GB2302090A - Immobilisation of metal-containing waste using cement - Google Patents

Description

2302090 1 WASTE IMMOBILISATION This invention relates to the disposal of

metal-containing waste sludges.

Metal-containing sludges are a common by-product of metal-working industries. Their effective and environmentally-acceptable disposal represents a major problem. Chemical treatment to render sludges disposable in normal waste water disposal channels is very expensive, because of the large volumes involved and the wide variety of materials in the sludges, the latter meaning that a chemical treatment which is effective in one case may not be effective in another.

A proposal for handling such sludges has involved their being blended with cement and cast into blocks which can then be stored or even used. In theory, this idea has much to commend it; it is relatively cheap (portland cement is cheap) and it can accomodate all types of metal-containing waste. However, in practice, there are limitations - many of the metals act as retarders for cement (in some cases, the cement does not harden at all), and the strength of the blocks is often not sufficient. Moreover, some metals, particularly zinc, are highly mobile and can leach from concrete.

It has now been found that such wastes can be successfully immobilised by the use of a particular cementitious composition. There is therefore provided, according to the present invention, a process for the immobilisation of a fluid, metal-containing waste, comprising the rnixing of the waste with a cementitious composition, wherein the cement comprises at least 5% by weight of high alumina cement.

By "immobilisation" is meant that the metal content either will not leach out or will leach out only to an acceptably small extent. For the purposes of this invention, the strict Swiss standards (Technische Verordnung fiber Abfgle (TVA), December 1990) have been taken as the standard. Under the testing regime laid down in this requirement, the maximum tolerable leaching of zinc, as hereinabove mentioned, one of the most mobile metals, is 2 1Orng/1. This is the maximum permitted leaching for the purposes of this invention.

For the purposes of this invention, the cement should comprise a minimum of 5% by weight of high alumina cement (HAQ. In theory, all of the cement may be HAC, but this is very expensive and for most purposes completely unnecessary. In addition, better performance may be obtained by blending the HAC with much cheaper ordinary portland cement (OPQ. The preferred weight ratio of HAC to OPC is from 5:95-90:10, preferably from 10:90-60:40.

In a further embodiment of this invention, it has been found that a number of additives significantly and surprisingly enhance the already good performance of the invention. For example, the addition of very fine silica, such as fly ash which occurs as a residue from furnaces, reduces the water permeability of the solid material. Silica fume is used in the proportion of 2-40% by weight on HAC. A particularly convenient way of adding fine silica is to incorporate it into HAC and the combined additive can then be mixed with whatever quantity of OPC is required. The invention therefore also provides an admixture which comprises HAC and fine silica in the weight ratio of 95:5 - 85:15.

The addition of plasticisers and superplasticisers improves the compressive strength of the hardened cementitious compositions. The plasticisers or superplasticisers useful in the present invention may be selected from the very wide range of such materials known to and used by the art, typical examples including lignosulfonates, hydroxycarboxylic acids, carbohydrates, melamine-formaldehyde condensates, Pnaphthalene sulfonate- formaldehyde condensates ("BNS) and styrene-maleic anhydride copolymers. The last-named are especially preferred, and the most preferred of these are styrene-maleic anhydride-derived copolymers in free acid or salt form and selected from the group consisting of those having the following types and numbers of monomer units:

3 CH- CH-2 -H-7H- - 1 0c 11 0' c, 0 z 1 j -7H-CH- 1 CO Llu 1 1 OH U L_ Y -0)ni---JR in which R is an C2-6alkylene radical R, is a C,-21-, C6.9cycloalkyl- or phenyl group, x, y and z are numbers from 0.01 to 100 m is a number from 1 to 100 and n is a number ftom 10 to 100 with the provisos, that i) the ratio of x to (y + z) is from 1: 10 to 10: 1 inclusive, ii) the ratio of z:y is from 3:1 to 100:1 and iii)m + n = 15-100 r and those having the following t:ypes and numbers of monomer units:

- 1 -C) X - CH- H- 1.;u CO 1 1 UM U A m Y H- CH- 1 CO CO 1 1 OH 0 (R-O)W R, n in which M is hydrogen or the residue of a hydrophobic polyalkylene glycol or polysiloxane, R,, m and n are as hereinabove defined, x, y and z are numbers from 1 to 100 15 with the provisos that 4 i) the ratio of x to (y + z) is from 1: 10 to 10: 1 inclusive, ii) the ratio of z:y is from 5: 1 to 100: 1 and iii) m + n = 15-100.

