GB2224024A

GB2224024A - Immobilising wastes containing heavy metals

Info

Publication number: GB2224024A
Application number: GB8923465A
Authority: GB
Inventors: Walter Lukas; Hans Spannbauer; Berndt Meixner
Original assignee: SALZBURGER STADTWERKE AG
Current assignee: SALZBURGER STADTWERKE AG
Priority date: 1988-10-18
Filing date: 1989-10-18
Publication date: 1990-04-25
Anticipated expiration: 2009-10-18
Also published as: CH678018A5; ATA237489A; AT396558B; FR2638109B1; GB2224024B; FR2638109A1; GB8923465D0

Description

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1- 2224024 Process for immobilizing and consolidating solid and liquid materials having a heavy metals content .DTD:

Description .DTD:

A process for immobilizing and consolidating materials containing heavy metals is known from WO 88/02,739. In this process, the material to be disposed of is mixed with an ash from a fluidized-bed furnace and/or with an electrostatic precipitator ash from a power station, the water/hydrate phase-forming substances ratio being adjusted to a value between 0.23 and 0.35.

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The known process has proved itself in principle, but the proportion of binder is as a rule far more than 50 % by weight and the water permeability values (k values) are restricted to orders of magnitude of 10-I" cm/second.

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In the light of this, the invention is based on the object of immobilizing in a suitable binder solid and liquid materials containing heavy metals, such as arise, for example, as waste materials in refuse incinerators, special refuse incinerators, in effluent treatment sludges or the like, and consolidating them therein, and in particular in such a way that the water permeability values, known from the state of the art, of the consolid- ated product are exceeded, and especially even if the binder proportion is lower.

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To achieve this object, which is described in more detail below, the invention exploits the following findings:

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As compared with a pure fly ash as is used in WO 88/02,739, the immobilization and consolidation of residues containing heavy metals and also organic residues can be improved if the binder comprises the following components:

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an at least latent hydraulic silicate component of a fly ash, boiler ash, amorphous silica, slag (for example granulated blast-furnace slag) or the like in a grain size smaller than i00 #m, at least 80 % by weight being smaller than 40 #m, an alkaline and/or preferably alkaline-earth component based on CaO, Ca(0H)2, MgO or Mg(0H)2 as initiator for the latent hydraulic component, and calcium sulphate and/or calcium sulphite, anhydrous or in the hydrate form.

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Starting from such a binder mixture, it was also found that the desired result is obtained only if the weight ratio between the silicate component and the alkaline/alkaline-earth component together with calcium sulphate and/or calcium sulphite is adjusted to 60- 92 to 8-40, where the proportion of the alkaline and/or alkaline-earth component must be at least 3 to 5 % by weight.

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If, for example, a fly ash is used which already has a content of free CaO, Ca(OH)2, MgO or Mg(OH), the corresponding component is then separately added only proportionately. This also applies to the proportion of calcium sulphate and/or calcium sulphite (if appropriate in the hydrate form).

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In order to obtain the desired result, careful matching of the individual components is necessary. The calcium sulphite and/or calcium sulphate content in the binder should be between 3 and 25 % by weight. In fact, it has been found that, at a lower content, the leachability of the heavy metals is increased and the water permeability rises. If components containing calcium sulphite and/or calcium sulphate are used in greater proportions, the mechanical strength of the consolidated products deteriorates, since blowing phenomena then occur which lead to bursting of the consolidated body and unfavourably increase its leachability.

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The minimum content of alkaline and/or alkalineearth components of from 3 to 5 % by weight is important in order to initiate the hydraulic setting of the latent hydraulic binder component and, at the same time, to achieve adequate immobilization of the pollutants in the hydrate phases.

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The latent hydraulic component fulfils its action i0 within the said binder in a preferred manner whenever it is particularly finely dispersed, which is the reason why a grain size smaller than 100 m, preferably finer, is proposed. The immobilization of the heavy metal ions within the hydrate phases, formed after addition of water, is then particularly intensive.

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Using the abovementioned binder, both solid and liquid materials having a heavy metals content can be immobilized in the hydrate phases. In this case, the proportion of binder should be between 20 and 60 % by weight of the material to be disposed of, corresponding to between 40 and 80 % by weight. A water content of 15 to 32 % byweight, relative to the substances participating in the formation of hydrate phases, permits, on the one hand, homogeneous thorough mixing of binder and material to be disposed of, but leads at the same time also to a complete hydration of the reacting fractions and to a more favourable immobilization of the heavy metal ions to be disposed of.

