Tgchnical Field

This invention relates to the method of vitrification of dusty waste, especially of fly ash from the incinerators for municipal wastes containing a high amount of sulphur compounds, organic matters and heavy metals. This dusty waste will be mixed with meltable constituents of the glass batch that will be melted in the glass electrical furnace to obtain the molten glass. The molten glass will be drained through the QLttlet channel in the bottom or close to the bottom of the electric furnace and will be processed to the glassy mass with the ability to be normally stored or further utilized.

Bar,kground Art

The trend to the environmental protection being applied to date tends to the waste processing in order to acquire sLtbstances as inert as possible. One of the fields being considered is the disposal of dusty waste from the incinerators, especially of fly ash being collected in the electrostatic filtres which contains harmful substances, above all heavy metals, harmful organic substances, sulphur, etc.

A number of disposal methods of fly ash have been published in the literature: one of them is the

vitrification. The final product of vitrification is the glass mass which is resistant, features a very low extractivεness along with the ability to bind a lot of harmful SLibstances in its structure. Moreover, when comparing with the initial raw material the volume of such glassy mass will be considerably reduced. The waste processed by the vitrification can be deposited in common tips and if need it can be utilized as a secondary raw material.

The vitrification of waste is described e.g. in BRD-patεnt No. 359 003, in BRD-patent No. 38 41 SS9 and in BRD-patent application No. 39 04 613. In this inventions the method of vitrification is presented and in some cases the equipment used to the realization of the corresponding method.The method presented is based on the preparation of a mixture of fly ash with one or more meltable additives such as sand, sodium, phonαlite, dolomite, limestone, etc. The glass batch composed in such a way will be processed into molten glass, from the smaller part the waste gas is generated which is led back to the said batch where it will be cooled under the condensing temperature of heavy metals and their compounds: in consequence of that the content of these harmful substances in the waste gas will be substantially reduced. The . cooled waste gas will be purified by some of known methods. The dust being separated from the waste gas during PLtrification will be returned to the melting process. For example, in the patent application BRD- No. 39 04 613 is presented that after separation of dList the waste gas is purified by a wet process or sludge can be recycled into the batch

similarly as the dust. After the purification the waste gas will be led through the active coal filter and at the outlet of this filtration the gas features sutch a cleanliness that it can be emitted directly into atmosphere without cautsing any contamination of the environment.

In the European patent No. 359 003 and BRD-patent application No. 39 04 613 is presented that a certain amount of ^' the molten glass surface will be maintained withoutt a layer of the glass batch and besides the molten glass a layer of alkali or alkaline earth - the sulphate foam will be produced that is maintained on the molten glass surface in the thickness of 20 to 50 mm. Thus, the furnace for melting of suich wastes must be equipped with an opening in the height of s tlphate foam layer to enable the draining-off this sulphate foam during the melting process. The sulphate foam is composed of salts whose limit of solubility will be exceeded d iring the melting. In both patents cited the controlled keeping of the sulphate foam is assumed. This method of control of the melting process is very demanding concerning the qualification of operators with regard to the need that considerable variation of the composition of input raw materials must be followed.The rise of sulphate foam and creating of sulphate pools can lead to the lost of insulating effect and to the lowering of the meltiftπg performance. Besides that the sulfates are highly corrosive and, concerning their low viscosity, the penetration of these compoLinds even to the narrowest joints of refractory lining can occLir with the consequent drop of

the furnace life. Also, the handling is dangerous when draining the sulphate foam because the considerable contraction arises by its chilling and by the contact with a cold body the large quantity of heat is led away and the sulphates squirt.

The equiipment designed to the vitrification of waste is specified in BRD-patent No. 38 41 918. It is a glass melting furnace, fully electric one, e.g. heated by vertical electrodes extending from above and dipped into the molten glass. The furnace consists of the melting end and separated working end in which the further cuirtain wall is placed. Between the first and second curtain walls the closable outlet for the SLilphatε foam is situated whose lower edge ex ^' teπds above the level of molten glass, the position of the opening itself corresponds with the level of the SLtlphate foam. The first separating wall between the melting and working ends extends to the level of the molten glass only and can be advantageouisly vertically adjusted. The overflow edge is covered by an electrically conductive material and can be heated up. To the waste the melting agents are added which amount 30% of maximum content in the glass batch.

