NL1010590C2 - Method for the non-leachable immobilization of molybdenum compounds in a slag. - Google Patents

Description

Method for the non-leachable immobilization of molvbdenum compounds in a slag.

The invention relates to a process for the non-leachable immobilization of molybdenum compounds in a slag by melting waste streams containing iron and molybdenum compounds and subsequently cooling them.

Molybdenum compounds are present in a number of fairly specific waste streams. Examples are fly ash from waste incineration plants (waste incineration plants) and fly ash in the processing of chemical waste via incineration in, for example, drum ovens. The source of the molybdenum is probably partly due to corrosion of the used installation and partly present in the waste streams processed in these installations.

Many residues, whether or not after a pre-treatment, are generally used in construction. Molybdenum, like many other elements, is subject to a leaching requirement when a processed waste stream is used as a building material. This requirement has been laid down in the Building Materials Decree in the Netherlands.

Chemically, molybdenum is one of the oxyanion formers and the compounds containing these ions are usually very soluble in water. This makes it more difficult to comply with the leaching requirements of the aforementioned Building Materials Decree. This problem is virtually independent of the technical approach followed in the processing of the waste streams. Both with cold immobilization (mixing waste streams with, for example, cement) and with processing in the form of a melt (thermal immobilization or pyrometallurgical processing), it is repeatedly found that the leachability of molybdenum exceeds the standard stated in the Building Materials Decree 25. This standard established for molybdenum concerns a maximum immission value of 150 mg / m2 per 100 years.

Surprisingly, it has now been found that the above-mentioned problem can be completely or largely solved by carrying out the pyrometallurgical processing of molybdenum-containing waste streams under reducing conditions, whereby the oxygen pressure is adjusted within a relatively narrow range, so that slags which crystallize after obtaining a exhibit greatly reduced leaching of molybdenum. The molybdenum-containing waste streams to be used in the method according to the invention must also contain iron compounds, which is the case in virtually all cases in 101 0590 2. However, if such a waste stream does not contain iron compounds, these must still be added separately.

More particularly, the invention relates to a method mentioned in the preamble 5, characterized in that the processing temperature is above the liquidus temperature of the slag; the oxygen pressure is set to a value between 10'6 and 10'12; this can be achieved by adjusting such a fuel / oxygen ratio that a CO2 / CO2 ratio in the furnace gas of 0.25-10 is obtained; 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 the resulting slag is cooled after tapping to obtain a solid solution with a spinel structure.

Advantageously, the following conditions are set in the method according to the invention: the operating temperature should preferably be 50 ° C or more above the liquidus temperature of the slag; the oxygen pressure is adjusted to a value between 10'7.5 and 10'9.5. This can be achieved, for example, by setting a ratio of fuel and oxygen such that a CO / CO2 ratio in the furnace gas is created between 0.5 and 5; the slag composition is adjusted such that the thermodynamic activity coefficient of Fe (II) in the slag is> 1. This can be achieved by a careful composition of the mixture of materials to be fused. The method according to which the activity coefficients can be calculated is described, for example, in the following references: - Michels M.A.J. and Wesker E., 1985, "Thermodynamic modeling of the solid-liquid interaction in oxides and silicates": Solid State Ionics 16: 33-38, and 30 - Michels M.A.J. and Wesker E. 1987, "A network model for the thermo dynamics of multicomponent silicate melts, I. Binary mixtures MO-Si02", Calphad, 11 (4): 383-393.

1010590 3 the Fe (III) / Fe (II) ratio in the slag is set between 0.5 and 1.5, for which the residence time in the oven is used as a parameter.

The resulting slag is then drained and then cooled slowly.

Therefore, this cooling should not be done in a jerky manner (as in a "quench"), as this prevents crystallization. For example, crystallization can take place by allowing the slag vases, in which the slag is present, to cool in the air or by imposing a suitable cooling regime on the slag in a cooling oven. Normally the snail will be drained into large snail vases with a volume of about 1 ton and cooled in air for about 24 hours. Such a cooling procedure provides a cooling rate of about 60K / h on average. In general, therefore, with regard to the method according to the invention, a cooling rate of 10 to 200K / hour, advantageously 40 to 80K / hour, will be used. If such conditions are maintained, spinels are formed, which belong to the series 2FeO.Mo02-Fe304, in which molybdenum - in view of the low leaching - is very tightly bound.

The method according to the invention is further elucidated by means of the example below.

Example 20 Under reducing conditions, waste streams were melted in a melting plant. The furnace of the melting installation consisted of a cylindrical, water-cooled space, where the molten slag partially solidified on the wall and where the slag ran down on the inside and was collected in a slag bath. This slag bath was emptied with some regularity, the tapped snail being cooled in snail vases in air. This cooling took about 24 hours. The furnace was further equipped with an oxy-fuel burner in which oil with technical oxygen was burned. The feed was introduced into the oven through the flame. The off-gas was afterburned and then the acidic components were removed in a wet scrubber, after which the fly dust was separated.

Four different waste streams were processed into various mixtures.

The composition of these waste streams for the main elements is shown in Table A below (in% m / m, except for Mo and Pb, these in ppm).

