SK279287B6 - Process for fixing filter dust containing heavy metals in ceramic mouldings - Google Patents

Abstract

The filter dust is washed in an alk. reacting medium being mixed with clay and water forming mouldings from the mixture, and firing the green moulding at a temperature between 900 and 1100 °C. The firing temperature is selected in such a manner that a melt phase is formed below the softening point thus leading to vitrification and hence to permanent occlusion of the heavy metals in the moulding.

Description

The filtered dust is washed in an alkaline reacting medium, then mixed with clay and water, the mixture is formed into blanks, the blanks are baked at a temperature between 900 degrees Celsius and 1100 degrees Celsius, the baking temperature below the softening point being selected the molten phase which causes vitreous and therefore the permanent incorporation of the heavy metals and their compounds into the compact.

SK 279287 Β6

Technical field

The invention relates to a process for bonding filtered dust containing heavy metals into ceramic moldings by mixing the filtered dust with clay, forming the blanks from the resulting mixture, and curing or sintering the blanks.

BACKGROUND OF THE INVENTION

This method is known from DE-OS 36 12 381. The purpose of the known method is to prepare filtered dust containing poisonous heavy metals by binding it to a matrix material so that it can be permanently deposited in simple landfill sites. For this purpose, the filtered dust is mixed with clay and an additive material is added to the mixture which reduces the capillarity and water-binding capacity of the clay. Subsequently, the resulting mixture is compressed into shaped bodies of any size and shape that can be stored. The additive material, which may be a sulfonated, water-soluble oil, prevents the absorption of the shaped bodies and thus the scouring of heavy metals. Moldings can be stored in molded and air-dried molds, but to further improve their physical and chemical resistance, moldings can be fired at temperatures between 600 ° C and 900 ° C or sintered at temperatures between 1100 ° C and 1200 ° C . If the shaped bodies are only compressed but not burnt or sintered, they have only very low compressive strength and can only be deposited in landfills, but can no longer be used as building material. In addition, too much heavy metals can be permanently washed out of them. Too large scrubbing ratios of heavy metals can also be found when moldings or hard-burnt moldings are tested, and the sintered moldings have the further disadvantage that a significant proportion of compounds and heavy metals are ejected from the moldings during sintering.

It is known from DE-PS 36 30 697 to mix sludges containing heavy metals, in particular galvanized sludges, with clay and fluxes and then to burn them into clinkers at temperatures between 750 ° C and 1150 ° C, the residual porosity of the clinkers being less than 6%. However, the conditions that work best when the sludges containing heavy metals form clinkers cannot be transferred to the treatment of the filtered heavy metal containing dust, which occurs, for example, in waste incineration plants because the sludges containing heavy metals and the filtered dust, containing heavy metals, their composition is very different. For example, the sludge largely contains organic fractions which are almost completely absent from the filter dust. Since, for economic reasons, an attempt is made to maximize the proportion of sludge or filtered dust in admixture with clay, it is understandable that moldings made of sludges containing heavy metals behave differently than moldings made of filtered dust.

Experiments which used the method of binding filtered dust, known from DE-PS 36 30 697, have shown that mixtures of clay and filtered dust exhibit the necessary sintering behavior only at relatively high firing temperatures above 1150 ° C, leading to the fixing of heavy metals in ceramic mass. At such a high sintering temperature, a strong tendency to swelling can be observed, thereby increasing the porosity and with it the danger that heavy metals will be expelled from the ceramic moldings. In addition, it has been shown that a large proportion of heavy metal compounds contained in the filtered dust, in particular lead, copper and zinc, can escape with the furnace combustion gases into the free space, which must in any case be avoided.

It is known from EP-A-170 that from a mixture of clay and filtered dust at a baking temperature between 700 degrees Celsius and 1300 degrees Celsius, a granulate in which the filtered dust is bound can be fired. However, this granulate has a large pore volume as a ceramite and exhibits too high scrubbing ratios.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method particularly suitable for the permanent binding of filtered powders in ceramic moldings, which is associated with low expulsion of heavy metal compounds during firing.

SUMMARY OF THE INVENTION

This object is achieved by the method of binding the filtered heavy metal-containing dust in the ceramic moldings according to the invention, the principle being that the filtered dust is washed in an alkaline reacting medium, the washed filtered dust is mixed with clay and water, formed into semi-finished products. at a temperature between 900 degrees Celsius and 1100 degrees Celsius, the firing temperature below the softening point being selected such that a molten phase is formed which causes glazing and therefore the permanent incorporation of the heavy metals and their compounds into the compact.

