METHOD FOR THE TREATMENT OF IRON-CONTAINING SLUDGE

Abstract

A method for the treatment of sludge containing iron, the method including a leaching step wherein the sludge containing iron is mixed with an acid and an oxidation agent so as to create an oxidized leachate, and a step of precipitation of iron wherein the oxidized leachate is mixed with a neutralizing agent so as to create a mixture composed of a solid part including precipitated iron and of a liquid part, the neutralizing agent including at least 30% in weight of dust recovered from a bag filter treatment of ironmaking, steelmaking, coke making or sintering gas.

Claims

1-6 (canceled)

7. A method for the treatment of sludge containing iron, the method comprising: leaching the sludge containing iron by mixing the sludge containing iron with an acid and an oxidation agent so as to create an oxidized leachate; and precipitating the iron by mixing the oxidized leachate with a neutralizing agent so as to create a mixture composed of a solid part including precipitated iron and a liquid part, the neutralizing agent including at least 30% in weight of dust recovered from a bag filter treatment of ironmaking, steelmaking, coke making or sintering gas.

8. The method as recited in claim 7 wherein the neutralizing agent includes less than 65% in weight of lime.

9. The method as recited in claim 7 further comprising, after the precipitation of the iron, separating the mixture so as to recover separately the solid part including the precipitated iron and the liquid part, the solid part further including carbon.

10. The method as recited in claim 9 wherein the solid part includes at least 8% in weight of iron, at least 15% in weight of carbon, less than 0.4% in weight of zinc and less than 0.1% in weight of lead.

11. The method as recited in claim 7 wherein the solid part includes at least 8% in weight of iron, at least 15% in weight of carbon, less than 0.4% in weight of zinc and less than 0.1% in weight of lead.

12. The method as recited in claim 7 wherein the dust includes less than 0.1% w of zinc, less than 1% w of lead, between 0.5% w and 2.5% w of silica SiO.sub.2, between 2 and 5% of potassium, between 2% w and 5% w of chloride, less than 2% w of sulphur, at least 8% w of iron, at least 10% w of carbon, at least 25% w of calcium oxide and between 1 and 3% w of magnesium oxide, a balance being oxygen and unavoidable impurities.

13. The method as recited in claim 10 wherein the dust includes less than 0.1% w of zinc, less than 1% w of lead, between 0.5% w and 2.5% w of silica SiO.sub.2, between 2 and 5% of potassium, between 2% w and 5% w of chloride, less than 2% w of sulphur, at least 8% w of iron, at least 10% w of carbon, at least 25% w of calcium oxide and between 1 and 3% w of magnesium oxide, a balance being oxygen and unavoidable impurities.

14. The method as recited in claim 11 wherein the dust includes less than 0.1% w of zinc, less than 1% w of lead, between 0.5% w and 2.5% w of silica SiO.sub.2, between 2 and 5% of potassium, between 2% w and 5% w of chloride, less than 2% w of sulphur, at least 8% w of iron, at least 10% w of carbon, at least 25% w of calcium oxide and between 1 and 3% w of magnesium oxide, a balance being oxygen and unavoidable impurities.

15. The method as recited in claim 7 wherein the sludge containing iron is blast furnace sludge.

16. The method as recited in claim 8 wherein the sludge containing iron is blast furnace sludge.

17. The method as recited in claim 9 wherein the sludge containing iron is blast furnace sludge.

18. The method as recited in claim 10 wherein the sludge containing iron is blast furnace sludge.

19. The method as recited in claim 11 wherein the sludge containing iron is blast furnace sludge.

20. The method as recited in claim 12 wherein the sludge containing iron is blast furnace sludge.

21. The method as recited in claim 13 wherein the sludge containing iron is blast furnace sludge.

22. The method as recited in claim 14 wherein the sludge containing iron is blast furnace sludge.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to illustrate the invention, trials have been performed and will be described by way of non-limitative examples, notably in reference to figures which represent:

[0019] FIG. 1 is a schematic view of one embodiment of a method according to the invention

DETAILED DESCRIPTION

[0020] In FIG. 1 is illustrated a device to perform a treatment method according to an embodiment of the invention. Sludge 1, such as blast furnace sludge, is poured into a tank 11 where it is mixed with an acid 2, such as hydrochloric acid HCI and an oxidizing agent 3, such as an aqueous solution of NaCIO.sub.3. Mixing the leaching agents with the sludge produces a leachate 21, which is composed of a solid and a liquid part. The duration of the leaching step 2 is preferably comprised between 30 min and 2 h. A specific leaching step is here described at matter of illustration but the invention encompasses any leaching step using an acid and an oxidizing agent.