1010590 4 _TABLE A_ waste stream- 12 3 4 number H20__56 9.2__66 / 7_ 7.2 5 _Si02__30.3 22.7__22_ 2.1 A1203__4.9 4.3_ .3 .2

FexOy 4.0 9.2 16.8 27.2

CaO_ 2.3 8.4 7.3 5.2

ZnO__02__LL2__1_ 44.9 10 _Pb__3Π__31350__36_ 8850

Mo 1.1 1366 111 243

Of these, two blends were made with the following mixing ratio (in%) listed in Table B: 15

TABLE B

starting material 12 3 4 mixture 1 71.4 14.3 14.3 20 mixture 2 78.6 7.1 - 14.3

To these two mixtures a further 21 kg of magnesium hydroxide and 15 kg of coke per 100 kg of feed mixture were added. The magnesium hydroxide served to obtain the correct slag composition with the associated desired crystallization behavior.

The calculated liquidus temperature (this was the temperature at which the entire mixture was liquid) for both mixtures was 1250 ° C. The calculated activity coefficient of the FeO was 1.4 resp. 1.2 in the given slag composition.

Melting of mixture 1 was carried out at significantly higher oxygen pressure than in mixture 2, namely at 105 and 109 bar, respectively. The temperature in the oven was at least 1350 ° C. The tapped snails both crystallized well and were black in color.

1010590 5

The composition of the slags with regard to the relevant elements and the associated fly dust is shown in Table C below.

TABLE C

5 '_

Element Snail mixture 1 Snail mixture 2

Zn,% m / m 0.77 1.0

Mo, ppm 351 157

Pb, ppm 3000 1550 10 fly dust fly dust

Zn,% m / m 11.4 14.9

Pb,% m / m 14.3 14.6 From the results reported in Table C, the volatilization of the elements zinc and lead can be calculated. These values are for mixture 1: 71% and 59%. For the test with mixture 2, these are 93% and 81%. The higher volatilization in the mixture 2 test matches the stronger reducing conditions imposed.

In the crystallized slag, the following crystalline phases can be observed with X-ray diffraction: in mixture 1: diopside, forsterite / fayalite; these belong to the pyroxene and olivine family, respectively; in mixture 2: diopside, forsterite / phayalite, and magnetite; the latter is one of the 25 spinels.

The leaching of molybdenum on the slag is determined in accordance with NEN 7343 as laid down in the Building Materials Decree. In general, the material is ground and sieved in this leaching test. The 4 mm screen fraction is placed in a column, through which a nitric acid solution with a pH of 4 is passed. Seven samples are taken from passing a number of (specified) volumes. The composition of the lye liquor is then measured and the determined concentrations are converted into an emission, which is then compared with the standard 1010590 6. Further details regarding the leaching test are stated in NEN 7343.

The ratio of the leaching of molybdenum from the slag to the permissible form is 1: 7.6 for mixture and 2: 0.64 for mixture. This makes it clear that the slag crystallizes with the spinel magnetite (Fe0.Fe203), which contains a solid solution of MoO2, which exhibits a leaching of molybdenum which is clearly lower than the standard. As a result, such a slag can be used freely in construction with regard to molybdenum leaching.

101 0590

Claims (6)

Method for the non-leachable immobilization of molybdenum compounds in a slag by melting waste streams containing iron and molybdenum compounds and subsequently cooling them, characterized in that the processing temperature is above the liquidus temperature of the slag; the oxygen pressure is adjusted to a value between 10'6 and 10'12; 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 the resulting slag after cooling is cooled such that a solid solution with a spinel structure is obtained. 2. Method according to claim 1, characterized in that the processing temperature of the waste stream is at least 50 ° C above the liquidus temperature of the slag. Method according to claim 1 or 2, characterized in that the oxygen pressure is adjusted to a value between 10'7.5 and 10'9.5. Method according to one or more of claims 1 to 3, characterized in that the thermodynamic activity of Fe (II) in the slag is> 1. Process according to one or more of Claims 1 to 4, characterized in that the Fe (III) / Fe (II) ratio in the slag is between 0.5 and 1.5. 25 Process according to one or more of claims 1 to 5, characterized in that the resulting slag is converted into a solid solution with a 2FeO.MoO2-Fe304 spinel structure by cooling. 101 0590 COOPERATION TREATY (PCT) REPORT ON NEWNESS RESEARCH OF INTERNATIONAL TYPE IDENTIFICATION OF THE NATIONAL APPLICATION Characteristic of the applicant or of the authorized representative NO 42250 / md Dutch application no. Date of the request for an examination of an international type Granted by the International Research Authority of the USA) to the request for an examination of an international type No SN 32474 EN I. CLASSIFICATION OF THE SUBJECT When using different classifications, indicate all the classification symbols) International clatsification IIPC) Int. Cl. 6: A 62 D 3/00, C 22 B 7/04 II. FIELDS OF TECHNIQUE EXAMINED Minimum Documentation Examined Classification System Classification Symols Int. Cl.6 A 62 D, C 22 B Documentation examined other than the minimum documentation to the extent that such documents are included in the areas investigated HL II NO STUDY MAY BE FOR CERTAIN CONCLUSIONS (comments on supplement sheet) IV- I 1 LACK OF UNITY OF INVENTION ( comments on supplement sheet) Form PCT / ISA / 20 1 fa) 07.1979