Silicon-rich clays are best suited for binding heavy metals from sludge after galvanization into ceramic moldings. Therefore, the inventors have also worked with these clays in their first attempts to bind the filtered powders to the ceramic moldings. However, it has been shown that with these silicon-rich clays, the filtered powders cannot in fact be bound in a satisfactory manner: cadmium and lead were almost completely expelled during the firing, the volatile proportions of copper and especially zinc present in the filtered dust in large quantities, god unacceptably high. However, the inventors have found that small expulsion of heavy metals can be achieved with the aid of aluminum-rich clays. Therefore, in the process according to the invention a clay is used which contains at least 25% by weight, preferably more than 30% by weight, of aluminum oxide in the dry matter. As a result, the ejection of heavy metal compounds can be further reduced by the proposed use of fluxes. However, this is redeemed by increasing the firing temperature, which leads to higher swelling, and reducing the silicate content, which is important for the silicate bonding of heavy metal compounds into the ceramic mass and for the resistance to scouring from the compacts.

The inventors have found a surprisingly simple possibility to reduce the ejection of heavy metal compounds during firing and at the same time to achieve a high resistance of ceramic moldings to scrubbing of the heavy metal compounds. In order to achieve this, it is sufficient to wash the filtered dust with an alkaline reacting medium. In the simplest case, scrubbing can be done with water, since the filtered dust from the combustion plants regularly contains metal oxides, in particular CaO, which form hydroxides with water. Therefore, an alkaline medium is already formed during water scrubbing. If the alkalinity produced is not sufficient in the individual case, it can be increased by adding lime (CaO) in the simplest way. Although it is also possible to add NaOH or KOH, calcium Ca on firing is less disruptive than sodium Na or potassium K. It is preferable to ensure that the pH is at least pH = 9.

The scrubbing of filtered dust has decisive advantages: Heavy metal compounds which have been largely expelled during firing, in particular lead, zinc, copper and cadmium chlorides, become less volatile

Compounds a will decrease from the alkaline medium. The sulphates, which contribute to the swelling of the moldings to a greater extent, will wash to a large extent (on the order of 40%). Together with chloride ions, alkali metal ions and alkaline earth ions, they pass into solution and are substantially no longer contained in the filtered and practically dehydrated filtered dust, as opposed to heavy metal compounds which are ceramic-fixed.

If the washed-out filtered dust blanks together with clay - and naturally with water to be used for crusting - are pressed and fired, then the ejection of heavy metals is already extremely small in advance, since the previously present slightly volatile heavy metal compounds have been converted into heavier volatile compounds. Therefore, there is no reason to use A1 ₂ O _3- rich clays, and rather - which is advantageous for the permanent silicate fixation of heavy metals - SiO _2- rich clays with a SiO content of 60-80% by weight in the dryer are used.

In addition, it has been shown that semifinished products produced with laundered and filtered dust are sintered at a lower temperature than the semifinished products produced with unsampled filtered dust, since the fractions of filtered dust converted by alkaline washing and not evaporated by firing appear to act as flux. In this case, lead and iron should play an important role in particular, since their hydroxides are converted into oxides in the firing process and can act as fluxes. Since, when working with the washed filtered dust, flux is produced during the mixture firing, the advantage of the sintering temperature reduction and, in addition, the described additional advantages, is achieved even without external fluxes. Thus, without special addition of flux, the most impermeable sintered compact can be fired at temperatures below 1100 degrees Celsius, which has an excellent resistance to scrubbing of heavy metal compounds, by means of the filtered dust treatment process. Since, on the other hand, when working with washed filtered dust, the small baking of heavy metals during firing and the reduced tendency to swelling no longer require the lowest firing temperature, it is possible to work with the firing temperature just below the softening temperature. a connection with a relatively small pore volume and a higher compression strength of the moldings fired in the form of clinkers.

While mixing, enough water is added to give the mixture the plasticity necessary to form the blanks, and the blanks are best extruded. The water content must not be too high in order to produce blanks of higher density and strength. Preferably, the water content is not more than 350 g, based on 1000 g of the composition prior to shaping the blanks. Preferably, the blend is produced such that the cylindrical blanks have a 30 mm killing height (corresponding to the Pfeferkom method of 1.3) when measuring the deformation of the cylindrical bodies by the pressure between two plates by the Pfeflerkom method. The Pfefferkom measurement method is described, for example, in S. Scholze's publication "Keramik", Volume 2, entitled Keramische Werkstoffe on p. 35th

For economical reasons, the proportion of washed filtered dust in the mixture is chosen as large as possible. It has been shown that excellent filter dust binding and only small scrubbing ratios are achieved even under extreme conditions where the fraction of washed filtered dust in the mixture of clay, flux and washed filtered dust is not more than 50% by weight (dry weight), preferably this proportion of washed filtered dust in the mixture is one third (based on dry matter).

