Method for recycling waste water from a stainless steel slag treatment process

Abstract

The present invention relates to method for recycling alkaline waste water from a stainless steel slag treatment process wherein stainless steel slag is brought into contact with water thereby producing said waste water, which waste water contains heavy metals, including at least chromium, and has a pH of at least 12. The waste water is recycled by using it for treating an alkaline granular carbonatable material, which contains aluminum metal, in order to oxidize the aluminum metal contained therein. This material is in particular municipal waste incinerator bottom ash which can, after the treatment of the present invention, safely be used as fine or coarse aggregate in bonded applications such as concrete, mortar and asphalt. During the treatment with the alkaline waste water, hydrogen gas is produced which is captured and used to produce energy by means of a cogeneration device.

Claims

1. A method for recycling alkaline waste water from a stainless steel slag treatment process wherein stainless steel slag is brought into contact with water thereby producing said waste water, which waste water contains heavy metals, including at least chromium, and has a pH of at least 12.5, in which method said waste water is used for treating an alkaline granular carbonatable material, which contains between 0.1 and 5% by dry weight of aluminium metal, in order to oxidise the aluminium metal contained therein so as to produce hydrogen gas, the method comprising the steps of: introducing the alkaline granular material in a bath containing a portion of said waste water to produce said hydrogen in said bath, the amount of alkaline granular material introduced in said bath being less than 100 wt. % of the amount of waste water contained therein; maintaining the pH of the waste water contained in said bath at a predetermined level higher than 12.5 by adding additional waste water from said stainless steel slag treatment process and in case of an excess of waste water in the water bath, at least a portion of this excess of waste water is recycled; removing the alkaline granular material from said bath after having produced the hydrogen gas therein; subjecting the alkaline granular material which is removed from said bath but which still contains water to an aging process wherein the alkaline granular carbonatable material is brought in contact with a gas containing carbon dioxide to carbonate said carbonatable material until its pH, measured in accordance with the standard DIN 38414-S4, is lower than 10; allowing water which is contained in the alkaline granular material to evaporate during said aging process; and replacing at least a portion of the evaporated water by a further portion of said waste water which is applied during said ageing process onto the alkaline granular material.

2. The method as claimed in claim 1, wherein the produced hydrogen gas is captured in and/or above said water bath.

3. The method as claimed in claim 2, wherein the alkaline granular material is passed through said water bath.

4. The method as claimed in claim 3, wherein the alkaline granular material is maintained in a fluidized bed when passing through said water bath.

5. The method as claimed in claim 3, wherein said waste water is passed in counter-current with said alkaline granular material through said water bath.

6. The method as claimed in claim 1, wherein said predetermined level at which the pH of the waste water contained in said bath is maintained is higher than 12.75.

7. The method as claimed in claim 1, wherein said waste water has a pH higher than 12.75.

8. The method as claimed in claim 1, wherein the amount of alkaline granular material introduced in said bath is less than 50 wt. % of the amount of waste water contained therein.

9. The method as claimed in claim 1, wherein the introduction of said alkaline granular carbonatable material in said water bath reduces the pH of the waste water contained therein, the pH of this waste water being maintained at a predetermined level by adding additional waste water from said stainless steel slag treatment process, in case of an excess of waste water in the water bath, at least a portion of this excess of waste water is recycled preferably to said stainless steel slag treatment process.

10. The method as claimed in claim 1, wherein the produced hydrogen gas is captured and used to produce energy.

11. The method as claimed in claim 1, wherein after having produced said hydrogen gas but before being carbonated the granular carbonatable material is pelletised to produce a coarser granular material wherein the particles of said carbonatable material are agglomerated.

12. The method as claimed in claim 1, wherein at least a portion of the carbonated material is used as a construction aggregate.

13. The method as claimed in claim 1, wherein said granular material contains at least bottom ash of an incinerator.

14. The method as claimed in claim 1, wherein non-ferrous metals are removed from the alkaline granular material before treating this material with said waste water.

15. The method as claimed in claim 1, wherein at least a portion of said waste water is produced by bringing stainless steel slag in contact with water in order to neutralise (hydrate) free lime contained therein.

