METHOD FOR THE NON-LEACHABLE IMMOBILISATION OF MOLYBDENUM COMPOUNDS IN A SLAG

Abstract: The invention relates to a method for the non-leachable immobilisation of molybdenum compounds in a slag by treating iron- and molybdenum-containing waste streams under reducing conditions, for example under an oxygen pressure of 10<-6> to 10<-12> bar, advantageously 10<-7.5> to 10<-9.5> bar, at a temperature which advantageously is 50 DEG C above the liquidus temperature of the slag and under a thermodynamic activity of Fe(II) in the slag of more than 0.5, the Fe(III)/Fe(II) ratio in the slag being adjusted to a value of between 0.25 and 2. The resulting slag is then cooled in such a way that a solid solution of MoO2 in a spinel structure is obtained, which spinel structure exhibits a very low leaching value for molybdenum. By reason of this, such slags can be used, for example, as material in the construction industry.

Description: Method for the non-leachable immobilisation of molybdenum compounds in a slag The invention relates to a method for the non-leachable immobilisation of molybdenum compounds in a slag by melting and subsequently cooling waste streams containing iron and molybdenum compounds. Molybdenum compounds are present in a number of fairly specific waste streams. Examples of these are fly ashes from WIIs (waste incineration installations) and fly ashes from the processing of chemical waste by means of incineration in, for example, rotary furnaces. The source of the molybdenum is partly to be ascribed to corrosion of the installation used and partly present in the waste streams processed in said installations. Many residues are generally used, before or after a pretreatment, in the construction industry. Molybdenum, just like many other elements, has to comply with a leaching requirement if a processed waste stream is to be used as building material. In the Netherlands this requirement is laid down in the Bouwstoffenbesluit (Building Materials Order). In chemical terms molybdenum is one of the elements that forms oxyanions and the compounds that contain these ions are usually highly soluble in water. This makes it more difficult to comply with the leaching requirements under the abovementioned Building Materials Order. This problem is virtually independent of the technical approach followed when processing the waste streams. Both in the case of cold immobilisation (mixing of waste streams with, for example, cement) and in the case of processing in the form of a melt (thermal immobilisation or pyrometallurgical treatment) it is repeatedly found that the leachability of molybdenum exceeds the standard laid down in the Building Materials Order. Said standard specified for molybdenum is a maximum emission value of 150 mg/m per 100 years. Surprisingly, it has now been found that the abovementioned problem can be completely or substantially solved by carrying out the pyrometallurgical treatment of molybdenum-containing waste streams under reducing conditions, the oxygen pressure being set within a relatively narrow range, so that slags are obtained which - after crystallising out - exhibit substantially reduced leaching of molybdenum. The molybdenum-containing waste streams to be used with the method according to the invention must also contain iron compounds, which is the case in virtually all cases that arise. If, however, such a waste stream does not contain any iron compounds, these still have to be added separately. More particularly, the invention relates to a method as specified in the preamble, which is characterised in that the treatment temperature is above the liquidus temperature of the slag; - the oxygen pressure is adjusted to a value of between 10 " and 10 " bar, this can be achieved by setting a fuel to oxygen ratio such that a CO/CO2 ratio of 0.25 - 10 is obtained in the furnace gas; the thermodynamic activity of Fe(II) in the slag is >0.5; the Fe(III)/Fe(II) ratio in the slag is between 0.25 and 2; and - after tapping off, the resultant slag is cooled such that a solid solution with a spinel structure is obtained. Advantageously, the following conditions are set for the method according to the invention: the operating temperature must preferably be 50 °C or more above the liquidus temperature of the slag; the oxygen pressure is adjusted to a value of between 10 " and 10 * bar. This can be achieved, for example, by setting a fuel to oxygen ratio such that a CO/CO2 ratio of between 0.5 and 5 is obtained in the furnace gas; the slag composition is adjusted such that the thermodynamic activity coefficient of Fe(II) in the slag is > 1. This can be achieved by careful composition of the mixture of materials to be melted. The method by which the activity coefficients can be calculated is described, for example, in the following references: Michels M.A.J. and Wesker E., 1985, "Thermodynamic modelling of the solid- liquid interaction in oxides and silicates": Solid State Ionics 16:33-38, and - Michels M.A.J. and Wesker E. 1987, "A network model for the thermodynamics of multicomponent silicate melts, I. Binary mixtures MO-SiO 2 ", Calphad, l l(4):383-393. the Fe(III)/Fe(II) ratio in the slag is adjusted to be between 0.5 and 1.5, the residence time in the furnace being used as the parameter for this. The resultant slag is then tapped off and then slowly cooled. Said cooling must therefore not take place suddenly (as in a "quench"), because this prevents crystallisation. For example, crystallisation can take place by allowing the slag pots, in which the slag is present, to cool in air or by imposing a suitable cooling regime on the slag in a cooling oven. Usually the slag will be tapped off into large slag pots with a capacity of approximately 1 tonne and cooled in air for approximately 24 hours. Such a cooling procedure gives a cooling rate of, on average, approximately 60 K/hour. In general, therefore, for the method according to the invention a cooling rate of 10 - 200 K/hour, advantageously 40 to 80 K/hour, will be used. If such conditions are maintained, spinels are formed, which belong to the 2FeO.Moθ2-Fe 3 O 4 series, in which molybdenum is very firmly bound, in view of the low degree of leaching. The method according to the invention is explained in more detail on the basis of the following example. Example Waste streams were melted in a smelter installation under reducing conditions. The furnace of the smelter installation consisted of a cylindrical, water-cooled chamber, where some of the molten slag solidified as a thin layer on the wall and where the slag on the inside thereof ran down and was collected in a slag bath. Said slag bath was emptied with some regularity, the tapped slag being cooled in slag pots in the air. Said cooling took approximately 24 hours. The furnace was further equipped with an oxy-fuel burner in which oil was burnt with technical grade oxygen. The feed was fed into the furnace through the flame. The off-gas was post-combusted and the acid components were then removed in a wet gas scrubber, after which the fly material was separated off. Four different waste streams were treated to produce different mixtures. The composition of said waste streams as far as the most important elements are concerned is given in Table A below (in % (m/m,) except for Mo and Pb, which are given in ppm). TABLE A Two mixtures, with the mixing ratio specified in Table B (in %), were made from each starting material: TABLE B In addition, 21kg magnesium hydroxide and 15kg coke per 100kg feed mixture were also added to these two mixtures. The magnesium hydroxide served to obtain the correct slag composition with the associated desired crystallisation behaviour. The calculated liquidus temperature (this was the temperature at which the entire mixture was liquid) was 1250°C for both mixtures. The calculated activity coefficient of the FeO was 1.4 and 1.2, respectively, in the given slag composition. Mixture 1 was melted under a significantly higher oxygen pressure than mixture 2, the oxygen pressures being, respectively, 10 "5 and 10 "9 bar. The temperature in the furnace was at least 1350°C. The tapped slags both crystallised well and were black in colour. The composition of the slags as far as the relevant elements are concerned and that of the associated fly material are given in Table C below. TABLE C The volatilisation of the elements zinc and lead can be calculated from the results given in Table C. For mixture 1 these values are: 71% and 59%. For the test with mixture 2, these values are 93% and 81%. The higher volatilisation in the case of the test with mixture 2 is in line with the more highly reducing conditions which were imposed. The following crystalline phases can be observed in the crystallised slags by means of X-ray diffraction: in mixture 1 : diopside, forsterite/fayalite; these belong, respectively, to the pyroxene family and the olivine family; in mixture 2: diopside, forsterite/fayalite and magnetite; the latter belongs to the spinels. Leaching of molybdenum from the slag was determined in accordance with NEN 7343 as specified in the Building Materials Order. In general, the material is ground and screened for this leaching test. The 4 mm screen fraction is introduced into a column, through which a nitric acid solution at pH 4 is passed. Seven samples are taken after a number of (specified) volumes have passed through. The composition of the leached liquid is then determined and the concentrations determined are converted to obtain an emission, which is then compared with the standard. Further details relating to the leaching test are given in NEN 7343. The ratio of the amount of molybdenum leached from the slags to the permitted standard is 7.6 for mixture 1 and 0.64 for mixture 2. It is thus clear that the slag that crystallises with the spinel magnetite (FeO. Fe 2 O 3 ), in which a solid solution of M0O2 is present, exhibits leaching of molybdenum that is distinctly lower than the standard. Consequently, as far as molybdenum leaching is concerned, such a slag can freely be used in the construction industry.

