Method for the treatment of steelwork 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 agent 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 elementary iron is mechanically separated from the solidified melt. The solidified melt is then supplied for use as hydraulic mineral binder. Furthermore the invention relates to a 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, wherein the MnO may be contained in the steel slag, processing the feed product as melt, incorporating reducing agent 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 incorporation of the reducing agent is carried out in a non-oxidizing atmosphere, slow cooling, wherein the melt solidifies in 15 minutes at the earliest, mechanical separation of elementary iron from the solidified melt, and subsequent supply of the solidified melt with a reduced iron content for use as hydraulic mineral binder.

2. The method according to claim 1, characterized in that the feed product has 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, and 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 1600 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, or 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 borax is incorporated into the melt.

10. The method according claim 1, characterized in that liquid elementary 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 4 hours at the latest.

12. The method according to claim 1, characterized in that the defined cooling takes place in cooling receptacles.

13. The method according to claim 1, characterized in that the mechanical separation of the elementary 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 investigations into the strength 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. Reducing agent can already be added to the feed product in this step. Petroleum coke is particularly 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 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 1650 C. and 1750 C. before the addition of reducing agent is stated 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 foaming, 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 part of the reducing agent 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) In step IV, the liquid melt is conveyed, for example via a pouring apparatus, into special cooling units such as ingot moulds and slowly cooled there in a time period of at least fifteen minutes to approximately two hours. A part of the ironapproximately 80%is deposited both in the melt unit and in the cooling units as a separate phase at the bottom. It can be separated here still in the liquid state. Another portion of the metal phase remains, however, after cooling, in the form of drops and inclusions in the mineral part. In this case, mechanical processing thereof is necessary to increase the metal yield.

(11) 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.

(12) 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.

(13) 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.

(14) TABLE-US-00001 Base slag LDS (untreated) binder SiO.sub.2 13.9 19.6 A1.sub.20.sub.3 1.7 2.7 Fe2O3 28.8 2.7 CaO 42.7 62.3 MgO 3.3 3.4 TiO.sub.2 0.47 0.72 MnO 5.2 3.89 K.sub.2O 0 0.04 Na.sub.2O 0.02 0.29 SO.sub.3 0.1 0.1 S.sup.2 0.1 0.31 P.sub.2O.sub.5 1.07 1.12

(15) Table 1: Chemical analysis of the base slag and the LDS binder in wt. %

(16) According to Table 1 there is a lime saturation factor of 70.1 for the base slag and of 104.3 for the LDS binder. Table 2 reproduces the crystalline composition of the base slag and the LDS binder in wt. %.

(17) TABLE-US-00002 Base slag LDS (untreated) binder Alite, C.sub.3S 5.1 66.1 Belite, C.sub.2S 22.2 9.8 C.sub.12A.sub.7 0.6 C.sub.3A 2.2 5.3 C.sub.4AF 23.2 1.2 XRD amorphous 38.6 11.8

(18) Table 2: Phase composition of the base slag and the LDS binder according to Rietveld in wt. %.

(19) As can be deduced from Table 2, it is possible with the method according to the invention to obtain a high alite portion of up to 66 wt. % in the LDS binder. It is also to be emphasised that in the method according to the invention the formation of other less reactive phases such as for example belite (C.sub.2S) is reduced. The belite phase does indeed also make a contribution to the strength of the LDS binder but to a lower extent and at later times than the alite phase. The higher the alite portion in a hydraulic mineral binder is, the higher is its hardening capacity and the more universal is its suitability as a construction material.

(20) The good reactivity of the LDS binder has been demonstrated by investigating strength in accordance with DIN EN 196 on standard mortar prisms after 2, 7 and 28 days. The results of the strength studies are shown in FIG. 2.

(21) Three different samples were formulated for this purpose and the results thereof were compared with each other. Reference cement CEM 142.5 R was used as the first sample. The second sample had a composition of 70% reference cement and 30% quartz sand, fraction 0-2 mm, wherein the quartz sand was used as non-reactive inert aggregate. The third sample comprised 70% reference cement and 30% LDS binder. The LDS binder was hereby ground to a specific surface of 4000 cm.sup.2/g Blaine.

(22) It follows from the results of this investigation shown in FIG. 2 that the sample 3 with the LDS binder lies above the strength level of the comparative sample 2 with quartz sand. It can be concluded from this that already after 2 days the LDS binder provides an independent contribution to the strength. After 7 days, the sample 3 with LDS binder almost reached the strength level of the reference cement and after 28 days even exceeded it.

(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.