[0021] The hydrochloric acid HCI notably reacts with zinc and lead oxides according to following reactions:

ZnO+2HCl.fwdarw.ZnCl.sub.2+H.sub.2O

PbO+2HCI.fwdarw.PbCl.sub.2+H.sub.2O

Zinc and lead chlorides thus produced are water soluble.

[0022] The hydrochloric acid HCI also reacts with iron oxides according to the following reaction:

Fe.sub.2O.sub.3+6H+.fwdarw.2 Fe.sup.3++3H.sub.2O

The Fe.sup.3+ ions thus formed may react with zinc according to the following reactions:

Zn.sup.o+2 Fe.sup.3+.fwdarw.2 Fe.sup.2++Zn.sup.2+

ZnS+2 Fe.sup.3+.fwdarw.Zn.sup.2++2 Fe.sup.2++S.sup.0

[0023] In the meantime, the oxidizing agent 3 reacts with zinc and lead present in the initial sludge 1 so as to turn into water soluble elements which are removed from the sludge 1 and transferred to the liquid part of the leachate 21. This leachate 21 contains notably Fe.sup.2+, Fe.sup.3+, Zn.sup.2+, Pb.sup.2+.

[0024] To recover iron, it is necessary to have this element only under the form of Fe.sup.3+, meaning that Fe.sup.2+ions have to oxidize. This oxidation step can be done by using chlorate ions:

6 Fe.sup.2++6 H.sup.++CIO.sup.3−.fwdarw.6 Fe.sup.3++CI.sup.−+3H.sub.2O

It can occur through a specific oxidation step or together with the leaching step by having an initial concentration of oxidizing agent 3 higher than necessary for the mere reaction with zinc and lead.
If done through a specific oxidation step, it consists in adding an oxidizing agent such as chlorate, for example by using a solution of NaCIO.sub.3, to the leachate 21. The product of this oxidation is an oxidized leachate 21 comprising notably Fe.sup.3+, Zn.sup.2+, Pb.sup.2+.

[0025] To recover iron, the oxidized leachate 21 is sent to a second tank 12 to be subjected to an iron precipitation step. This iron precipitation step is realized by mixing a neutralizing agent 4 with the oxidized leachate 21. This addition results in an increase of pH up to a value comprised between 2 and 3, at which Fe.sup.3+ precipitates as goethite FeOOH. The product of this iron precipitation step is a first mixture 22 composed of a solid part, the leached sludge comprising iron and a liquid part, a residual liquid. Such first mixture 22 is sent to a separation device 13, such as a filter press or a decanter, where solid 23 and liquid 24 phases are separated. The solid phase 23, also called cake, is a concentrate of iron and carbon which can be subjected to further washing and drying steps and recycled to the sintering plant.

[0026] According to the invention, the neutralizing agent 4 used for the precipitation step contains dust recovered from a bag filter treatment of gas. A baghouse (BH, B/H), bag filter (BF) or fabric filter (FF) is an air pollution control device that removes particulates out of air or gas released from industrial processes, such as steelmaking industry. Most filter bags use long, cylindrical bags (or tubes) made of woven or felted fabric as a filter medium. Exhaust gas or air enter the bag through hoppers and is directed into the baghouse compartment. The gas is drawn through the filters, either on the inside or the outside depending on cleaning method, and a layer of dust accumulates on the filter media surface until air can no longer move through it. Those dusts are then recovered by cleaning of the filters. According to the invention, the neutralizing agent 4 contains at least 30% in weight of dusts recovered from the filter bag treatment of gas. These dusts are dusts recovered from the filter bag treatment of ironmaking, steelmaking, coke making or sintering gas.

[0027] The sinter plant is a plant within the steelmaking shop where, basically, raw iron ore is mixed notably with a carbon-containing material and a fluxing agent, the mix thus created being then sintered so as to form iron agglomerates. These agglomerates are then loaded to the blast furnace to create pig iron. This process emits a lot of combustion gases which contain dust coming from the different materials used. Those gases are captured to avoid their release into the atmosphere and treated to remove those dusts by filter bag treatment. The use of these dusts has proven several advantages, notably for the environment. Indeed, the consumption of an external neutralizing agent, such as lime, is reduced without having a detrimental impact on the treatment of the sludge, unwanted compounds such as zinc and lead being still removed within the required limits. Moreover, the use of those dusts allows obtaining a cake with higher carbon and iron content. It so improves the recycling rate of the cake 23 to the sinter plant, where it replaces raw iron material. It so also reduces the consumption of raw materials at the sinter plant. In a preferred embodiment the solid part comprises at least 8% in weight of iron, at least 15% in weight of carbon, less than 0.4% in weight of zinc and less than 0.1% in weight of lead.