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Surprisingly, it has been found that, when the said binder is used in the said quantities, the water permeability values (k values) are about one to two powers of ten below those mentioned in WO 88/02,739. In numerous test mixtures, it was no longer even possible to force measurable quantities of water at all through the set specimens when applying a pressure of 5 bar. The leachability of set products, which had been ground, was also so low that it was far less than the statutorily prescribed maximumquantity for landfill sites (landfill class II according to the draft guidelines of the state of North-Rhine Westphalia). In most cases, even the potable water quality requirements according to Austrian Standard M 6250 were exceeded.

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The setting times of the mixtures of the material containing heavy metals and the said binder admittedly depend on the particular composition, but are in general in the range from a few hours to a few days. At the latest after a week, the consolidation is concluded to such an extent that the products can be transported or dumped without problems, unless immobilization and consolidation initially already took place in a landfill site.

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It is of course possible to add additives of generally known types to the mixture of material to be disposed of and binder. These include, for example, slag from metallurgical plants, which, in contrast to granulated blast-furnace slag, is not hydraulic and, in this respect, is introduced as an inert additive material into the system.

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It has found to be particularly preferable to use the binder component containing calcium sulphite/calcium sulphate in the form of dry residues from spray absorption units, such spray absorption units being operated with calcium oxide or aqueous solutions of calcium hydroxide or calcium carbonate, and the residues also containing unspent calcium hydroxide in addition to calcium sulphite and calcium sulphate, if appropriate in the hydrate form, so that the corresponding quantity of separately added alkaline and/or alkaline-earth component can be reduced to the same extent.

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According to an advantageous embodiment of the process, the use of fused alumina cement is proposed as the initiator for the latent hydraulic component. In contrast to the conventional silicate cement such as Portland cement, it consists essentially of calcium aluminates. It is used above all as a binder for refractory mortars and concrete. To this extent, it must be surprising that the use of preferably from 3 to 6 % by weight of fused alumina cement within the binder described leads, evidently due to a synergistic interaction with the other components, to quite outstanding leaching and water permeability values, some of which are even better than those mentioned above. Moreover, it has found to be advantageous to use only those latent hydraulic components the SiO2 content of which is at least 40 % by weight. These include the said fly ashes from power station furnaces, boiler ashes or latent hydraulic slags (granulated blast-furnace slags). Binders prepared from these (without separately added amorphous silica) contain, for example, 40-55 % of SiOz, 15-25 % of Alz03, 5-30 % of CaO and 3-12 % of S03/S04 (all in % by weight).

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From these ashes (latent hydraulic binders), those ashes must be distinguished which are themselves contaminated with pollutants, for example ashes from refuse incineration including special refuse incineration. In these, the SiO2 is in most cases below 40 % by weight and sometimes below I0 % by weight. As stated above, both solid and liquid materials can be disposed of. The solid materials include, for example, effluent treatment sludges, materials polluted by heavy metals or organic substances, such as soils, refuse tips and flotation residues and the like. Liquid materials include, for example, effluents from flue gas desulphurization units, industrial or municipal effluents and seepage waters from landfill sites. If liquid materials are immobilized in the binder, the proportion of separately added water can be reduced correspondingly.

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To facilitate compaction and further reduction in leaching, plasticizers and/or sealants such as melamine resin, naphthalenesulphonate, ligninsulphonate or alcohol-soluble silicones can be added to the mixture. The added quantity, relative to the substances forming hydrate phases, is between 0.2 and 3 % by weight. The water content of the total mixture is preferably chosen such that it is slightly above the Proctor value, but the mixture is still Just capable of compaction on a large scale.

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These measures lead to particularly advantageous compaction and reduction of the capillary porosity.

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If the latent hydraulic binder component is not present in adequate fineness, it is ground up separately. In this case, grinding should preferably be taken directly to particle sizes smaller than 10 #m, because it has been found that the immobilization of the pollutants can be further improved with increasing fineness of the latent hydraulic component.

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Further features of the invention can be seen i0 from the features of the subclaims and the other application documents.

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The invention is explained in more detail below by reference to various examples: Example 1:

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A mixture 1 was prepared from a binder consisting of (all data in % by weight):

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a) 80 % of boiler ash (grain size smaller than 40 #m, of which 80 % smaller than i0 m) b) 8 % of calcium hydroxide c) 12 % of dry residues from a Niro spray absorption unit operated with an aqueous calcium hydroxide solution, which also contain 55 % of ash and free CaO of a grain size smaller than 40 #m in addition to 45 % of the component containing calcium sul- phate/calcium sulphite.

