METHOD OF PRODUCING GLASS FROM INCINERATED SOLID URBAN WASTE

Abstract: The method is implemented starting with a feedstock of dry, solid material composed predominantly of a ternary mixture of a first oxide constituted by SiO2, a second oxide constituted by Na2O and/or K2O , and a third oxide constituted by CaO and/or PbO. In the case of ash produced by and incinerator, the second oxide is scarce and is added to the feedstock in a proportion such as to give a composition having a pour point of the order of 1450 DEG C. If the mixture is constituted by dust obtained from electrostatic precipitators for the smoke discharged from an incinerator, the first oxide is scarce and, in this case also, is added in a proportion such as to give a composition having a pour point of the order of 1520 DEG C. In both cases, the feedstock is melded in a furnace to produce a bath of vitreous material and the bath is subjected to a refining step. Upon completion of the refining step, the vitreous material is cast. As a result, at least substantially stable, solidified glass which can be re-used for the production of products or as an inert material is obtained.

Description: METHOD OF PRODUCING GLASS FROM INCINERATED SOLID URBAN WASTE with the method The present invention relates to a method of producing glass from a feedstock of dry, solid material resulting from the combustion of solid urban waste, according to the preamble of Claim 1. The incineration of solid urban waste is a waste-disposal technique which is becoming ever more widespread since, by combustion, this waste completely loses its bacterial charge and is transformed into an essentially inorganic product . The products of the combustion of solid urban waste in incinerators are constituted by powders of two different types: a true ash and a dust coming from the electrostatic precipitators used to separate the solid components of the combustion smoke from the gaseous components before they are admitted to the atmosphere. Ash is classified by legal regulations as "special non- toxic and non-harmful waste", whereas dust is classified by the same regulations as "special toxic and harmful waste" . Although ash is not harmful, it is a completely useless material which is generally stored in a dump. As well as causing problems with regard to the protection of the landscape, ash dumps also have high management costs. These problems are even more serious in the case of dust, the storage of which requires expensive dumps for special toxic waste . To solve these problems, various attempts have already been made to convert the ash and dust resulting from the combustion of solid urban waste into inert vitreous elements which, on the one hand, can be used as aggregates, for example, instead of gravel and which, on the other hand, in the case of ash, fix or imprison the harmful elements such as lead and mercury which are contained in the dust in large proportions. A method according to the preamble to Claim 1 is known from the document EP-A-0 294 362. This document relates to a method in which ash and dust resulting from the combustion of solid urban waste are first dried by heat treatment at about 500°C. Additives in the form of alkali -metal salts, amongst which is sodium chloride NaCl, and/or metals such as lead, are then added to the feedstock of dried product so as to form a eutectic composition. The feedstock thus supplemented is melted at a temperature which may reach up to 1200°C in a fuel or electric furnace. The molten material is then cast and solidified so as to form a granulate or pieces of other kinds which have a vitreous and/or sintered structure and are insoluble in water . The final vitreous products thus obtained have an unstable, that is, a crystallisable structure which makes them unsuitable for transformation into glass products. The known method is also expensive since, in order to obtain a eutectic composition, it requires quite expensive addi ives . A method according to which ash from solid urban waste is heated to a temperature above its melting point so that its components are at least partially fused and form aggregates is known from the document EP-A-0 330 872. This method provides for the addition of additives in order to lower the melting point. These additives fall within the group of sand, glass, calcite and dolomite. This known method produces a material which is in any case not homogeneous although it has stable chemical characteristics. A method of producing glass from fly ash, dust and sludges coming from the purification of water and solid urban waste, in which various additives, amongst which is calcite, CaC0 3 , are added to the feedstock of these materials is known from the document US -A- 4 191 546. This method produces glasses which crystallize easily, that is, which are unsuitable for transformation into products. A method for converting solid urban waste into glass in which ash is mixed with cullet and an alkaline- earth salt in order to form a mixed feedstock which is added to a bath of molten glass is known from the document EP-A-0 359 003 and the corresponding document US-A-4 944 785. The final result of this method is a glass only partially produced from waste. None of the methods according to the prior art considered above produce economically from the combustion products of solid urban waste, glass which, in the case of ash, is completely stable, that is, which does not crystallize easily or spontaneously and which can be re-used to produce products such as cullet and which, in the case of dust, is sufficiently stable to be re-usable at least as an inert material . The invention is directed precisely towards this object. According to the invention, this object is achieved by means of a method as claimed. The invention is based