Claims: CLAIMS 1. Method for the non-leachable immobilisation of molybdenum compounds in a slag by melting and subsequently cooling waste streams containing iron and molybdenum compounds, characterised in that the treatment temperature is above the liquidus temperature of the slag; the oxygen pressure is adjusted to a value of between 10 " and 10 " bar; the thermodynamic activity of Fe(II) in the slag is >0.5; the Fe(III)/Fe(II) ratio in the slag is between 0.25 and 2; and - after tapping off, the resultant slag is cooled such that a solid solution with a spinel structure is obtained. 2. Method according to Claim 1 , characterised in that the treatment temperature for the waste stream is at least 50°C above the liquidus temperature of the slag. 3. Method according to Claim 1 or 2, characterised in that the oxygen pressure is adjusted to a value of between 10 " and 10 ' bar. 4. Method according to one or more of Claims 1 - 3, characterised in that the thermodynamic activity of Fe(II) in the slag is > 1. 5. Method according to one or more of Claims 1 - 4, characterised in that the Fe(III)/Fe(II) ratio in the slag is between 0.5 and 1.5. 6. Method according to one or more of Claims 1 - 5, characterised in that the resultant slag is converted into a solid solution having a 2FeO.MoO2-Fe 3 O 4 spinel structure by cooling.

Inventors: LOTENS JAN PIETER, VAN DER GIESEN ABRAHAM BAREND, LOTENS, JAN, PIETER, VAN DER GIESEN, ABRAHAM, BAREND
Applicants: DROAN B V, GIESEN ABRAHAM BAREND V D, LOTENS JAN PIETER, DROAN B.V., LOTENS, JAN, PIETER, VAN DER GIESEN, ABRAHAM, BAREND
Application year: 1999
Main classification: A62D3
IPC codes: A62D3, A62D101, B09B3, C04B5, C22B7