica and 
the chalk. 
Immediately after treatment the sludge is in the form of a viscous fluid 
that can be pumped and that is allowed to harden for several weeks at its 
storage site. 
This process was developed for treating sludge resulting from SO washing of 
thermal power plant smoke and was subsequently used to solidify liquid 
waste produced by chemical and metal surface treatment industries. 
Variants of this process are used in the Research-Cottrell process (see R. 
B. POJASEK reference above). The treatment system includes a gravity 
thickener, a centrifuge for dehydrating the sludge, a fly ash conveyor, a 
chalk conveyor and a dehydrated sludge/fly ash/chalk mixer. 
In the case of waste sludge from metal surface treatment, a centrifuge or a 
conveyor belt press produces a residue containing approximately 80% by 
weight water. A filter press produces residue containing approximately 70% 
water. 
This high water content has disadvantages because large quantities of chalk 
and pozzolan are needed to make it set and this results in large weights 
and volumes of solid waste to be stored. 
What is more, the Research-Cottrell mixer must be a powerful and rugged 
unit, as its function is to intimately mix with the fly ash and chalk a 
thick medium consisting of the centrifugally concentrated sludge. 
An object of the invention is to propose a method of treating relatively 
low concentration sludge (containing, for example, approximately 3 to 20% 
by weight of dry material relative to the weight of the sludge) that is 
simpler in that it does not require a powerful mixer, but which secures 
improved separation between the solid phase and the liquid phase of the 
sludge. In other words, the process according to the present invention, 
recovers 35 to 40% of the aqueous phase of the initial sludge. This 
aqueous phase can be recycled or purified, for example by means of ion 
exchanger resins. 
SUMMARY OF THE INVENTION 
The present invention consists in a method of chemically of fixing liquid 
aqueous sludge by means of a pozzolanic reaction, wherein said liquid 
aqueous sludge is first mixed with a pozzolanic material and optionally 
with chalk if the sludge does not have a sufficiently high initial content 
of calcium hydroxide to react with all of the pozzolanic reagent to obtain 
a substantially homogeneous mixture which is then dried mechanically to 
produce a solid cake which sets slowly and can be stored. 
In more detail, the above process is carried out in the following 
successive stages: 
a) if necessary, the pH of the sludge is adjusted to a value greater than 
9; 
b) the product obtained in step a) is mixed with the pozzolanic material 
and if necessary with chalk so that the resulting homogeneous mixture has 
a content of sludge dry material, calcium hydroxide, pozzolanic material 
in the weight ratio of calcium hydroxide/dry material between 1 and 2 and 
pozzolanic material/calcium hydroxide of approximately 2; and 
c) this homogeneous mixture is dehydrated mechanically. 
In the context of this invention, the term "chalk" is to be understood as 
meaning either quicklime (CaO) or deadlime (Ca(OH).sub.2). 
By "mechanical dewatering" is meant the more or less complete elimination 
of the liquid phase from a sludge by application of a force or pressure 
field, for example by filtering or centrifuging. 
The method in accordance with the invention has many advantages: 
The agitation is applied to a relatively fluid dilute aqueous substance 
which quickly obtains better homogenization of the solids (waste, chalk, 
pozzolanic material) in the aqueous phase than is the case in the 
Research-Cottrell process. 
The pozzolanic material gives this mixture improved dehydration 
capabilities, in particular, by making the filter cakes less compressible 
and more permeable than those obtained merely by filtering the sludge. 
The recovered solid phase solidifies slowly into a kind of stone that has 
good resistance to erosion and leaching. 
Finally, this method can be used with no modification to an existing 
treatment station other than adding the installation for storing and 
metering the pozzolanic material. 
In addition to the arrangements already described, the invention comprises 
other arrangements that will emerge from the following description given 
with reference to examples of implementation of the method in accordance 
with the present invention and to the appended diagrammatic drawing in 
which the single figure is a schematic view of an installation for 
implementing the method in accordance with the invention. 
SLUDGE TREATMENT TESTING EXAMPLES 
1. Definition of the sludge by composition 
a) First sludge V.sub.1. 
To enable comparative testing a sludge V.sub.1 was synthesized. The V.sub.1 
sludge was obtained by using deadlime to neutralize an acid solution of 
metallic salts. 
Table 1 shows the composition of the solid and liquid phases of this 
sludge. 
This sludge had a dry materials content of approximately 261 g per kg of 
sludge (26% dry solid material). It was diluted to 6% by adding water to 
produce the sludge V.sub.1. 
b) Second sludge V.sub.2. 
This was waste sludge from the metal surface treatment industry 
(chromium-plating and nickel-plating of bicycle components) obtained from 
an effluent treatment system comprising in succession: 
chemical reduction of the chromates by addition of sodium bisulfite in an 
acid medium, and 
neutralization of the quicklime to a pH of around 10 at the rate of 14.1 g 
of quicklime/kg of effluent. 
The essential characteristics of this sludge were as follows: 
suspension liquid pH: 9.8 
dry residue at 110.degree. C.: 60.7 g/kg 
concentration of ionizable salt metals (mg/kg of dry material): 
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Sodium 8 518 
Calcium 6 175 
Trivalent chromium 
980 
Iron 192 
Nickel 1 310 
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2. Nature of pozzolanic materials used. 
Various pozzolanic materials were selected. 
They are respectively identified hereinafter by the reference letters A 
through E. 
A is a silica smoke marketed by the French company SOFREM. 