on the observation, on the one hand, that the ash produced by an incinerator for solid urban waste has a composition which almost corresponds to that of a silica- soda- lime glass and that, to achieve this composition it suffices to increase its proportion of fluxes in the form of soda, which is very cheap, or potash, whereas the dust recovered from the electrostatic precipitators of an incinerator has a composition which usually almost corresponds to that of a lead glass; one of the constituent oxides PbO and the flux Na 2 0+K 2 0 are already present in the necessary proportions in the dust and, to achieve the composition of a lead glass, it suffices to add a suitable proportion of the other constituent oxide, silica, that is, a very cheap material. The constituent oxide which is scarce may be added before or during melting. The invention will become clearer from a reading of the following description, given with reference to the single appended drawing which shows a conventional phase diagram for a ternary silica- soda- lime mixture on which the melting points corresponding to certain test compositions are given. Test I A homogeneous sample of about 2 kg of material was taken from an incinerator for solid urban waste and was left to dry for a few days with daily mixing to facilitate its complete dehydration. Even though the waste had been deferrized before combustion there were still latten residues, rusty containers and other fine ferrous materials in the ash collected. These metallic residues were eliminated from the ash. From analysis, the ash had the composition according to Table I below (the percentages are by weight relative to the total weight) : Table I silica 36.2% iron 2.9% nickel 54 pp copper 480 ppm lead 0.52% calcium 11.28% potassium 1.08% aluminium 5.48% zinc 0.14% chromium 0.0 2% sodium 6.12% magnesium 2.15% manganese 0.07% organic C 33-41% It should be understood that the metals listed above were in the ash in the form of oxides and oxygenated inorganic salts It was found by analysis that 52% by weight of the mixture was composed of the constituent oxides and fluxes of a silica- soda- lime glass mixture. If one considers the oxides which are present in the ash in the greatest concentrations, it can be seen that a phase diagram to be considered should comprise at least six species : Si0 2 - Na 2 0 - A1,0 3 - CaO - K,0 - organic C. However, since the melting of the ash for the purposes of the present invention has to take place at high temperatures, a few observations may be made: - in the melting step, any oxygenated inorganic salts still present in the ash will be transformed almost completely into oxides during the heating; - the C (carbon residue) which is present and which is subjected to heat treatments in an atmosphere of more than 6% oxygen will be transformed completely into C0 2 by complete combustion at 1000°C. At this point, the phase diagram to be selected is reduced to five components, but further observations may be made: - within a vitreous structure, sodium oxide and potassium oxide can replace one another and can therefore be considered together; - aluminium oxide is present in smaller percentages than calcium oxide and, by a substantial approximation, it is therefore permissible to ignore it. On this basis, the ternary soda- lime- silica diagram of the drawing can be examined in order to evaluate the melting point of the ash: in this case, if the three percentages relating to the oxides of the diagram are considered as the only ones present in a feedstock intended for a melting furnace, the expected melting point is found from the diagram. In general, it can be said that ash resulting from the combustion of solid urban waste contains silica, soda and/or potash, lime and magnesia in the following proportions by weight: Si0 2 : 30-40% Na 2 0+K 2 0 : 7-8% CaO+MgO : 10-15% The phase diagram of the drawing shows that, for a composition relating solely to ash with these proportions and if the foregoing simplifications are considered valid as seems permissible, the expected melting point is between 1200°C and 1250°C (point El of the drawing, or thereabouts) . The ash was then subjected to a melting and refining heat treatment, the molten bath being heated to 1600°C to give a melt which could be cast. The heating programme used is given in Table II below. Tabl e I I The solidified material produced by the melting of the ash alone was a shiny substance with a greenish-black colour most probably due to the presence of iron. Upon diffractometric analysis, it was found that glass was actually produced by treating the ash alone at 1600°. A sample of this glass subjected to differential thermal analysis and subsequent diffractometric analysis, however, showed that the material had become polycrystalline with the formation of aluminium, calcium and sodium silicates, iron silicate, calcium silicate and silica. Upon the basis of these results, it was concluded that it is certainly possible to produce glass starting with ash alone but this involves reaching high temperatures (1600°C) and, moreover, the product obtained devitrifies easily, which makes it unusable as a raw material for transformation into glass products etc. Test II A flux was added to ash having the composition of Test I in order to lower its melting point as well as to eliminate the crystallization problem. The most common fluxes being potassium, lithium and sodium carbonates, the latter was preferred because it is easily obtainable and less expensive. It was intended to achieve a pour point of about 1450°C and thus to have a temperature slightly lower than the melting point of a furnace for industrial glass, with a consequent energy saving. In Test II, the starting mixture was prepared on the basis of a certain quantity of