Method for selective extraction of calcium and secondary magnetic separation of iron from steel slag

Abstract

The present invention relates to the technical field of steel slag solid waste utilization, and in particular to a method for selective extraction of calcium and secondary magnetic separation of iron from steel slag, comprising the following steps: adding a leaching agent into steel slag, controlling the leaching temperature to be 95-125℃ and the pH of a leaching system to 5.5-8, and continuously leaching calcium for 0.5-2 h; in the process of leaching the calcium or after the leaching of the calcium is finished, adding an oxidizing agent into a reaction system, and oxidizing part of the original non-magnetic ferrous oxide in the steel slag into magnetic ferroferric oxide; and performing magnetic separation on residues obtained after leaching to obtain magnetically separated powder having a total iron content of greater than 40% and tailing powder having an iron oxide content of greater than 40%. According to the method, calcium in the steel slag can be efficiently and selectively extracted, and a steel slag ore phase can be changed while the calcium is extracted, so that most of iron in the residues obtained after leaching can be enriched by magnetic separation again, thereby realizing grading and efficient utilization of calcium and iron elements in the steel slag, and improving the recycling rate of the iron.

Description

A method for selectively extracting calcium from steel slag and secondary magnetic separation of iron Technical Field

[0001] The invention relates to the technical field of steel slag solid waste utilization, in particular to a method for selectively extracting calcium from steel slag and performing secondary magnetic separation of iron. Background Art

[0002] Steel slag is the molten slag discharged during the steelmaking process. It contains oxides formed by the oxidation of various elements in the metal charge, impurities in the metal charge, and calcined products of tonic materials such as limestone. To improve iron utilization, the molten slag is typically crushed and screened after undergoing various treatments, such as hot stuffing, water quenching, and air quenching. Then, it undergoes magnetic separation. Approximately 20% of the slag iron (particle size greater than 30 mm and total iron content greater than 80%) is returned to the steelmaking process, while the magnetic separation powder (particle size no greater than 8 mm and total iron content greater than 40%) is returned to the sintering process. The remaining slag is primarily composed of calcium, magnesium, and aluminum oxides and silicates, but still contains less than 2% free iron and approximately 20% iron oxide, which cannot be utilized. Besides being used in cement and road paving, most of this remaining slag is still stored and disposed of, which not only consumes land resources but also poses significant safety risks to the ecological environment and wastes iron resources.

[0003] The chemical composition of steel slag after magnetic separation mainly includes CaO (34%-48%), Fe2O3 (7%-12%), SiO2 (9%-15%), MgO (2.5%-10%), Al2O3 (0.9%-2.8%), as well as small amounts of oxides such as MnO and TiO2. These oxides mainly exist in the form of tricalcium silicate (C3S), dicalcium silicate (C2S), calcium ferrite (Ca2Fe2O5), RO phase (a solid solution of metal oxides such as MgO and FeO), and small amounts of calcium aluminoferrite (C4AF) and free calcium oxide (f-CaO). The calcium in the steel slag can be used as a calcium source for fixing carbon dioxide, and by extracting the calcium, it can further enrich elements such as iron.

[0004] At present, the extraction agent used for calcium in steel slag mainly contains acidic solvents such as acetic acid, hydrochloric acid, and ammonium salts. Calcium in steel slag is extracted with hydrochloric acid, which is fast in efficiency and has a high leaching rate. However, when calcium is leached, impurity elements such as iron, magnesium, aluminum, and silicon in the steel slag are also leached together in ionic form. If the purity of calcium in the solution is to be improved, it is necessary to adopt methods such as alkali precipitation to remove impurities. This process inevitably causes the loss of calcium and consumes a large amount of acid and alkali. Calcium in steel slag is extracted with ammonium salts such as ammonium nitrate, ammonium acetate, and ammonium chloride. Due to its own weak acidity, the extraction rate of calcium in the steel slag is low, and only free calcium oxide can be extracted usually. Calcium in the mineral phases such as tricalcium silicate (CS) and dicalcium silicate is further extracted. Generally, a higher ammonium chloride concentration is required, and the leaching process is strengthened by microwaves, ultrasound, and other means. During the strengthening process, impurity elements such as iron, magnesium, aluminum, and silicon are also leached together in ionic form.