According to the invention, the blanks are fired at a temperature between 900 degrees Celsius and 1100 degrees Celsius. Below 900 degrees Celsius, gas-forming reactions, such as the decomposition of calcium carbonate into CaO and CO _{2, still take place} . Below 900 degrees Celsius, the product made of the blank is still so porous that the gases formed can escape without difficulty, but it is too porous to permanently bind heavy metals. Above 900 degrees Celsius, the product becomes more impermeable and heavy metal bonding is stronger. However, with the temperature approaching 1100 degrees Celsius, heavy metals are becoming increasingly volatile, which is in principle undesirable and leads in addition to swelling of the products, since these are already sealed and heavy metals cannot escape without further action. Swelling of the products will result in failure of their shape and strength. This is counteracted if, according to the invention, the filtered dust is washed and optionally flux is added, since then a molten phase is formed during firing below 1070 degrees Celsius to 1100 degrees Celsius. The type and amount of flux is optionally selected such that at the selected firing temperature a molten filament is formed which leads to vitrification and hence permanent incorporation of the heavy metals.

However, the flux content should not be too high, since with increasing flux content the plasticity of the blend from which the blank is made decreases and the foam molded from the blend or the blank formed therefrom no longer has sufficient strength in the undried state. Preferably, the proportion of flux is at most 20% by weight, based on the total sum of clay, flux and washed filtered dust. Common fluxes which can be used for the purposes of the invention and are found in the ceramics, glass and enamel industries are in particular silicic acid, nepheline, syenite, glass powder, borax, fluorspar or enamel.

By adding flux, the temperature at which thickening occurs, in particular the softening temperature, is reduced to such an extent that the sintering can be carried out below the firing temperature of 1100 degrees Celsius, in particular still below 1070 degrees Celsius, and yet silicate bonding of heavy metals. This limitation of the firing temperature significantly reduces the observable expulsion of the heavy metal compounds during firing. Heavy metal losses, which still occur at lower firing temperatures, can be practically completely filtered off from the flue gases of the firing furnace, in particular by using molecular sieves (zeolites), which are particularly advantageous as seizure filters for this purpose. They are hardly clogged sufficiently, they are preferably added to the mixture to form further blanks, whereby the heavy metal compounds filtered from the flue gas are returned to the disposal circuit. Molecular sieves do not interfere with the firing process; on the contrary, they even act as fluxes in the desired manner.

It has been shown that sludge after galvanization can be added instead of conventional flux. The addition of sludge after electroplating also leads to a drop in the temperature at which local glazing occurs. The addition of sludge after galvanization is particularly advantageous because it also contains heavy metals, which can be permanently destroyed by binding, so that two problems are solved. As a particular advantage, it has further been found that the addition of sludge after electroplating can be well coupled to otherwise very problematic zinc. The amount of post-electroplating sludge to be added should not be greater - based on dry matter - than the fraction of filtered dust itself.

Preferably, the preforms made of washed filtered dust are fired at a temperature between 1070 degrees and 1090 degrees Celsius. At higher temperatures, the ejection of heavy metals increases.

How and to what extent heavy metals can be washed from sintered parts again depends not only on the choice of clay, flux and

However, to a certain extent also from the porosity of the sintered compacts. Therefore, there is an effort to keep the porosity as low as possible and to adapt the firing conditions to this, in particular for the time course of the firing temperature (firing curves). It has already been pointed out that, during the firing process, gases escape from the compact. Preferably, at a temperature of 900 degrees Celsius, a firing curve is selected such that gases, as well as water vapor and carbon dioxide, are expelled as slowly as possible. This ensures that the escaping gases do not cause any cracks and large pores in the compact. In contrast, other temperature ranges should run as quickly as possible in order to achieve the sintering temperature range and thus local melting at the grain boundaries as quickly as possible, which leads to the solid binding of heavy metals before they are gasified in larger quantities.