ash with an addition of 15% by weight of Na 2 0 in the form of Na 2 C0 3 (point E2 of the diagram of the drawing) . The mixture thus obtained was ground in an electric mill with agate balls. The mixture thus ground was melted and refined in the same furnace as in Test I in accordance with the programme of Table III below. Table III As can be seen, in this test, as in all of the other tests which will be described below and in which additional flux is used, the gradient of the temperature rise was low in the step from 650 to 950°C, that is, in the carbonate- removal zone, whereas a more rapid rise took place from 950°C to 1450°C with successive stationary periods. The pour point was 1450°C. After cooling, the cast material was in the form of a shiny transparent mass of dark green colour, again probably due to the presence of bivalent iron ions and carbon in colloidal form. Diffractometric analysis carried out showed that the material produced had all of the properties of a vitreous substance. When the sample on which the diffractometry was carried out was subjected to differential thermal analysis, it was noted that, at about 1000°C, there was an endothermic reaction signal which could be expected to be associated with the start of the melting of the material. Further diffractometric analysis of the dust subjected to heat treatment (that is, the dust coming from the differential thermal analysis) did not show crystallization peaks. On the basis of the results it was therefore possible to confirm that the glass produced from ash with the addition of 15% by weight of soda had not undergone any devitrification as a result of the heat treatment. Tests III, IV and v Test II showed that, with a flux (Na 2 0) in a percentage of 15% by weight, it is possible to obtain a homogenous glass at a melting point of about 1400°C. The same result can be achieved in a likely way with a flux constituted by Na 2 0+K,0 or by the substitute K,0 or Li,0 alone, or by any mixture of such oxides. It was therefore of interest to know what types of glass could be obtained with a further reduction in the percentage of flux but without a substantial rise in the pour point . Three further compositions were therefore prepared by adding respective soda percentages of 13% by weight (Test III), 11% by weight (Test IV), that is, having characteristics close to those of the glass of Test II, and another composition having 8% by weight of soda (Test V) and thus having characteristics between those of glass with ash alone (Test I) and of glass with the addition of 15% by weight of soda (Test II) . The heating programme of Table III was again implemented in the furnace in these cases. The melting points of the mixtures with the additions of 13%, 11% and 8% by weight of soda are indicated E3 , E4 and E5 , respectively, in the phase diagram of the drawing. Prolonged maintenance of the molten mixture at 1450°C permitted optimal refining in all three of Tests III, IV and V. Casting in brass moulds took place without difficulty in Tests III and IV. Both of these tests produced a true, dark green glass the vitreous nature of which was confirmed by X-ray analysis. The vitreous dust produced by grinding from the glasses of Tests III and IV was subjected to differential calorimetric analysis which showed no significant endothermic or exothermic reaction peaks. It was therefore possible to conclude that, in contrast with the glass produced by the melting of ash alone (Test I) , which was easily devitrified, a stable vitreous substance, that is, a substance which cannot be devitrified, is obtained with the use of a mixture containing incinerator ash with the addition of either 13% or 11% by weight of soda. In Test V (8% by weight of soda added) a glass paste was cast at 1450°C but was uniform only if all of the metallic material had previously been removed from the ash with maximum precision. In the glass obtained, the uniformity of the dark green colour was disturbed by small percentages of metallic material. Since these remained undissolved in the molten paste, they left reddish coloured regions in the solidified material . As with the sample of Test II, diffractometric analysis of the samples of Tests III, IV and V showed no crystalline phases . In the case of the sample of Test V, differential calorimetric analysis showed solely an endothermic reaction at about 620°C but this left the vitreous structure of the material obtained unchanged. It can therefore be concluded that a vitreous material which is not likely to spontaneously devitrify is also obtained with 8% of soda. The samples of Tests II, III, IV and V were subjected to chemical resistance tests. For analysis of this type, the glass samples were crushed with a steel pestle to give a particle size smaller than 9.5 mm. The crushed samples were subjected to eluate analysis consisting of a transfer test with 0.5N acetic acid by the IRSA-CNR technique, as well as a transfer test by atomic absorption to determine the heavy metals in the eluate. The IRSA-CNR transfer tests are described in the IRSA publication "Metodi analitici per le acque" (Analytical methods for water), Quad. 1st. Ric, Acque, 11, 1972, to which reference should be made. All of the transfer tests on the glasses obtained by Tests III, IV and V confirmed that all of the concentrations of heavy metals were below the permissible limit concentration set by legal regulations. In other words, these glasses could be classified as inert, non- toxic and non-harmful substances which can be stored as such or recycled for the production of products. In particular, if these glasses were to be stored, they would go, together with construction waste, to Type B dumps provided for by the legal regulations. Test VI The preparation of glass with the