16. The method as claimed in claim 1, wherein at least a portion of said waste water is produced in a wet jigging apparatus wherein stainless steel slag particles are separated based on their density in particles which contain stainless steel content and particles which contain no or less stainless steel.

17. The method as claimed in claim 1, wherein the alkaline granular material which is brought in contact with said waste water to produce said hydrogen gas comprises less than 2.5% by dry weight of aluminium metal.

18. The method as claimed in claim 1, wherein the alkaline granular material which is brought in contact with said waste water to produce said hydrogen gas contains particles with a size greater than 1 mm.

19. The method as claimed in claim 1, wherein the amount of alkaline granular material introduced in said bath is less than 25 wt. % of the amount of waste water contained therein.

20. The method as claimed in claim 1, wherein the amount of alkaline granular material introduced in said bath is less than 10 wt. % of the amount of waste water contained therein.

Description

(1) Other particularities and advantages of the invention will become apparent from the following description of a particular embodiment of the method according to the present invention. The reference numerals used in this description relate to the annexed drawings wherein:

(2) FIG. 1 is a flow chart of a bottom ash treatment process in accordance with the present invention; and

(3) FIG. 2 is a schematic drawing of a water bath wherein bottom ash is treated by the method according to the present invention and wherein the produce hydrogen gas is captured underneath a bell.

(4) The present invention generally relates to a new method for recycling waste water generated during the processing of stainless steel slag. Stainless steel contains mainly iron and further at least chromium and optionally other heavy metals such as nickel and molybdenum. During the production of stainless steel, calcium and magnesium oxides/carbonates (f.e. burned lime, calcite, dolomite and magnesite) are added to the furnace (in particular an electric arc furnace) to produce a liquid slag on top of the molten slag. This slag acts as a destination for oxidised impurities. After the steel making process, the slag is poured into pits and is allowed to cool down. To accelerate the cooling process, water is sprayed onto the hot slag. During the cooling process, different amorphous and crystalline phases are formed, including calcium silicates.

(5) The solidified pieces of stainless steel slag are crushed to produce fine or coarse aggregates which can be used in particular for producing concrete or asphalt (=bituminous concrete) (see EP 0 837 043 which is incorporated herein by reference). As disclosed in EP 2 160 367 the crushed stainless steel slag particles can also be further grinded or milled to a very small particle size, in particular to a particle size smaller than 63 m so that the grinded stainless steel slag particles can be used as a filler in concrete (in particular self-compacting concrete) or in asphalt. Crushing/grinding of the stainless steel slag enables to recover as much as possible of the valuable stainless steel which is contained in stainless steel slag. This can be done by hand picking, magnetic separation techniques or density separation techniques. A preferred density separation technique is the wet jigging technique which is disclosed in EP 1 312 415. This European patent application is also incorporated herein by reference. In this wet jigging technique, the stainless steel slag particles are made to float in water so that they can be separated based on their density.

(6) Fresh stainless steel slag always still contains some free lime (i.e. CaO). This free lime may be present in the form of small or larger inclusions in the stainless steel slag particles. When used as aggregate for concrete or asphalt, it is important that this free lime is neutralised since when the free lime inclusions come into contact with water, they may start to swell thus causing cracks in the concrete or asphalt. To solve this problem, the crushed/grinded stainless steel slag particles are brought in contact with water to neutralise the free lime contained therein. As disclosed in EP 1 146 022, which is incorporated herein by reference, this can be done by immersing the stainless steel slag particles in a bath of water or the water can be sprayed onto the stainless steel slag particles.

(7) By being brought in contact with the stainless steel slag particles, the pH of the neutralisation water rises and also its heavy metal content. The neutralisation water is therefore collected in one or more reservoirs and is re-used for the neutralisation process. Although part of this water also evaporates, there is a surplus of water during rainy periods. Consequently, part of the neutralisation water needs to be discharged. Also the water used in the wet jigging installation needs to be refreshed from time to time and needs thus also to be discharged.

(8) The waste water produced by neutralising the free lime in the stock piles has a pH higher than 12, in particular higher than 12.5 and usually even higher than 12.75. Depending on the amount of rain fall, the pH may even be higher. The pH of the water contained in the wet jigging installation is not dependent on the rain fall, and is usually about equal to 14. The pH of this waste water is thus higher than 13 and in particular higher than 13.25. Due to its very high pH, it can be used to raise the pH of the waste water (or a portion thereof) which is used to neutralise the free lime in the stainless steel slags.