From these furnaces the molten glass is drained by means of various types of outflow channels that are situated directly in the bottom or close to it, by means of overflow, etc., and the coolεd glassy mass is processed by using roLttiπe methods to the fritted glass. pellets, fibres, etc., which a.re deponable in common tips or can be Lttilized in various consequent

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technological processes.

When using the current methods of fly ash vitrification the sulphur included passes mostly to the sulphate foam, the organic matter mineralizes and their carbon matter passes to a great extent into the waste gas. The major product of the vitrification is glass so that the release of metals and further suibstances contained in the glass is reduced to a minimum.

The disadvantage of the present also very progressive soluitions of the vitrification of dusty fly ash types with the higher content of sulphur is the controlled layer of the sulphate foam being maintained above the level of the molten glass, often with at least partly uncovered surface of the molten glass which means very difficult melting when taking the corrosiveness of sulphates into account. The advantage of a layer of foam being maintained above the surface of the molten glass for reason to catch a part of harmful substances could be regarded as disputable because the problem of the disposal of harmful substances is simply transferred from the sphere of gas purification to the sphere of disposal only of the soluble waste. The recycling of this foam to the melting process is questionable especially with the higher content of sulphur: e.g. the incineration waste contain up to 20% by weight of sulphur. In the present solutions the process of gas purification is also very demanding namely in the solutions where the partly opened surface of molten glass is presumed and where large quantities of waste gases occur.

Disclosure* of the. Invention

According to this invention the disadvantages of the vitrification of dusty waste mentioned above are eliminated or substantially reduced by using the method in which the dusty waste, especially fly ash from the incinerators for municipal wastes with the high amount of sulphur compounds, organic matter and heavy metals will be mixed with the meltable components of the glass batch, which will be melted in the glass furnace to obtain the molten glass, which will be drained through the outlet channel in the bottom or alternatively close to the bottom of the furnace. According to this invention the method consists of the cold non-molten layer of glass batch being maintained on the surface of the molten glass and the sulphur compounds will be subjected to the reduction onto the lowest stage of oxidation in the considerably reducing medium by adding the reductive components into the glass batch and/or by the melting temperature in the molten glass being kept on a minimum of 1420°C, the molten glass being heated—up immediately before the outlet channel by the additional electric heating.

The major advantage of this solution is the elimination of the sulphate foam formation, the increase of the amount of sulphur compounds in the glass product and the limitation of the rise of harmful substances in waste gases along with the reduction of leakage of the heavy metal products. The final product of vitrification is partly the solid πon-extractiblε product, namely the glassy phase along with the

segregated phase of sulphides and metals, partly the gaseous phase, namely waste gas.

The considerably reducing medium with the lowered partial pressure will be. reached during the melting by adding the reducing components into the glass batch which contribute to the reduction of sulphates and/or by the melting temperature at least 1420°C, which accelerates the thermal decomposition of sulphate sulphur and carbon from the organic compounds. During the melting the contingently reduced heavy metals and iron can be concentrated in the reducing medium at the bottom of the furnace or immediately before the outlet channel so that here the molten glass with low heat conductivity rises whose difficult melting will be supported by the additional electric heating.

Example of Carrying out Invention

During incineration of municipal wastes the dusty fly ash is collected on the electrostatic filtres installed for the purification of the gases, the said fly ash represents the harmful waste. Its composition ranges e.g. in the following limits, from 35 to 50% by weight of silica dioxide, from 7 to 18 % by weight of oxides of alkali metals, from 6 to 14 % by weight of oxides of alkali earth metals, from 8 to 15 % by weight of alumina oxide, from 2 to 5% by weight of iron oxides, from 7 to 15% by weight of sulphur oxides, from 4 to 6% by weight of carbon in organic compounds. The fly ash contains further capper . cobalt, cadmium, niobium, lead, zinc, chromium, manganese, fluorides and

chlorides.

For the purpose of vitrification, e.g. three parts of this fly ash will be mixed with one part of the meltable constituent which is represented by non—regenerable cutllet of amber glass that contains also the reducing compounds, namely bivalent iron and sulphides.

Concerning the meltable components being added to the glass batch it is favourable to choose their type, composition and quantity according to the initial composition of the waste which varies considerably especially in fly ash types.