addition (to the ash) of 6% by weight of Na 2 0 in the form of Na 2 C0 3 , the melting point of which is indicated E6 in the phase diagram of the drawing, was also attempted. The techniques for the preparation and heat treatment of this mixture corresponded to those of Examples II, III, IV and V. The glass obtained, the melting point of which was again above 1200°C (point E6) , was brought to 1450°C but it was impossible to cast it in a mould at this temperature. It should be remarked that the ash used in the tests described above was particularly rich in refractory oxides and therefore the addition of high amounts of flux oxides was necessary. One should however take account of the fact that the composition of ashes highly depends on both the territory and the seasons of the year. It may happen therefore that some ashes contain refractory oxides Si0 2 , A1 2 0 3 , CaO, MgO in lower amounts or in any case in such propositions as to lead to a lower melting point and to a greater facility to be converted into glass in comparison with the ashes of the above said tests. In such cases the flux (Na 2 0, K 2 0, Li 2 0) may be added in percentages as low as about 2% by weight, still permitting to obtain homogeneous and stable glasses. Test VII In this test, first of all, the composition of the dust obtained from electrostatic precipitators for the smoke discharged from an incinerator for solid domestic waste was analysed. Data for assessing whether the dust could give rise to glass or not were obtained from the results of the analysis . For the analysis, the five oxides present in the largest quantities were considered: Si0 2 - Na 2 0 - CaO - PbO - K 2 0 Since in the literature there is no phase diagram relating to this group of five oxides, some approximations were used to address the problem and these led to a simplified model on the basis of which information relating to the mixture could be obtained: sodium and potassium are ions which can replace one another within a vitreous structure and their concentrations can therefore be considered as a single quantity relating to K 2 0 and/or Na 2 0; - the second approximation, which was somewhat greater than the first, was that the presence of CaO (12.6% by weight in the samples analysed) within the mixture was not taken into consideration. Upon the basis of these approximations, it can be said that an examination of the ternary phase diagram with the following oxides: PbO - K 2 0 - SiO, could suffice to assess the vitrification of a sample of dust . If the percentages of the other oxides are omitted, it can be seen from the diagram shown in Figure 2 whether the dust alone will be able vitrify at high temperatures. An analysis of the dust restricted to the aforementioned oxides gave the following values (all of the percentages are by weight) : Na 2 0+K 2 0: 22% SiO, : 7.4% PbO : 12.4% In general, it can be said that dust separated from the combustion smoke of an incinerator for solid urban waste contains soda (and/or potash) , silica and lead oxide in the following proportions, by weight: Na 2 0+K 2 0: 20-25% Si0 2 : 7-8% PbO : 10-15% It has been observed experimentally that, if the dust alone is heated to 1600°C, a melt is obtained which, after solidification, gives rise to a system constituted by two solid phases. Upon the basis of this experimental observation and in view of the composition of a normal lead glass, an increase in the silica content was considered, with the (Na 2 0+K 2 0) /PbO ratio kept constant. Test VIII In order to prepare the mixture, 77g of silica was added to lOOg of dust. The application of the approximations listed in Test VII in this case produced a mixture of which the oxides with the greatest concentrations by weight were those given below: SiO, : 48% Na 2 0+K 2 0: 12.4% PbO : 7% The composition of Test VIII was subjected to melting and refining heat treatment in accordance with the programme given in Table IV below. Table IV The melt produced in a platinum crucible and with the use of an electric furnace in accordance with the programme of Table IV was cast in a mould, although with a certain amount of difficulty, at 1520°C and was green in colour. Diffractometric analysis demonstrated the vitreous nature of the compound obtained. By eluate analysis effected by the transfer tests with acetic acid and by atomic absorption as in Tests II, III, IV and V, it was found that the glass obtained from lOOg of dust with the addition of 77g of silica (glass from dust with 77% of silica) satisfied the limit concentrations set by the legal regulations to classify it as inert, non- toxic and non-harmful material, in contrast with the glass obtained from dust alone in Test VII which, owing to the presence of lead and other heavy metals, is classifiable as special toxic or harmful material. This material is suitable for use as an inert substance, for example, as aggregate or stone in the civil engineering field. Test IX A mixture of dust was then prepared with the addition of 25% by weight of silica to the ash. In this case also, the melt was cast after heating to 1520°C at 60°C/h in accordance with Table IV. Diffractometric analysis also confirmed the presence of a vitreous structure in this case, although it was susceptible to crystallization in the long term. When subjected to eluate analysis according to the transfer tests with acetic acid and by atomic absorption, the sample of glass gave results similar to those of the composition obtained in Test VIII, that is, in this case also, an inert, non- toxic and non-harmful material was obtained from a special toxic and harmful waste such as dust from electrostatic precipitators. This material as well is suitable for use as an inert substance, for example, as aggregate or stone in the civil engineering field.