[0028] In a preferred embodiment the dusts comprise less than 0.1% w of zinc, less than 1% w of lead, between 0.5% w and 3.5% w of silica SiO.sub.2, between 2 and 11% of potassium, between 2% w and 10% w of chloride, less than 3% w of sulphur, at least 8% w of iron, at least 15% w of carbon, at least 25% w of calcium oxide and between 1 and 3% w of magnesium oxide, balance being oxygen and unavoidable impurities.

[0029] The residual liquid 24 still contains zinc and lead which can be recovered. To do so, the residual liquid 24 is submitted to a zinc and lead precipitation step. This zinc and lead precipitation step may be realized by pouring the residual liquid 24 in a third tank 13 where it is mixed with an alkaline component 5, such as lime. This addition results in an increase of the pH of the liquid, preferably until 9,5, at which zinc and lead hydroxides Zn(OH).sub.2 and Pb(OH).sub.2 precipitate.

[0030] The product of this zinc and lead precipitation step is a second mixture 25 composed of a solid part, a concentrate of zinc and lead 26, and of a liquid part, an effluent 27. Such second mixture 25 is sent to a separation device 14, such as a filter press or a decanter, where solid 26 and liquid 27 phases are separated. The concentrate 26 of zinc and lead may be recycled to a Waelz furnace, and the effluent 25 may be treated with other effluents from the plant.

Results

[0031] Sludge coming from the blast furnace has been subjected to a treatment according to the prior art (Method 1) and according to an embodiment of the invention (method 2). Results are presented in table 1. The initial composition of the sludge was (in % weight), remainder being oxygen:

TABLE-US-00001 Zn Pb SiO.sub.2 K Cl S Fe C CaO MgO 7.34 0.57 4.84 0.55 0.03 3.1 13.4 48.0 4.13 0.64

[0032] In both methods sludge was first subjected to a leaching step in a first tank during which it was mixed with hydrochloric acid HCI and with an aqueous solution of sodium chlorate NaCIO.sub.3. Reactions previously described occurred and a leachate was formed. NaClO.sub.3 was added in sufficient quantity to oxidize this leachate so as to get the iron ores under their required oxidized form Fe3.sup.+. This oxidized leachate was then poured in a second tank to perform the iron precipitation step.

[0033] In method 1, according to the prior art, this iron precipitation step was performed using lime as neutralizing agent. In method 2, according to the invention, the neutralizing agent used was a mixture of lime and dust from the filter bag treatment of sinter gas. The composition of the dust used was as follows (in % weight), remainder being oxygen:

TABLE-US-00002 Zn Pb SiO.sub.2 K Cl S Fe C CaO MgO 0.03 0.66 1.9 4.1 3.8 1.0 9.3 19.9 29.2 1.4

[0034] Then, in both methods, solid and liquid parts resulting from the iron precipitation step were separated in a filter press. The solid part, also named cake, was analyzed to determine its iron and carbon content. The liquid part was then sent to a third tank where it was subjected to a zinc and lead precipitation step by mixing with lime. The products of this zinc and lead precipitation step are Zn/Pb concentrate and wastewaters which are separated by filter press. The Zn/Pb concentrate was analyzed to determine its zinc and lead content.

TABLE-US-00003 TABLE 1 Method 1 Method 2 (prior art) (Invention) Sludge to be treated (kg) 1000 1000 Fabric Filter dust used in iron — 80 precipitation step (kg) Lime consumption (kg) 160 138 HCl consumption (kg) 670 670 C and Fe in cake (kg) 749 775 Zn and Pb in cake (% w) 0.15 0.14

[0035] As can be seen from table 1 the method according to the invention allows reduction in consumption of lime, while keeping the removal rate of zinc and lead in the required limits. Moreover, the method according to the invention allows obtaining a cake with a higher content in carbon and iron, which increase the quantity of external carbon and iron sources it can substitute to at the sinter plant. All these advantages contribute to a reduction of the environmental impact.