FR2732249A1 - Stabilising ash from incineration of mud from water purificn. plant for use as filler - Google Patents

Abstract

Ash resulting from incineration of muds from a purificn. plant for town water is stabilised by adding the ash to an aq. binder formed by a powder prod. based on blast furnace slag, Mg lime and clay. Also claimed is the prod. obtd. in the process.

Description

 The present invention relates to a method for stabilizing ash resulting from the incineration of sludge from urban water treatment plants.

 It is known to treat urban water in sewage treatment plants. After a complex physico-chemical and biological treatment intended to digest and decompose the organic matter contained in these waters, at the end of the treatment, sludge is obtained which still has a certain polluting potential due to the presence of heavy metals such as lead. , zinc, or chromium, even arsenic.

 Some of this sludge is currently used in agricultural spreading. They indeed present an interesting fertilizing power.

 Another part, whose tonnage is constantly increasing, is cremated. However, if the incineration of this sludge is a treatment solution making it possible to reduce the mass and volume of the waste produced, it also has the effect of concentrating the heavy metals in the resulting ash.

 We also know the fill materials obtained by mixing an aqueous suspension of cement, thermal plant ash and possibly clay.

 The cohesion and the stability of the grounds treated by this type of mixture is thus improved.

 The compressive strengths that are sought for this type of application are commonly 2.5 MPa at 90 days.

 The present invention aims to provide a solution to the problem of recovering ash from the incineration of wastewater sludge by proposing a simple and economical process which makes it possible to use this ash to make a product which is sufficiently solid and non-polluting for be usable as backfill material.

 The subject of the present invention is a process for stabilizing ash resulting from the incineration of sludge from urban water treatment plants, which is characterized in that it consists in incorporating these ash in an aqueous binder formed mainly by a powder of a product based on blast furnace slag, with magnesian lime and clay.

 This process makes it possible to obtain a material which exhibits, after hardening, a continuous and homogeneous compact matrix having good mechanical strength and capable of effectively retaining pollutants such as heavy metals. It can thus constitute a good quality backfill material.

 Apart from the fact that the method according to the invention makes it possible to recover the ash from the incineration of wastewater sludge, one of the advantages of the product thus obtained compared to conventional backfill materials is that, with identical performance , the product obtained according to the invention is currently more economical to manufacture than the conventional backfill material.

Furthermore, it turns out, as yet unexplained, that to treat these ash from incineration of sludge, a combination of slag and magnesium lime allows to obtain performance levels that cannot be achieved with cements. classics such as CLK, CLC, or
CHF.

 Indeed, with equal amounts of ash, binder and water, a mixture prepared from a binder composed of slag and magnesium lime has a compressive strength at 90 days twice as high as a prepared mixture from CLK45 for example.

 According to the invention, the term "slag powder" means ground blast furnace slag, blast furnace slag cement or any other powder of a product containing blast furnace slag.

 According to the invention, the slag-based product advantageously has grain sizes less than 200 Hm, and preferably less than 100 Sm.

 The proportion by weight of slag powder is advantageously between approximately 5 and 30% relative to the dry ash, and preferably between 10 and 25%.

 According to the invention, magnesian lime preferably contains about 66% by weight of calcium oxide (CaO) and 33% by weight of magnesium oxide (MgO).

 The proportion by weight of magnesium lime is advantageously between 5 and 50% relative to the aqueous binder.

 According to the invention, it is also possible to incorporate into the mixture clay such as bentonite or any other clay compound capable of fixing a large amount of water so as to stabilize the suspension and avoid the formation of supernatant water or " risen ".

 The proportion of clay is advantageously less than 5% and is preferably between 0.5 and 4.5% relative to the amount of water contained in the mixture consisting of the ash and the aqueous binder.

 Finally, depending on the needs and depending on the applications, it is possible to add to the mixture an accelerator for setting cements based on slag, chosen from those known to those skilled in the art, for example Cacl2, Li2CO3, Na2CO3, Aluminate. or sodium silicate, etc.

 The ratio by weight of the quantity of dry matter to the quantity of water in the mixture constituted by the ash and the aqueous binder is advantageously between 1 and 1.35, and preferably between 1.1 and 1.3.

 The present invention also relates to the material obtained by implementing the method described above, as well as the use of this material as backfill material.

 In order to better understand the invention, we will now describe two examples given without limitation.