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% of this binder were mixed with 50 % of solid residues from a refuse incinerator (ash/filter cake = 80/20 %), adjusting the water/hydrate phases-forming substances ratio to 0.28. The mixture was then compacted. After setting, the following compressive strengths were measured:

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28 days: 17 N/mm2 days: 25 N/mm2 days: 31 Nlmm2 The water permeability value (k value) could not be determined, that is to say it was smaller than 10"12 cm/second. The k value was determined analogously to DIN 18130. Example 2:

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Mixture 2 was prepared in a manner analogous to that described in Example I. The binder consisted of (all data in % by weight): a) 65 % of fly ash from a coal-fired power station, of a grain size smaller than 40 m, 80 % by weight again having a grain size smaller than i0 #m, c) 35 % of dry residues from a spray absorption unit analogous to Example i.

Component b), that is to say the initiator for the latent hydraulic component a), was already present in the fly ash according to component a), in a proportion of 18 % of free CaO. 0.6 % of a ligninsulphonate was added, relative to the fly ash proportion.

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% of the abovementioned binder were then mixed with 70 % of solid residue from aspecial refuse incinerator (70 % of slag, 25 % of ash, 5 % of filter cake). The water/hydrate phases-forming substances ratio was 0.32. The mixture was compacted. After setting, the following compressive strengths were measured: 28 days: 14 N/ram2 90 daysz 23 N/ram2 days: 33 N/ram2 The k value in cm/second was initially still approximately 10"11 and, after 90 days, it could no longer be determined, that is to say it was below 10-12. This sample shows an even smaller water penetration than the mixture according to Example I. Since a determination of the k value was not possible, the water penetration was measured according to DIN 1045.

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Claims

Patent Claims i. Process for immobilizing and consolidating solid and

liquid materials having a heavy metals content with the use of an at least latent hydraulic binder, characterized in that, to obtain the lowest possible leaching values and low water permeability values, a binder having the following features is used:

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a) an at least latent hydraulic silicate component of fly ash, boiler ash, slag or the like having a grain size smaller than i00 #m, of which at least 80 % by weight is smaller than 40 #m, b) an alkaline and/or alkaline-earth component based on CaO, Ca(OH)2, MgO, Mg(OH)2 as initiator for the latent hydraulic component, and c) calcium sulphite and/or calcium sulphate, anhydrous or in the hydrate form, the weight ratio of component a) relative to the total of components b) and c) being 60 to 92: 8 to 40 and the content of component b) being at least 3 to 5 % by weight, and this binder being mixed with the material to be disposed of in a weight ratio of 20 to 60: 40 to 80 at a water content of between 15 and 32 % by weight, relative to the substances which form hydrate phases, and being compacted and then hardened.

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2. Process according to Claim 1 with the proviso that the weight ratio of component a) relative to the total of components b) and c) is adjusted to 60 to 80 20 to 40.

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3. Process according to Claim 1 or 2 with the proviso that a binder having a content of component c) between 3 and 25 % by weight is used.

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4. Process according to one of Claims 1 to 3 with the proviso that, as component c) of the binder, the dry residues from a spray absorption unit operated with calcium oxide or aqueous solutions of calcium hydroxide or calcium carbonate are used, which also contain unspent calcium hydroxide in addition to calcium sulphite and calcium sulphate, if appropriate in the hydrate form.

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5. Process according to one of Claims 1 to 4 with -- 9 -- S i02 AI203 Ca0." S03, S04" the proviso that a latent hydraulic component a) having a predominant grain fraction smaller than i0 #m is used.

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6. Process according to one of Claims 1 to 5 with the proviso, that component b) is replaced wholly or partially by fused alumina cement.

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7. Process according to Claim 6 with the proviso that fused alumina cement is used in a quantity of from 3 to 6 % by weight, relative to the binder in total.

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8. Process according to one of Claims 1 to 7 with the proviso that a latent hydraulic component a) is used, the Si02 content of which is at least 40 % by weight.

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9. Process according to one of Claims 1 to 8 with the proviso that a binder having an amorphous silica content and the following chemical analysis in % by weight is used:

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to 85 5 to 45 8 to 35
i0.

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the proviso that a plasticizer and/or a sealant is added to the mixture of binder and material to be disposed of.

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ii. Process according to one of Claims 1 to 10 with the proviso that the water content of the mixture is adjusted such that it corresponds to the highest possible compaction in the Proctor test.

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12. Process according to one of Claims 1 to ii with the proviso that filter dusts, filter residues from refuse incinerators, including special refuse incinerators, effluent treatment sludges, electroplating sludges, liquid process waters, ashes having an Si02 content of less than 40 % by weight or the like are used as the material containing heavy metals which is to be disposed of.

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3 to 12 Process according to one of Claims 1 to 9 with Puhlmhed 1990 at The Patent Office, State House, 66/71 Hgh Holborn, London WC 1R 4TP. Further copies may be obtained,om The P&tent Office. Sales Branch, St Mary Cray, 0rpmgton, Kent BP,,5 3RD. Printed by Muluplex techniques ltd, St Mary Cray. Kent. Con. 1/87