B is a commercial thermal silica having the following properties: 
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Chemical analyses: 
ZrO.sub.2 
1.3% 
Al.sub.2 O.sub.3 
3.4% 
Fe.sub.2 O.sub.3 
0.2% Moisture content 0.22 at 100.degree. C. 
Na.sub.2 O 
0.2% Fire loss (excluding H.sub.2 O) 0.3% 
CaO 0.04% between 100 and 1 000.degree. C. 
C 0.07% pH of a 4% aqueous suspension: 4.4 
K.sub.2 O 
0.06% 
SO.sub.3 0.006% 
SiO.sub.2 
94% 
Physical properties: 
Theoretical density: 2.2 g/cm.sup.3 
Apparent density (uncompacted): 0.24 g/cm.sup.3 
Specific surface area (B.E.T.): 14 m.sup.2 /g 
Particle size: 
90% less than 2 .mu.m 
80% less than 1 .mu.m 
10% less than 0.2 .mu.m 
.phi..sub.50 = 0.55 .mu.m 
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This thermal silica consists essentially of microspheres of vitreous 
silica. 
C is a pozzolan marketed by TUBAG. 
D is a natural pozzolan from VOLVIC (France). 
E is a silica-based material available in large quantities as a waste 
byproduct of the glass tempering process used in earlier times by the 
glassmaking industry. 
Chemical analyses (% weight): 
at least 90% SiO.sub.2 
1.5% hydrated Na.sub.2 O 
0.02% carbon-based organic material 
Fe.sub.2 O.sub.3 remainder 
The particle size of this powder is less than 5 .mu.m. 
The use of the material E (for which no use has previously been found) is 
of great economic benefit as compared with the use of commercially 
available pozzolanic materials which are generally relatively costly. 
I. Examples of the use of the invention to treat the synthetic sludge 
V.sub.1 
The figures for each test are adjusted to relate to 
1 kg of untreated sludge V.sub.1. 
Tests 1 through 6 were control tests. 
Tests 7 through 22 were carried out using the method in accordance with the 
invention. 
To 1 kg of sludge V.sub.1 there were added each time either 60 g or 120 g 
of quicklime CaO and double this quantity of the pozzolanic material A, B, 
C, or E. The mixture was then homogenized by agitation before filtering at 
a pressure of 196 or 392 kPa. 
As recorded in table 2, the weight of the filtrate, the weight of the final 
waste product and its volume were measured for each test. 
The results of these tests show the favorable effect of the filter pressure 
which reduces the final weight of solid waste product and increases the 
volume of the filtrate obtained, the final waste product weight decreasing 
in the order of materials A, B, E, C. 
Other tests described hereinafter show the favorable influence on the 
filterability of the sludge V.sub.1 of the pozzolanic materials added to 
it. 
The filterability of a sludge may be characterized by the following 
parameters: 
1. The specific resistance to filtration: .alpha. 
2. The compressibility coefficient of the filter cake: S 
3. The limit-dryness of the cake: sl 
These parameters are discussed in the AFNOR (French Standards Institution) 
publication T97-0001 of November 1979 (pages 1 through 14) on the subject 
of: Experimental standards--sludge tests--determination of characteristics 
related to capacity for concentration. 
.alpha. is defined as the specific resistance to filtration of the filtered 
suspension. It is independent of the concentration of the suspension and 
depends essentially on the size, shape and degree of agglomeration of the 
solid particles constituting the filter cake. 
Table 3 shows the specific resistance obtained by adding chalk and 
pozzolanic materials to the V.sub.1 sludge in comparison with that for the 
V.sub.1 sludge alone. 
The table shows that pozzolanic materials C and D improve filterability. 
Table 4 shows that adding pozzolanic reagents decreases the value of S very 
significantly. 
Table 5 shows the limit dryness results of adding pozzolanic materials 
compared with the sludge V.sub.1 alone. 
The results summarized in tables 2 through 5 show that the pozzolanic 
materials improve the filterability of the synthetic sludge V.sub.1. 
Further tests summarized in table 6 show the results achieved with regard 
to solidifying cakes obtained from tests numbers 1 through 22 as listed in 
table 2. 
Method: 
Immediately after filtration cylindrical samples (diameter 3.5 cm, height 
5.5 cm) were taken from the various filter cakes and allowed to solidify 
for 28 days in the case of a first batch and for six months in the case of 
a second batch. The samples were then placed in water with no agitation 
and their appearance checked periodically for up to 15 hours from 
immersion. 
The results are summarized in table 6, which uses the following notation: 
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0 material crumbled immediately placed in the 
water or very shortly afterwards; 
+ material entirely disintegrated after 15 hours; 
++ material partly disintegrated to a final state 
comprising small (&lt;15 mm) fragments; 
+++ partial disintegration yielding a final state of 
large (&gt;15 mm) fragments; 
++++ material cracked; 
+++++ physical state unchanged; 
++++++ material unbreakable by hand. 
______________________________________ 
Table 6 shows that the materials treated by the method in accordance with 
the invention are generally much more durable than the sludge alone or the 
sludge treated only with chalk. 
Further tests (leaching tests) showed that, in relation to retention of 
pollutants, after six months maturing the metals Cr, Ni, Cu, Fe, Cd, Mn 
were no longer detectable in the aqueous phases obtained by leaving 100 g 
of solidified cake in contact with 1 liter of water with no agitation for 
15 hours. 
II. Example of the use of the method in accordance with the invention to 
treat the sludge V.sub.2