Method for processing steel slag and hydraulic mineral binder

Abstract

The invention relates to a method for processing steel slag to produce a hydraulic mineral binder with a high hardening potential and to recover iron. There is provision for this purpose to provide a feed product comprising steel slag with MnO. This feed product is further processed as a melt by introducing reducing agents into the melt. A lime saturation factor of between 90 and 110 is hereby to be achieved in the mineral melt portion. Subsequently the melt is cooled in a defined manner and elemental iron is mechanically separated from the solidified melt. The solidified melt is then supplied for use as hydraulic mineral binder.

Claims

1. A method for processing steel slag to produce a hydraulic mineral binder with high hardening potential and to recover iron, comprising the steps: providing a feed product comprising steel slag with iron compounds, and MnO, whereby the MnO may be contained in the steel slag, processing the feed product as melt in a furnace, incorporating reducing agents into the melt to reduce the iron compounds in order to achieve a lime saturation factor in a mineral part of the melt between 90 and 110, wherein the reduction is carried out in a non-oxidizing furnace atmosphere, cooling the melt such that the melt solidifies in less than 15 minutes, and mechanically separating at least part of the elemental iron from the solidified melt, wherein the solidified melt following the mechanical separation has a reduced iron content and an alite content of at least 40 wt. % with a content of crystalline phases of at least 60 wt. %, and wherein the solidified melt with the reduced iron content is for use as a hydraulic mineral binder.

2. The method according to claim 1, characterized in that the feed product comprises 0.1 to 10 wt. % of MnO.

3. The method according to claim 1, characterized in that the feed product contains one or more of the compounds in the group consisting of: up to 5 wt. % of Al.sub.2O.sub.3, 30-50 wt. % of CaO, or 10 to 20 wt. % of SiO.sub.2.

4. The method according to claim 1, characterized in that the melt has a temperature of approximately 1450 C. to approximately 1800 C. before and/or during the reduction.

5. The method according to claim 1, characterized in that the non-oxidizing atmosphere is a reducing atmosphere.

6. The method according to claim 1, characterized in that at least one of carbon, silicon and other metals or semi-metals are used as reducing agents.

7. The method according to claim 1, characterized in that at least part of the reducing agent is blown into the melt.

8. The method according to claim 7, characterized in that the reducing agent blown into the melt is blown in by means of an inert gas flow.

9. The method according to claim 1, characterized in that a flux is incorporated into the melt.

10. The method according to claim 1, characterized in that liquid elemental iron is separated after the reduction and before solidification of the melt.

11. The method according to claim 1, characterized in that the melt has solidified after 3 minutes at the latest.

12. The method according to claim 1, characterized in that the defined cooling is carried out by means of devices for dry or wet granulation.

13. The method according to claim 1, characterized in that the defined cooling is carried out by means of active cooling devices.

14. The method according to claim 1, characterized in that the mechanical separation of the elemental iron takes place by means of a grinding process and a classifying process.

Description

(1) The invention will be explained in greater detail below with the aid of a schematic exemplary embodiment by reference to the figures, in which:

(2) FIG. 1 shows a schematic flowchart of an embodiment of the method according to the invention; and

(3) FIG. 2 shows a bar chart revealing the heat production rate of the hydraulic mineral binder according to the invention.

(4) A feed product is provided in step I in the flowchart according to FIG. 1. This feed product comprises essentially LD slag. The feed product has a MnO content in the range of between 1 wt. % and 5 wt. %. Many LD slags, which are also described as SWS, already have a MnO content in the desired range. If this is not the case, the MnO is added to the slag. Further correcting substances, for example SiO.sub.2-containing substances, can also be added at this time or at another time in order to achieve the subsequently necessary lime saturation factor. Reducing agents can already be added to the feed product in this step. Petroleum coke, for example, is suitable for this purpose.

(5) In the subsequent step II, the processing of the feed product to the melt takes place, if required. The slag can either be obtained already in the melt-liquid state from an upstream process or can also be present in the cold solid form. Melting and/or heating of the slag can take place in an electric arc furnace. It can be operated in resistance operation with a fire-resistant composition of graphite or carbon-containing fire-resistant material. The electric arc furnace can also be described as a melt unit.

(6) The melt should reach a temperature of between approximately 1600 C. and 1750 C. before the addition of reducing agents is started in step III.