Claims: CLAIMS 1. A method of producing glass from a feedstock of dry, solid material resulting from the combustion of solid urban waste, comprising the steps of: - adding an additive to the feedstock, - melting the feedstock in a furnace to give a bath of vitreous material, and - casting and solidifying the vitreous material, characterized in that - the method starts with a feedstock of dry, solid material composed predominantly of a ternary mixture of a first oxide constituted by Si0 2 , a second oxide constituted by Na 2 0 and/or K 2 0, and a third oxide constituted by CaO and/or PbO, in which the first or the second oxide is scarce, in order to form an at least substantially stable solidified glass , - before or during the melting, a proportion of the scarce oxide such as to give a composition which has a pour point equal to or less than 1600°C and which is at least substantially stable in the solidified state, is added to the feedstock as an additive, - the bath is subjected to a refining step, and - upon completion of the refining step, the vitreous material is cast at the said temperature equal to or less than 1600°C. 2. A method according to Claim 1, characterized in that ash from urban waste in which the scarce oxide is constituted by Na,0 and/or K,0 is used as the solid material and, before or during melting, the scarce oxide is added to the feedstock in the said proportion, in the form of one of its compounds . 3. A method according to Claim 2, characterized in that, the initial ternary mixture having the following composition by weight relative to the total feedstock: Si0 2 30-40% Na 2 0+K 2 0 7-8% CaO+MgO 10-15 ζ and the rest of the feedstock being constituted predominantly by organic C, a proportion of flux of between 2% and 15% by weight of the total feedstock is added and mixed with the feedstock, the flux being constituted by Na,0+K 2 0 or by the substitute K 2 0 or Li 2 0 alone, or by any mixture of such oxides. 4. A method according to Claim 3, characterized in that the vitreous material is cast at a temperature of the order of 1450°C. 5. A method according to any one of Claims 2, 3 and 4, characterized in that the said component is added in the form of Na,C0 3 and/or K,C0 3 and/or LiC0 3 . 6. A method according to any one of Claims 2 to 6 , characterized in that the said proportion of Na 2 0 and/or K 2 0 and/or LiC0 3 is added to the ash and the mixture thus obtained is subjected to grinding before melting. 7. A method according to Claim 1, characterized in that dust which has been separated from the combustion smoke of solid urban waste, and in which the third oxide is constituted at least partly by PbO and the first oxide Si0 2 is scarce, is used as the solid material, and in that Si0 2 is added to the feedstock in the said proportion before or during melting. 8. A method according to Claim 7, characterized in that, the ternary starting mixture having the following composition by weight relative to the total feedstock: Na 2 0+K 2 0 : 20 - 25% Si0 2 : 7 - 8% PbO : 10 - 15% , a proportion of Si0 2 of between 15% and 50% by weight of the total feedstock is added and mixed with the feedstock. 9. A method according to Claim 8, characterized in that the vitreous material is cast at a temperature of the order of 1520°C. 10. Glass produced by a method according to any one of the preceding claims . 11. An element or product made of glass produced by a method according to any one of Claims 1 to 9. 12. Use of a solid material resulting from the combustion of solid urban waste for the production of substantially stable glass by a method according to any one of Claims 1 to 9.

Inventors: DALLARA ENRICO, MONTENERO ANGELO, DALLARA, ENRICO, MONTENERO, ANGELO
Applicants: CO R I VE CONSORZIO RICERCA IN, DALLARA ENRICO, MONTENERO ANGELO, CO.R.I.VE. CONSORZIO RICERCA INNOVAZIONE VETRO, DALLARA, ENRICO, MONTENERO, ANGELO
Application year: 1997
Main classification: B09B3
IPC codes: C03C1