These copolymers (hereinafter "SMA") are described in United States Patent 5,158,916 and French Published Application 2 671 090 the contents whereof are incorporated herein by reference.

Plasticisers andlor superplasticisers may be added at a rate of 0.05-2.5% by weight on total cement. In an especially preferred embodiment of the invention, the plasticiser or superplasticiser may be incorporated into HAC, or more preferably into a mixture of HAC and fine silica as hereinabove mentioned, to give an admixture which is especially useful in the immobilisation of wastes. This can of course only be done if the plasticiser or superplasticiser is dry, that is, essentially free of water, as the presence of water will have an adverse effect on storage stability. The invention therefore. provides an admixture for cementitious waste immobilisation compositions, which comprises a mixture of HAC and dry plasticiser or superplasticiser in the proportions of 100:0.25 100:15 by weight. The invention also provides such an admixture which additionally contains fine silica. These admixtures are especially good when the superplasticiser is a melamine- formaldehyde condensate or an SMA type as hereinabove defined.

In a further preferred embodiment, the addition of paraffin gives enhanced performances.

In particular, it reduces leaching of metals, even highly mobile metals such as zinc, to hitherto unattainable low levels. By "paraffin" is meant a higher alkane having a fusion point above ambient temperature (see, for example, R6mpp "Chernie Lexikon", 9th edition (Thieme Verlag 1989), volume 1, page 102, the contents of which are incorporated herein by reference). A preferred form of paraffin is a paraffin emulsion of a type which forms a film, but not a coherent film such as that formed by a paint. The emulsion may be stabilized by any convenient means, but it is preferable to use an anionically-emulsified paraffin mixture (fusion point of 45-51C) with a particle size of less than 2p. Examples of such paraffin emulsions are "Mobilcer" 55 or "Mulrex" 62 from Mobil and "Ubatol" FPG 860 from Cray Valley and "Tecol" BC 60/40 from Trfib Emulsion Chemie. Another form of paraffin useful in the working of this invention is solid, particulate paraffin, such as the so-called "microparaffins". A typical example of such a paraffin is "Luwax" (trade mark) ex BASF.

Such paraffins do not perform quite as well as the emulsions, but they do have one substantial advantage; being dry, they can be incorporated into any one of the three admixtures (HAC + silica, HAC + plasticiser/superplasticiser, HAC + silica + plasticiser/superplasticiser) hereinabove described to give a stable material with good properties. The admixture comprising all four components (HAC, silica, plasticiser/superplasticiser and paraffin) with its convenience of handling and its good all round properties is a particularly desirable material.

In some situations, it may be necessary to mix the cementitious composition at a location remote from where it will be used and transport it there. This may involve an appreciable lapse of time between mixing and use, something which may not be advisable with some combinations of HAC and OPC which are known to harden very quickly. This can be overcome by including in the cementitious composition on mixing a retarding admixture.

This can be selected from any such admixtures known to the art, for example, lignosulphonates, carbohydrates, and melamine-forrnaldehyde condensates. A thorough discussion of such admixtures may be found in standard texts known to the art, for example "Concrete Admixtures Handbook" (ed. Ramachandran, Noyes, 1984), the contents whereof are incorporated herein by reference. However, an especially preferred class of materials which may usefully be used with the materials of the present invention are the phosphonic acid derivatives which are used to retard the set of cement and concrete. Preferred phosphonic acid materials are those which comprise carboxy, hydroxy or amino groups.

An example of the carboxy group-containing class is "Bayhibit" AM (trade mark) (ex Bayer AG, Leverkusen, Germany).

Examples of the hydroxy or amino group-containing material are the commerciallyavailable "Dequest" (trade mark) material (ex Monsanto Co. St. Louis, USA). Typical examples of "Dequest" materials suitable for use in the working of this invention (more than one of which may be used) include- 6 - "Dequest" 2000: aminotri(methylenephosphonic acid) "Dequest" 2006: aminotri(methylenephosphonic acid)-pentasodium salt "Dequest" 2010: 1-hydroxyethylidene-l,l-diphosphonic acid - "Dequest" 2016: 1-hydroxyethylidene-l,l-diphosphonic acid-tetrasodium salt 5 "Dequest" 2041: ethylenediarffinetetra(methylenephosphoiiic acid) "Dequest" 2047: ethylenedianiinetetra(methylenephosphonic acid)calciunilsodium salt - "Dequest" 205 1: hexamethylenedianiinetetra(methylenephosphonic acid) - "Dequest" 2054: hexamethylenediaminetetra(methylenephosphonic acid)potassium salt "Dequest" 2060: diethylenetriaminepenta(methylenephosphonic acid) "Dequest" 2066: diethylenetriaminepenta(methylenephosphonic acid)- sodium salt.