[0005] After calcium extraction, the iron in steel slag exists primarily as non-magnetic ferrous oxide in the mineral phase, making it difficult to separate using magnetic separation. Some literature reports on extracting iron from the remaining residue through high-temperature reduction. For example, Chinese invention patent application number CN202010726836.5 describes reducing the residue from ammonium chloride leaching at 1300-1500°C to extract iron, and then converting it into diopside-phase microcrystalline glass in a molten state. This process is energy-intensive and requires high-quality equipment.

[0006] In view of this, the present invention proposes a method for selectively extracting calcium from steel slag while strengthening magnetic separation of iron. This method can efficiently and selectively extract calcium from steel slag. While extracting calcium, it can also change the ore phase of the steel slag, so that most of the iron in the residue after leaching can be re-enriched through magnetic separation, thereby obtaining magnetic separation powder and tailings with practical uses, realizing the graded and efficient utilization of calcium and iron elements in steel slag, and improving the recovery rate of iron.

[0007] Summary of the Invention

[0008] The object of the present invention is to provide a method for selectively extracting calcium from steel slag and performing secondary magnetic separation of iron, which realizes the graded and efficient utilization of calcium and iron elements in the steel slag and improves the recovery rate of iron.

[0009] The present invention provides a method for selectively extracting calcium from steel slag and performing secondary magnetic separation of iron, comprising the following steps:

[0010] S1. Add leaching agent to steel slag, control the leaching temperature to 95-125°C, the pH of the leaching system to 5.5-8, and continue leaching calcium for 0.5-2 hours;

[0011] S2. During or after the calcium leaching process, an oxidant is added to the reaction system to oxidize a portion of the non-magnetic ferrous oxide (FeO) in the steel slag into magnetic ferrosoferric oxide;

[0012] S3. After the oxidation treatment, the reaction system is subjected to solid-liquid separation to obtain a leached residue and a calcium-containing solution, and the leached residue is subjected to magnetic separation to obtain a magnetic separation powder with a total iron content greater than 40% and a tail powder with an iron oxide content greater than 40%;

[0013] Wherein, in step S1, the leaching agent is an ammonium chloride-ammonia buffer solution, and during leaching, the extraction rate of calcium in the silicate mineral phase of the steel slag is controlled to be greater than 30%;

[0014] In step S2, the oxidant includes any one or more of hydrogen peroxide, calcium peroxide, air and oxygen.

[0015] The present invention first adds an ammonium chloride-ammonia buffer solution to the steel slag to leach calcium in the steel slag. The main chemical reactions during the leaching process are the calcium extraction reactions between ammonium chloride and free calcium oxide and calcium silicate in the steel slag:

[0016] CaO+2NH4Cl=CaCl2+2NH3+H2O

[0017] CaX n O m + 2NH4Cl=CaCl2+X n O m-1 +2NH3+H2O

[0018] (Here X is silicon, iron, aluminum iron, etc.)

[0019] At the same time, the leaching agent can also dissolve part of the iron and aluminum oxide in the steel slag:

[0020] Al2O3+6NH4Cl=2AlCl3+3H2O+6NH3

[0021] FeO+2NH4Cl=FeCl2+H2O+2NH3

[0022] The present invention achieves selective calcium extraction based on the different solubilities of different elements at different pH values. Aluminum and iron require a pH of less than 4 to dissolve in large quantities from steel slag. However, ferrous iron dissolves at a pH close to that of calcium. Therefore, within the pH range of the present invention, ferrous iron dissolves significantly, increasing the iron content in the extraction solution. This can severely impact the quality of the calcium carbonate when the extract is subsequently used for mineralization to produce calcium carbonate.