(7) By reducing the iron compounds in the melt, carbon monoxide and/or carbon dioxide can be produced which escape from the melt as gases. This can lead to foaming of the melt. In order to reduce the foaming, a flux, for example a small quantity of borax, can be added to the melt. The viscosity of the melt is hereby reduced.

(8) In order to suppress the re-oxidation of the reduced iron, the furnace atmosphere is enriched with an inert gas, for example with argon. The argon can also be directly introduced into the melt. A proportion of the reducing agents can then also be blown with the argon flow directly into the melt. The argon flowing through the melt causes swirling of the melt bath and this has a positive effect on the metal separation.

(9) As soon as essentially all the iron compounds present in the feed product have been reduced, the remaining mineral melt part should have a lime saturation factor of between 90 and 110. This is to be noted with the composition of the feed product. The desired lime saturation factor can be achieved with many LD slags.

(10) The majority of the ironapproximately 80% to 90%settles at the bottom of the melt unit as a separate phase. This phase can be separated still in the liquid state. In step IV, the remaining liquid melt is then removed and subjected to cooling so that it solidifies in less than 15 minutes. This cooling can be realised for example through dry granulation by means of air cooling within less than two minutes.

(11) Since part of the metal phase remains in the solidified granulate, for example in the form of droplets or in inclusions in the mineral part, mechanical processing is necessary to increase the metal yield.

(12) This mechanical separation of elementary iron takes place in stage V through a grinding process by means of a LOESCHE roller mill and subsequent classifying. In this case the iron can be separated due to the difference in density from the mineralogical part. The method described in WO 2011/107124 A1 is particularly suited for this purpose.

(13) The remaining mineral part is the LDS binder according to the invention, which is present in stage VI. It can be utilised as a high-quality hydraulic mineral binder. Since it features a high share of clinker phases, a sinter or combustion process is not necessary any more.

(14) Table 1 lists the chemical composition of a feed product which is an untreated LD slag and the LDS binder obtained by means of the method according to the invention. The values are given here in wt. % in each case. The LDS binder obtained here for example through wet granulation has been cooled by means of water within a few minutes.

(15) TABLE-US-00001 TABLE 1 Chemical analysis of the base slag and the LDS binder in wt. % Base slag (untreated) LDS binder SiO.sub.2 13.9 21.8 Al.sub.2O.sub.3 1.7 4.7 Fe.sub.2O.sub.3 28.8 0.6 CaO 42.7 69.6 MgO 3.3 1.1 TiO.sub.2 0.47 1.05 MnO 5.2 0.23 SO.sub.3 0.2 0.81 P.sub.2O.sub.5 1.07 0.04

(16) According to Table 1 there is a lime saturation factor of 70.1 for the base slag and of 104.6 for the LDS binder. Table 2 reproduces the crystalline composition of the base slag and the LDS binder in wt. %.

(17) TABLE-US-00002 TABLE 2 Essential phase composition of the base slag and the LDS binder according to Rietveld in wt. %. Base slag (untreated) LDS binder Alite, C.sub.3S 5.1 56.3 Belite, C.sub.2S 22.2 19.9 XRD amorphous 38.6 21.0

(18) As can be deduced from Table 2, it is possible with the method according to the invention to obtain a high alite portion of 56.3 wt. % and at least 76.2 wt. % of crystalline phases in the LDS binder.

(19) It is also to be ascertained, however, that only approximately 20 wt. % of glass phases are produced, although similar cooling is used to that in the case of slag sand production, which normally consists of far more than 90 wt. % of glass phases.

(20) FIG. 2 shows a bar chart of the heat production rate in the case of setting during the early hydration of up to 48 hours of a reference cement (CEM I 42.5 R), of a mixture of 70% reference cement with 30% LDS binder and a mixture of 70% reference cement with 30% slag sand. The LDS binder is described in FIG. 2 as granulate.

(21) By reference to the heat production rate, conclusions can be drawn concerning the reactivity. As is clearly visible, the reactivity is clearly reduced through the addition of the slag sand. In contrast, the time of the heat production and thus the main reactivity, if the LDS binder according to the invention is added, is pushed essentially only further back.

(22) It can be concluded from the above that the LDS binder itself exhibits a high hydraulic activity and is therefore extremely well-suited as a composite material for cement or as an independent clinker material.

(23) In summary it can be ascertained that it is possible through the method according to the invention to recover iron from steel slag and to produce a hydraulic mineral binder having a surprisingly good hardening capacity.