In a preferred embodiment, these materials are used in admixture with hydroxycarboxylic acids, a particularly preferred material being citric acid. Without restricting the invention in any way, it is believed that these materials act to slow down, or even stop completely, the hydration reactions which take place when water is added to a hydraulic binder such as cement. In addition to these phosphonic acid derivatives, it is also possible to use an activator to recommence the hydration reaction. Such a system is described, for example, in United States Patent 4,964,917, the disclosures of which are incorporated herein by reference. Typical examples of materials which may be used include ASTM C 494 Class C type admixtures, preferably those which are chloride-free, such as calcium nitrate and formate, thiocyanates, triethanolamine and glycolurils.

The method and admixtures of this invention are useful in the treatment of fluid, metalcontaining wastes. By "fluid" is meant that the wastes are inherently fluid (usually the case) or that they can be rendered fluid, for example, by the addition of a suitable liquid such as water or organic solvent. Ideally, wastes should have a water content of about 40- 60% by weight; in most cases, this requires dewatering, which is readily achieved by conventional means. The process of the present invention simply involves mixing the waste with the cementitious composition and allowing it to harden. The proportions of the materials needed to immobilise a given waste will depend on a number of factors such as the nature of the waste and the solids content thereof, and it is therefore not possible to give precise quantities which will work in every case. However, the skilled person may dw 7 readily ascertain suitable quantities by simple experiment. Typical quantities required are HAC:OPC (weight ratio 5:95 90: 10) silica fume superplasticiser paraffin emulsion waste 5-80% by weight of total composition 2-20% by weight of OPC + HAC 0. 1 -5.0% by weight of OPC + HAC 03-5.0% by weight of OPC + HAC 20-90% by weight of total Water to make up a cementitious mixture with a minimum workability is not included in this composition. Generally, there is sufficient water present in the components to make any further addition of water unnecessary. Should this not be the case, the skilled person can easily provide the necessary (small) addition of water.

The superplasticiser quantity given here is based on commerciallyavailable materials which are aqueous solutions. The actives quantities referred to hereinabove should be observed.

The waste and the cementitious composition may be mixed in any suitable mixing apparatus (for example, a concrete mixer). It may then be rendered in any suitable form, for example, poured as a slab or cast into blocks. In all cases, the waste has been reduced from Swiss category "Sonderabfall" (waste requiring special stringent precautions in handling and storage) to 'Reststoff '(waste requiring minor precautions), and in many cases the waste may be classified as 1nertstoff '(waste requiring no precautions). In some cases, the blocks or slabs need not be merely rubbish, but are inert enough to be put to practical use.

The invention is further illustrated by the following non-limiting examples.

AM 8 Examples 1-10

A number of cement mixes, numbered I to 10 are prepared from the ingredients shown in Table I below. To each of these mixes is added a zinc-containing sludge (originating from the galvanising industry and having 131,800 mg Zn/kg waste (as measured by AAS)). The cement is cast into blocks and the strength tests are carried out according to German Test Method DIN 18555. The leaching test is performed by adding 1000ml deionised water to 100g broken solid cement (particle greatest dimension 5mm) and then injectingC02 into the system. The injection Of C02 is carried out for 24 hours so that the water is saturated. It is then filtered through a 0.45pm filter and the filtrate analysed by atomic absorption 10 spectroscopy.

The results of the testing are shown in Table 1.

h 9 Table 1

1 475 25 0 0 500 350 1.0 1617 0.36 0.13 3.50 3.14 0.36 2 475 25 0 500 325 0.6 1657 0.30 0.67 3.97 5.17 0.31 3 475 25 10 0 500 300 0.5 1683 0.57 0.55 5.27 7.09 0.34 4 475 25 15 0 500 275 0.4 1696 0.62 0.77 5.84 8.75 0.29 475 25 5 5 500 325 1.3 1634 0.45 0.42 5.51 5.84 0.20 is Mix No.

Mix Design HAC silica fume superplasticiser' paraffin emulsion2 Zn-Sludge Water Fresh mixture Air content Unit weight 6 475 25 10 500 325 2.9 160 8 0.32 0.53 3.62 5.22 0.20 7 475 25 15 500 325 4.0 1592 0.37 0.67 3.30 4.76 0.19 8 285 15 3 0 700 300 0.8 1548 0.16 0.29 0.72 1.25 0.46 9 665 35 7 0 300 385 0.6 1730 0.12 500 0 0 0 500 300 2.1 parts by wgt.