[0023] To this end, the present invention chooses to add an oxidant to the system during or after leaching to oxidize part of the original non-magnetic divalent iron in the steel slag into trivalent iron or ferroferric oxide. The main reaction is (taking oxygen as an example):

[0024] 6FeO+O2=2Fe3O4

[0025] 4FeO+O2=2Fe2O3

[0026] 4FeCl2+O2+4NH4Cl=4 FeCl3+4NH 3+ 2H2O

[0027] At this time, part of the FeO in the mineral phase is oxidized to Fe3O4 or Fe2O3, which will not dissolve significantly at the pH required by the present invention. However, the divalent iron ions in the leachate are oxidized to trivalent iron, which will further react with ammonia to form iron hydroxide precipitate and enter the residue:

[0028] FeCl3+3NH3+2H2O=Fe(OH)3+3NH4Cl

[0029] This removes iron ions from the solution and enables selective extraction of calcium.

[0030] During the oxidation process, the oxidation depth can be further controlled by controlling the amount and concentration of the oxidant so that the oxidation product is mainly concentrated in Fe3O4, thereby making the iron in the steel slag magnetic again.

[0031] Therefore, the method of selectively extracting calcium from steel slag and performing secondary magnetic separation of iron of the present invention can achieve efficient and highly selective extraction of calcium from steel slag through the synergistic effect of an oxidant and a leaching agent, as well as control of the leaching temperature and pH value. The calcium extraction rate can reach over 80%, and in particular, the calcium extraction rate from the silicate ore phase can reach over 30%. At the same time, the extraction of calcium from the silicate ore phase also changes the ore phase of the raw steel slag. The change in ore phase can, on the one hand, oxidize some of the original non-magnetic ferrous oxide (FeO) in the steel slag into magnetic ferrosoferric oxide through the oxidant, and on the other hand, can also release some of the iron present in the silicate ore phase. The combined effect of the two enhances the magnetism of the residue after steel slag leaching, creates conditions for secondary magnetic separation of iron, and significantly improves the secondary magnetic separation rate.

[0032] Furthermore, research has shown that only when the calcium extraction rate from the silicate phase of steel slag exceeds 30% (mass fraction) can the mineral phase be altered, and thus the iron secondary magnetic separation rate be significantly improved. Of course, this extraction rate can be achieved by using the technical solution provided by the present invention and appropriately optimizing it for different steel slags.

[0033] Finally, by reasonably controlling the magnetic field intensity, the magnetic separation powder with a total iron content (TFe) greater than 40% in the residue after leaching can be returned to the sintering section, and the remaining tail powder with a content of iron oxide greater than 40% can be used as an iron corrector for cement raw meal. No new solid waste is generated in the entire process, achieving 100% utilization of steel slag, which has high economic value.

[0034] As a preferred embodiment of the present technical solution, in step S1, the method for controlling the pH of the leaching system includes: adding ammonium chloride to the leaching system or promptly distilling off the ammonia generated during the leaching process.

[0035] As a preferred embodiment of the present technical solution, in step S1, the amount of leaching agent used per kg of steel slag is 18-76 L, and preferably 30-57 L, wherein the molar ratio of ammonium chloride in the leaching agent to calcium (calculated as calcium oxide) in the steel slag is (1.2-2.4):1, and the mass fraction of ammonium chloride in the leaching agent is 6-25%, and the amount of ammonia in the leaching agent is determined according to the control range of the pH of the leaching system.

[0036] As a preferred embodiment of this technical solution, in step S2, the oxidant of the present invention can be added during the leaching process or after the leaching is completed. Specifically, the oxidant can be added after the calcium has been leached for 0.5-1 hour. During the leaching process, the extraction of calcium from the silicate ore phase by the leaching agent has already changed the ore phase, releasing some of the iron present in the silicate ore phase. Adding the oxidant at this time can oxidize the divalent iron in the leachate to ferric iron and remove it, and oxidize the iron oxides released from the silicate ore phase to ferroferric oxide, thereby restoring the magnetic properties of the iron in the slag ore phase and enabling secondary magnetic separation.