(9) In accordance with the present invention, excess of waste water of the stainless steel slag processing plant is used to treat an alkaline granular material, in particular an alkaline granular carbonatable material, which contains aluminium metal in order to oxidise this aluminium metal so as to produce hydrogen gas. This alkaline granular material is in particular municipal waste incinerator bottom ash (MWI-bottom ash).

(10) MWI-bottom ash consists essentially of mineral material and is like a greyish gravel in which residues such as bottle glass, ceramics, scrap iron and non-ferrous metals can be identified.

(11) This is a fairly heterogeneous material since microscopic observation reveals the presence of two distinct zones: so-called slaggy zones, with low density because of its vacuolar structure and which comprises melting residues such as non-molten bottle glasses, metal debris, etc, and glassy zones which may either be in the completely amorphous state or contain mineral phases formed at high temperature (typically calcium silicates), the dendritic structure of which testifies to rapid cooling during a quenching step.

(12) The composition of the bottom ash therefore proves to be extremely complex and, among the main constituents, there are generally: a glassy matrix resulting from the quenching of a liquid silicate; minerals formed at high temperature that consist generally of silicates and oxides; species neoformed at low temperature at the discharge from the furnace, including mainly portlandite [Ca(OH).sub.2] issuing from the hydration of the lime that occurs during the quenching undergone by the bottom ash, carbonates and, to a lesser extent, chlorides; calcium sulphates, which may be present in residual form or be formed either at high temperature, by oxidation of the SO.sub.2 issuing from the combustion and reaction thereof with the calcium mobilised in the furnace, or at low temperature by precipitation during the quenching by capture of the SO.sub.2 by the water in combination with the available calcium; metals (Al, Cu, Fe) and alloys (PbAl) coming from residual fragments issuing from the incinerated waste; relic phases that are mainly constituent minerals such as quartz, potassium, feldspars and glass debris that has not melted; unburned materials that correspond to combustible organic material that has not resided for long enough in the furnace or that was protected by other compounds by an encapsulation effect.

(13) In the prior art, generally a natural aging of the bottom ash is carried out for several months before using it as a construction material. This aging step is highly complex since it comprises several phenomena: slow oxidation of the unburned materials, carbonation of the lime that leads to a reduction in pH responsible for the destabilisation of ettringite [Ca.sub.6Al.sub.2(SO.sub.4).sub.3(OH).sub.12.26H.sub.2O], oxidation/hydroxylation of the aluminium and oxidation/hydroxylation of the iron. One drawback of this natural aging is that it takes a great deal of time and space. The purpose of this aging processing of the MWI-bottom ash is intended firstly to stabilise it in particular on a dimensional level and secondly to fix the heavy metals within the neoformed phases. This is because the carbonates, in precipitating, are liable to trap the trace elements such as cadmium, lead and zinc whereas the same elements with in addition copper and manganese appear to have great affinity for the iron and aluminium (hydr)oxides.

(14) If the bottom ash is used without sufficient aging, swelling phenomena caused in particular by the subsequent formation of aluminium hydroxides from aluminium metal and ettringite still present in the bottom ash may take place.

(15) Bottom ash subjected solely to accelerated carbonation still contains a substantial amount of non-oxidated aluminium metal, which may pose swelling problems. Bottom ash is first of all subjected to an initial step of separation of aluminium metal by eddy currents in order to recover as much as possible of the aluminium metal. A magnetic separation is also carried out to recover iron. Further metal recovery steps can optionally be performed after having crushed the bottom ash to a smaller particle size. The thus obtained bottom ash still contains at least 0.1% by dry weight, in particular at least 0.3% by dry weight and more particularly at least 0.5% by dry weight of aluminium metal. Usually it contains less than 5% by dry weight of aluminium metal, in particular between 0.8 and 2.5% by dry weight of aluminium metal. Next, the bottom ash is treated with the alkaline waste water to oxidise this aluminium metal.

(16) FIG. 1 illustrates an example of a flow chart of a bottom ash treatment process.