Example 1
To recover 1,500 kg of ash comprising 498 kg of water and 1,002 kg of dry matter, including
35.7% by weight of Si02,
9.63% by weight of A1203,
20.99% by weight of CaO,
3.14% by weight of Fe2O3,
15% by weight of P205,
2.08% by weight of SO 3, and, per kg of dry matter
44 mg of As,
2200 mg of Fe,
200 mg of Cr,
1000 mg of Pb,
2900 mg of Zn,
10 mg of Ni,
10 mg of Cd,
less than 0.1 mg of Hg, they are incorporated into a mixture having the following composition
Slag powder 225 kg
Magnesium lime 75 kg
15 kg clay
Water 615 kg
The resulting mixture can be used as an embankment material.

Measurements are carried out on a test tube of this mixture and the monitoring of the compressive strength (Rc) over time gives the following results
Age (days) Rc (MPa)
28 1.46
45 1.72
60 2.13
90 2.53
The test piece, once solidified, is subjected to the leaching test according to standard X31211 and the analyzes on the leachate thus obtained show that the heavy metals present in the test piece are perfectly fixed by the binding matrix. Indeed, the concentrations found are reported in the following table.

Metal quantity detected (mg / kg)
As less than 0.15
Fe not detected
Cr 0.6
Cr less than 0.015
Pb less than 3
Zn 0.15
Ni 0.9
Cd less than 0.3
Hg less than 0.015
Example 2
1500 kg of ashes of the same composition as in Example 1 are incorporated into a mixture having the following composition
Slag powder 270 kg
Magnesian lime 80 kg
15 kg clay
Water 615 kg
On a test piece extracted from this mixture, the monitoring over time of the compressive strength gives the following results
Age (days) Rc (MPa)
28 2.42
45 3.3
60 3.35
90 4.55
The solidified test piece is subjected to the leaching test according to standard X31211 and the analyzes on the leachate thus obtained show that the heavy metals present in the test piece are perfectly fixed by the binding matrix. In fact, the concentrations found are reported in the following table
Metal quantity detected (mg / kg)
As less than 1.5
Cr less than 0.3
Cr less than 0.15
Pb less than 1.5
Zn less than 0.6
No less than 0.3
Cd less than 0.3
Hg less than 0.15
Comparative example
For comparison, 1500 kg of the same ash were mixed with a mixture comprising, instead of slag powder and magnesian lime, CLK 45 cement. The mixture therefore has the following composition
Ashes 1500 kg
CLK45 300 kg
15 kg clay
Water 615 kg
Monitoring the compressive strength of a test piece over time gives the following results
Age (days) Rc (MPa)
28 l, 11
45 1.19
60 1.30
90 1.38
It can be seen that replacing CLK cement with slag and magnesium lime, as in Example 1, considerably improves the mechanical properties of the material obtained. Failing to use slag and magnesian lime, the material obtained does not have a sufficiently high compressive strength to constitute an embankment material.

Claims (9)

 1 - Method for stabilizing ash resulting from the incineration of sludge from urban water treatment plants, characterized in that it consists in incorporating this ash in an aqueous binder formed mainly by a powder of a product based blast furnace slag, magnesian lime and clay.  2 - Method according to claim 1, characterized in that the powder produced from blast furnace slag has a particle size less than 200 Hm and preferably less than 100 Hm.  3 - Process according to claim 1, characterized in that the proportion by weight of the product based on blast furnace slag relative to the dry ash is between about 5 and 30%, and preferably between 10 and 25%.  4 - Method according to claim 1, characterized in that the proportion by weight of magnesian lime relative to the aqueous binder is between 5 and 50%.  5 - Process according to claim 1, characterized in that clay is added to the mixture constituted by the residues and the aqueous binder in proportions avoiding the rising of the mixture.  6 - Process according to claim 5, characterized in that the proportion by weight of clay relative to water of the mixture constituted by the ash and the aqueous binder is less than 5% and is preferably between 0.5 and 4.5%.  7 - Process according to claim l, characterized in that the mixture consisting of ash, aqueous binder and possibly other components, has a weight ratio of the amount of dry matter to the amount of water between 1 and 1.35, and preferably between 1.1 and 1.3.  8 - Material obtained by implementing the method according to any of claims 1 to 7.  9 - Use of the material according to claim 8 as backfill material.