[Kg/m31 Strength Flexural 7d 28d Compres. 7d 28d Leaching test Zinc [MPa] [MPa] [MPa] [MPal [mg/11 1. "Rheobuild" 3520 ex MBT (Schweiz) AG 2. "Masterkure" 122 ex MBT (Schweiz)AG am From these results, it can clearly be seen that Examples 11 - 14 - the zinc content of the leached solution decreases with increasing cement content (mixes 1, 8, 9) - the compressive strength increases with increasing quantity of superplasticiser (rnixes I - 4) - the zinc content of the leaching solution is reduced by the presence of paraffin emulsion (mixes 2 and 5) - the replacement of 10% of the cement with silica fume causes a reduction in zinc leaching (mixes 1 and 10) These examples utilise mixtures of HAC and OPC. The mixes are submitted to the same tests as are examples 1 - 10 and the results are shown in Table 2.

Table 2 is Mix No. 11 12 13 14 Mix Design OPC [g] 300 225 0 HAC [g] 0 75 300 superplasticiser' [g] 7.5 7.5 7.5 filter ash 2 [g] 100 900 600 600 Water [g] 500 550 550 Corriv. strength 7d [MPal 0 2.6 3.3 Leaching test L. [mglll 301.7 21.2 4.9 3.6 (1) superplasticiser as Examples 1 - 10 (2) residue from garbage incinerators It can be seen that the HACIOPC mixture gives zinc leaching and 7 day strength results which are very good, sufficiently good to place the waste in the best category.

Moreover, they are nearly as good as those of the HAC alone, and are achieved at a considerably reduced cost.

Claims (10)

CLAIMS:

1. A process for the immobilisation of a fluid, metal-containing waste, comprising the mixing of the waste with a cementitious composition, wherein the cement comprises at least 5% by weight of high alumina cement.

2.A process according to claim 1, wherein the cement is high alumina cement.

3. A process according to claim 1, wherein the cement is a mixture of high alumina cement and ordinary portland cement in the weight ratio of from 5:95-90:10.

4. A process according to any one of the previous claims, wherein the cementitious composition additionally contains very fine silica, preferably to the extent of from 2-40% by weight of the high alumina cement.

5. A process according to any one of the previous claims, wherein the cementitious composition additionally comprises paraffin, preferably in either solid particulate or aqueous emulsion form.

6.A process according to any one of the previous claims, wherein the cementitious composition additionally comprises plasticiser andlor superplasticiser.

A process according to claim 6, wherein the superplasticiser is selected from styrene-maleic anhydride-derived copolymers in free acid or salt form and having the following types and numbers of monomer units:

AM 12 CH- C+2- -H-H- -7H-CH 1 ,c, C. CV lu 0 0' 0 1 1 Y n in which R is an C2_.alkylene, radical R, is a C,_walkyl-, C,,. 9cycloalkyl- or phenyl group, x, y and z are numbers from 0.01 to 100 m is a number from 1 to 100 and n is a number from 10 to 100 with the provisos, that i) the ratio of x to (y + z) is from 1:10 to 10:1 inclusive, ii) the ratio of z:y is from 3:1 to 100:1 and iii) m + n = 15- 100 and those having the following types and numbers of monomer units:

[1 16 - X -cw-F+1 k-L) CO 1 1 Uri U 1 m Y (R-0); R m 1 CH- 1 L;0 UM U n in which M is hydrogen or the residue of a hydrophobic polyalkylene glycol or polysiloxane, R,, m and n are as hereinabove defined, x, y and z are numbers from 1 to 100 with the provisos that i) the ratio of x to (y + z) is from 1: 10 to 10: 1 inclusive, 13 ii) the ratio of z:y is from 5: 1 to 100: 1 and iii) m + n = 15-100.

8. A process according to any one of the previous claims, wherein the cementitious composition additionally comprises a retarding admixture.

9. A process according to claim 8, wherein the retarding admixture is a phosphonic acid derivative, preferably one with carboxy, hydroxy or amino groups.

10. A solid ccmentitious composition suitable for use in the immobilisation of fluid, mctal-containing wastes, comprising a mixture of high alumina cement, very fine silica, plasticiscr/supcrplasticiser and solid particulate paraffim.