(17) The MWI-bottom ash supplied to this process has preferable been subjected to a preliminary sieving/crushing operation so that it has a predetermined particle size ranging for example from 0 to 50 mm. The smallest fraction may optionally be removed from this bottom ash, in particular a fraction of 0 to x mm, with x being larger than 1 mm but preferably smaller than 5 mm. The preliminary process for preparing the bottom ash is preferably a dry process, wherein the smallest particles can be removed for example by means of a wind sifting process. The preliminary process may however also be a wet process, wherein the bottom ash is also washed, in particular to lower its content of water soluble salts, the smaller particles being preferably removed during this washing/sieving step.

(18) In a first step 1 of the flow chart illustrated in FIG. 1 the bottom ash is passed underneath a top belt magnet to recycle ferrous metals 2. The remaining bottom ash 3 is then subjected to a sieving step 4 wherein the bottom ash 3 is divided in a fraction 5 which has a particle size greater than 12 mm (i.e. which doesn't pass through a 12 mm sieve) and in a fraction 6 which has a particle size smaller than 12 mm (i.e. which passes through a 12 mm sieve). From the bottom ash fraction 5 the non-ferrous metals (in particular aluminium) 8 are removed by means of an eddy current separating device 7. The remaining bottom ash fraction 9 is then crushed in step 10 to achieve bottom ash having a particle size smaller than 12 mm. The crushed bottom ash is added to fraction 6 whilst the oversize is recycled to fraction 9 in order to be crushed again.

(19) In the next step 11, further ferrous 12 and non-ferrous metals 13 are removed from the finer bottom ash 6. This can be done by eddy currents, sink float, upstream column, jigging, top belt magnet, hydrocyclone and/or wind sifting techniques.

(20) The thus obtained bottom ash 14 is treated in the next step 15 with waste water 16 from the stainless steel slag treatment process. This very alkaline waste water is preferably contained in a water bath 100, as illustrated schematically in FIG. 2, and the bottom ash is passed, preferably continuously, through this water bath 100. In FIG. 2 the bottom ash is guided through the water bath 100 by means of a conveyor system 101. However, it is also possible to generate a fluid flow in the water bath conveying the bottom ash through the water bath. Preferably, the bottom ash is contained in a fluidized bed in order to optimize the contact between the alkaline waste water and the bottom ash and especially also to remove gas bubbles which are produced in the bottom ash and which have a tendency to adhere thereto (and thus disturb the chemical reactions). When passing the alkaline granular material through the water bath, the waste water can be passed in counter-current with this granular material through the water bath.

(21) In the water bath amphoteric metals contained in the bottom ash, in particular aluminium, reacts under the highly alkaline conditions to produce water. This reaction can be represented as follows:
2Al+2OH.sup.+4H.sub.2O.fwdarw.2[AlO(OH).sub.2].sup.+3H.sub.2

(22) The thus produced hydrogen gas is preferably captured. This can be done by guiding the bottom ash in tubes through the water bath, the tubes being inclined so that the hydrogen gas can be captured at one extremity of these tubes. In FIG. 2, the hydrogen gas is however captured in a bell 102 which is placed into the water bath. Initially, this bell is completely filled with water. When hydrogen gas is released from the bottom ash passing underneath the bell, this hydrogen gas raises and accumulates within the bell. When more hydrogen gas is captured, the bell may rise, the lower edge of the bell remaining in the water bath to form a water lock preventing the escape of hydrogen gas.

(23) The hydrogen gas captured in the bell is used to produce energy. This is preferably done by means of a combined heat and power generating device (cogeneration plant or installation).

(24) In the water bath most of the aluminium metal may have been oxidised, in particular more than 50 wt. %, preferably more than 75 wt. %, for example about 80 wt. %. The bottom ash 19 leaving the waste water treatment step 15 is preferably subjected to an accelerated carbonation step 20 wherein the bottom ash is brought in contact with a gas which contains more CO.sub.2 than air. This gas may contain more than 1 wt. %, preferably more than 5 wt. % and more preferably more than 10 wt. % of carbon dioxide. The gas may be a flue gas providing not only carbon dioxide but optionally also heat to dry the bottom ash.