DETAILED DESCRIPTION OF THE INVENTION

Example

Chemical analysis has shown the following substance content for fly ash (quantity data are in% by weight and refer to dry matter):

CD 0,03% Zn 2.1% On the 2.2% water-soluble anions Cr 0,04% fe 3.8% The 4% Cľ 13,7% Cu 0.09% Al 7% C 4.5% SO ₄ 3.8% Ni 0.01% ca 7.1 with 1,7% residue: predominantly silicon and Pb 0.5% mg 1.5 oxygen

The fly ash was washed with normal water and filtered in a filter press, the filtered fly ash having a residual moisture of about 40%. The fly ash is mixed with silicon-containing clay for the following composition: SiO ₂ 74.9% Al ₂ O ₃ 17.7%

TiO ₂ 1.3%

Fe ₂ Oj 1,0%

CaO 0,2%

MgO 0,5%

Na ₂ O 0.1%

K ₂ O 4.1% loss of 3.5% by burning% fly ash and 66% clay (dry matter) is first mixed for 5 minutes without water and then 5 minutes with water, adding so much water that the mixture contains 30% by weight of water.

A foam having a cross-section of 33 mm x 40 mm is pressed from the mixture in a vacuum extruder using a compression pressure of 0.65 MPa and 96% vacuum. So much water is squeezed out that the foam still has a moisture content of 26.6% after extrusion. The resulting foam is divided into test specimens (semi-finished products) which are dried by firing at 130 degrees Celsius for three days. Subsequently, after 6 hours of heating, firing is carried out at 1080 degrees Celsius for 3 hours.

Comparative example

Semi-finished products are fired and fired, unlike the above example, the fly ash has not been washed and an optimum firing curve with a maximum temperature of 1080 degrees Celsius has been selected for firing.

In this optimum firing curve, temperature ranges below 900 degrees Celsius were slower to occur, with strong degassing occurring, with an approximately uniformly long heating time of 6 hours, and standing at 1080 degrees Celsius for 3 hours.

The following table compares the most important properties of the moldings produced according to the given example and the comparative example, as well as the corresponding ejection of the heavy metals during firing.

comparative example example combustion conditions heating time in h 6 (best) 6 curve) downtime in h 3 3 max. temperature 1080 1080 in degrees C combustion values shrinkage by drying in% 1.7 2.9 shrinkage 7.8 10.5 % burning loss in% 9.3 6.6 compressive strength 46.4 154 vN / mm ² raw density 1.6 2.1 in g / cm ^a open porosity 32 8.2 in% eluate values in mg / kg CD N. N. N. N. Cr N. N. N. N. Cu 0.55+ 0.1 0.23 + 0.1 Ni N. N. N. N. Pb 2,45+ 0,2 0.19 + 0.2 Zn 1,07+ 0,2 1.54 + 0.2 expulsion in% IN in% IN heavy metals mg / kg mg / kg CD 67.5 73.7 10.9 10.7 Cr 6 1.8 0.2 0.3 Cu 30.4 137.3 0.6 2.7 Ni 3.2 1.4 0.5 0.2 Pb 49.5 1 433 3.5 79.5 Zn 41.5 3 257 0.2 19.3

Table clarification:

Comparing the quality of the mixtures produced with the washed and unprocessed filtered dust

N. N. - below the perception threshold

The eluate values were determined by leaching the samples according to DIN 38 414 (part 4, DEV S4) twice 10 hours with ten times the amount of moving water on the shaking table, while the conditions according to DIN 38 414 were sharpened by constantly saturating the water with gaseous CO ₂ and was therefore acid buffered.

The comparison shows the following:

The ejection of heavy metals in the firing of compacts made of washed filtered dust is considerably less than that of un washed washed filtered dust. Only a fraction of the mass of heavy metals is expelled with unswept filtered dust.

By using the washed filtered dust, even at a relatively low firing temperature of 1080 degrees Celsius, completely sintered moldings are formed which have a high compressive strength and are therefore suitable for use as a building material.

The raw density with the washed filtered dust increases strongly and the open porosity, which is associated with the ability to elute, decreases strongly. As regards the numerical values of the ability to elute, it should be taken into account that the heavy metal content of the moldings that are produced

SK 279287 Β6 due to the absence of expulsion, it is greater than in moldings which are produced with the non-washed filtered dust.

Analyzes of the washing liquid have shown that this liquid has taken up about 40% of the chlorides and sulphates, in addition to the alkali and alkaline earth metals, especially calcium, which is advantageous for the firing process, also the washing liquid contains only negligible amounts of heavy metals advantageous, since the heavy metals are to be bound by ceramic.