Concerning to the reducing components it is also favourable to consider the waste composition. For the chemical reduction can be used e.g. soot, furnace slag with reducing ability etc.

In such a way prepared glass batch will be charged into the fully electric glass furnace in which it will be melted at the temperature of appr. 1460°C with the covered surface of molten glass where the layer of non-molten glass batch will be maintained. During the reducing melting the slag and athermanαus molten glass which is melted with difficulty sinks to the bottom of the furnace which contains alternatively also reduced heavy metals and sulphidic sulphur so that this molten glass at the bottom of the furnace and immediately before the outlet channel will be heatεd-up by the additional electric heating which also positively

affects its homogeπization.

To create the optimal reducing medium the combination of reducing means should be advantageously chosen, i.e. both the reducing components in the glass bath and reducing action of the temperature value in the molten glass. The reducing means us be able to reduce the whole quantity of sulphur contained in the molten glass so that the danger of foamed surface of the molten glass cannot occur. Far example, in case that only the reducing component in the glass batch along with the low melting temperatures will be used, the sulphate foam can be formed with its consequent solidification and the melting output will be lowered. On the contrary under too high melting temperatures in the molten glass and lack of the reducing component in the glass batch which is not able to influence the sulphate foam forming between the surface of molten glass and the glass batch. The melting temperatures must be sufficiently high, at least 1420°C, in order to maintain the reducing atmosphere in the furnace. With the rise of temperature the effect of elimination of the sulphate foam on the surface of the molten glass will incerease but at the same time the requirements on the quality of refractory material of the furnace lining will be higher. The highest melting temperature is given substantially by the corrosion of refractory material of the lining.

The fly ashes from the incinerators usually contain a considerable amount of organic matter which will be decomposed at the melting temperatures of the molten

glass, and the organic carbon in the reducing medium creates further reducing components that affect the conditions in the molten glass.

By following the methods of controlling the melting process according to this invention the layer of sulphate foam will not be formed on the surface of the molten glass and the surface of the molten glass is covered by a compact layer of the glass batch. The sulphate foam from the fly ash will be converted during the vitrification in reducing medium to the lower oxidation stages of sulphur, up to the sulphidic sulphur, possibly in the segregated phase in the molten glass. The cold non-molten layer of the glass batch which is maintained on the surface of the molten glass prevents the products of heavy metal to be leaked into atmosphere and in this way the content of harmful substances in the waste gas will be lowered. The waste gases are exhausted out of the furnace and if necessary they will be purified by known methods, e.g. by a wet process. The products of purification along with the collected dust on the separator can be recycled into the glass batch. The purified waste gases can be emitted into atmosphere without danger.

For this method of vitrification the single-chamber continuous glass melting furnacε has been well proved, it is heated by electrodes spaced horizontally in side walls of the furnace and submerged under the surface of the molten glass. The exhaust channel for waste gases is situated in the crown of the furnace. The bottom of the furnace is inclined and the outlet channel for the

drainage of the molten glass is situated either in the middle of the bottom which is roof-shaped to the control axis or it is situated in the face of the furnace to which the bottom is uniformly inclined. The inclined bottom supports the flowing off the athermanous molten glass or slag. If the inclined bottom is not used the drains, channels etc. can be formed. Immediately before the outlet channel the additional electric heating is placed with horizontally spaced electrodes.

Against the highly corrosive molten glass the heating electrodes can be protected by introducing the low-frequency electric current. The stability and proper control of the melting process in the furnace can be maintained in such a way that in each horizontal electrode the constant intensity of current and at the same time the constant power input is being kept.

After converting the fly ash into the molten mass the molten glass will be drained via the outlet channel, will be cooled and the glass product acquired will be processed- e. g. to the he i-sphere spaced pellets which can be used as a filling matter in the civil engineering, in the road building or can be deposited in common tips.

Under the conditions described the glass mass was produced that was tested concerning the extractiveness and toxical harmlessnεss for the deposits on usual tips or for the further industrial utilization. The

The values of extractiveness and harmlessness fulfil the desired requirements.

Industrial Applicability

By means of vitrification the unstable and in most cases dusty and toxic wastes with the high content of sulphur, organic matter and heavy metals will be converted into the glassy mass which is normally storable or in a secondary way utilizable, e.g. in the civil engineering.