(25) The accelerated carbonation is preferably performed in a rotary drum, and this for example for 4 to 5 hours. Combustion gases, such as for example household waste incineration fumes or the fumes produced by the stainless steel plant, are introduced into the drum in order to obtain a hot atmosphere (approximately 50 C.) enriched with carbon dioxide (approximately 10% to 12%). However, a humidity level equal to or greater than 80% is maintained in the atmosphere of the drum rather than the usual 30% of household waste incineration fumes. Before being introduced in the rotary drum, the bottom ash can be put in a pile and the water contained therein can be allowed to drain. The recovered water can be returned to the water bath.

(26) After the carbonation treatment, the bottom ash has preferably a pH lower than 10, preferably lower than 9.5 and more preferably lower than 9. The pH of the granular material is measured in accordance with the standard DIN 38414-S4. Although the pH of the bottom ash material has been raised by the treatment with the alkaline waste water 16, the pH of this material can be lowered by the carbonation treatment quite easily again since the additional hydroxides which in the pore water react quickly with the carbon dioxide dissolved therein to produce carbonates.

(27) In a next step 21 the bottom ash is then subjected to a natural ageing process wherein further chemical reactions (such as a further destabilisation of the ettringite) and a further drying of the material take place. During this natural ageing the bottom ash is shielded of from the rain to be able to control its moisture content. However, due to the drying of the bottom ash, the water content may become too low so that it is necessary to spray water onto the bottom ash. This water is preferably also waste water from the stainless steel slag treatment process, possibly after this water has already been used in the water bath 100 (for example the water draining out of the treated bottom ash material). The bottom ash thus becomes sufficiently stable to be useful as coarse or fine (sand) aggregates, not only in unbound applications but also in bound applications such as concrete or mortar and asphalt (bituminous mixes).

(28) The coarse and fine aggregate fractions can be sieve out from the treated bottom ash in step 22 and can be used in step 23 in bound applications such as concrete.

(29) The stainless steel slag processing installation and the bottom ash treatment installation are preferably provided on one site. An advantage of the method of the invention is that the excess of waste water generated during the processing of stainless steel slag can be recycled for the treatment of the bottom ash so that no, or at least less, waste water needs to be purified and discharged.

(30) The use of the waste water for the treatment of bottom ash enables to produce valuable hydrogen gas, in contrast to only a natural weathering of the bottom ash. Moreover, the aluminium metal contained in the bottom ash is oxidised/removed to a much larger extent than in a natural weathering process, so that it is thus safer to use the treated bottom ash as fine or coarse aggregate. During a natural ageing process, most of the aluminium metal is passivated by an oxide layer whereas in the method of the present invention this oxide layer is dissolved and nearly all of the aluminium is oxidised.

(31) The method of the present invention also offers the advantage that during the carbonation step more metal oxides/hydroxides are available which can be carbonated and which can thus be used to immobilise the heavy metals. These metal oxide/hydroxides are not only those contained in the waste water but also those produced by the oxidation of the metals in the bottom ash. Moreover, the calcium (and magnesium) contained in the waste water also contributes to the formation of carbonates. These carbonates assist not only in immobilizing heavy metals but improve also the mechanical properties of the material. Moreover, when pelletizing the bottom ash particles before the carbonation step to produce a coarser granular material, the carbonates also contribute to a greater strength of this coarser granular material.

(32) Although the present invention has been described with reference to specific example embodiments, it is obvious that various modifications and changes can be made to these examples without departing from the general scope of the invention as defined by the claims. For example, other carbonatable materials than bottom ash could be treated in this way, such as for example slag from aluminium production, or residues from the extraction and/or processing of metals, alone or mixed with a carbonatable binder, such as cement. In addition, although the granulometry of the resulting material may normally be simply adjusted by screening of the treated material, in particular in such a way that the carbonatable material contains particles with a size greater than 1 mm, preferable greater than 2 mm, and even more preferably greater than 4 mm, it is also possible to obtain, from a starting material with an excessively fine granulometry, a material with such a granulometry by adding to the treatment process a pelletisation step before or during the carbonation, so that the calcic matrix formed during the carbonation functions as a binder of fine particles in grains with larger dimensions. Consequently the description and drawings must be considered in an illustrative rather than restrictive sense.