Complementary studies to optimize the firing conditions have shown that the selected compositions have a low metal ejected quantity at a firing temperature of 1080 degrees Celsius, and zinc is strongly ejected when the temperature rises. Surprisingly, it has been shown that the ability to elute the heavy metals Cd, Zn, Pb, Ni, Cr shows a minimum when a firing temperature of 1070 degrees Celsius to 1080 degrees Celsius is selected.

Furthermore, it has been found that the expelled amounts of heavy metals are less with a shorter heating time, and that the ability to elute heavy metals from the compacts is minimal when a heating time of 3 to 4 hours is selected.

When using a silicon-containing clay, as in the example above, the optimum firing conditions therefore remain at a heating time of 3 to 4 hours, a maximum temperature of 1070 Celsius to 1080 degrees Celsius, and stand for 2 to 3 hours.

The ability of the eluátovania clays containing SiO ₂ of 60 to 80% by weight and a Al-O ₃ just below 20% by weight minimum, such that the clays are preferred.

Claims (21)

Method for binding filtered dust containing heavy metals in ceramic moldings, characterized in that the filtered dust is washed in an alkaline reacting medium, the washed filtered dust is mixed with clay and water, the mixture is formed into blanks, blanks are baked at a temperature between 900 degrees Celsius and 1100 degrees Celsius, wherein the firing temperature below the softening point is selected such that a molten phase is formed which causes vitrification and therefore the permanent incorporation of the heavy metals and their compounds into the compact. Method according to claim 1, characterized in that the filtered dust is washed with a medium of pH ≥ 9. Method according to claim 1 or 2, characterized in that CaO is added to the medium to increase the alkalinity. Method according to claim 1, characterized in that the washed filtered dust is filtered off, dewatered by means of a filter press and then mixed with clay. Method according to one of Claims 1 to 4, characterized in that one or more fluxes are additionally added during mixing. Method according to one of Claims 1 to 4, characterized in that the compacts are fired at a temperature of 1070 degrees Celsius to 1090 degrees Celsius. Method according to claim 5, characterized in that the blanks are fired at a temperature between 950 degrees Celsius and 1020 degrees Celsius. Method according to one of the preceding claims 1 to 7, characterized in that up to 350 g of water, based on 1000 g of dry substance of the mixture, are added in the state before the semi-finished products are formed. Method according to one of the preceding claims 1 to 8, characterized in that a maximum of 50% by weight of washed filtered dust, i.e. dry substance, based on the total of clay, flux and washed filtered dust, is added. Method according to one of the preceding claims 1 to 9, characterized in that not more than 20% by weight of the flux, based on the total of clay, the flux and the washed filtered dust, i.e. the dry substance, are added. Method according to one of the preceding claims 1 to 10, characterized in that silicic acid, nepheline, syenide, glass powder, fluorspar or enamels are added as flux. Method according to one of Claims 1 to 11, characterized in that sludge is added as flux after galvanization. Method according to claim 12, characterized in that the proportion of sludge after galvanizing, calculated as dry matter, is at most as high as the proportion of washed filtered dust. Method according to one of the preceding claims 1 to 3, characterized in that the blanks are formed by partially dewatering the mixture by extrusion, then dried at a temperature of not more than 150 degrees Celsius for at least one day and then fired only. The method according to claim 14, characterized in that the blanks are extruded at a pressure of at least 0.5 MPa. Method according to claims 14 or 15, characterized in that the blanks are dried at 130 degrees Celsius for 3 days. Method according to one of the preceding claims 1 to 16, characterized in that the heavy metal compounds expelled in the firing process are filtered from the flue gas and used to prepare the mixture for further blanks. Process according to claim 17, characterized in that the heavy metal compounds are filtered by molecular sieves and the clogged molecular sieves are used to prepare the mixture for further blanks. Method according to one of the preceding claims 1 to 8, characterized in that the blanks are fired at a temperature below the softening temperature. Method according to one of the preceding claims 1 to 19, characterized in that the time course of the firing temperature rise below 900 degrees Celsius is selected such that the temperature ranges in which substantial amounts of water vapor and carbon dioxide are expelled are slower than the temperature ranges in which only minor amounts of heavy metal gases or heavy metal compounds are expelled. Method according to claim 1, characterized in that a clay is used which contains at most 25% by weight of Al ₂ O ₃ and 60 to 80% by weight of SiO in dry matter.