[0037] As a preferred embodiment of this technical solution, in step S2, during the oxidation process, the oxidation depth can be further controlled by controlling the amount and concentration of the oxidant so that the oxidation product is mainly Fe3O4. Specifically, the amount of the oxidant, measured based on the amount of oxygen contained in the oxidant, is 1 / 6 to 1 / 3 of the amount of iron contained in the steel slag.

[0038] As a preferred embodiment of the present technical solution, in step S3, during the secondary magnetic separation, the measure taken is to control the magnetic field intensity. The greater the magnetic field intensity, the higher the iron content of the magnetic separation powder. The present invention obtains magnetic separation powder with a total iron content greater than 40% and tail powder with an iron oxide content greater than 40% by controlling the magnetic field intensity and taking the maximum value of the magnetic field intensity when the iron oxide content in the remaining tail powder is greater than 40% as the critical value.

[0039] As a preferred embodiment of the present technical solution, the particle size of the steel slag is 100-300 mesh, and preferably 200-300 mesh.

[0040] As a preferred embodiment of the present technical solution, the steel slag is steel slag after magnetic separation, including any one of converter steel slag, electric arc furnace steel slag and waste slag treated outside the furnace, wherein the treatment method of the waste slag treated outside the furnace includes any one of hot stuffing method, hot pouring method, water quenching method, drum method and air quenching method.

[0041] As a preferred embodiment of the present invention, the calcium leaching device used during leaching is a liquid-solid two-phase reactor or a gas-liquid-solid three-phase reactor. Specifically, the calcium leaching device can be a kettle reactor or a bubble column reactor. To achieve the effects of the present invention, the calcium leaching device needs to have the following functions and structural units: a stirring or liquid forced circulation structural unit to ensure uniform mixing of liquid and solid; a heating unit to ensure that the leaching temperature is within the required range; and the reactor body of the calcium leaching device is provided with inlets for steel slag, leaching agent, and oxidant, as well as an outlet for leaching liquid and a gas outlet.

[0042] In addition, the calcium leaching device must also be equipped with a condensation reflux unit, and the gas outlet is connected to the condensation reflux unit to condense part of the ammonia in the reflux steam, thereby controlling the pH of the leaching system.

[0043] The method of selectively extracting calcium from steel slag and performing secondary magnetic separation of iron of the present invention has at least the following technical effects:

[0044] 1. The method of selectively extracting calcium from steel slag and performing secondary magnetic separation of iron can achieve efficient and highly selective extraction of calcium from steel slag through the synergistic effect of an oxidant and a leaching agent, as well as control of the leaching temperature and pH. The calcium extraction rate can reach over 80%, and in particular, the calcium extraction rate from silicate ore phase can reach over 30%. The extracted calcium and generated ammonia can be used as raw materials for CO2 mineralization and for the preparation of calcium carbonate.

[0045] 2. The present invention changes the mineral phase of the raw steel slag during the calcium extraction process. On the one hand, the oxidant oxidizes part of the original non-magnetic ferrous oxide (FeO) in the steel slag into magnetic ferroferric oxide. On the other hand, it releases part of the iron present in the silicate mineral phase. The combined effect of the two enhances the magnetism of the residue after steel slag leaching, creates conditions for secondary magnetic separation of iron, and significantly improves the secondary magnetic separation rate.

[0046] 3. After leaching, the present invention can reasonably control the magnetic field intensity, so that the magnetic separation powder with a total iron content (TFe) greater than 40% in the leached residue can be returned to the sintering section, and the remaining tail powder with a content of iron oxide greater than 40% can be used as an iron corrector for cement raw meal. No new solid waste is generated in the whole process, and 100% utilization of steel slag is achieved, which has high economic value. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0048] FIG1 is a process flow chart of selectively extracting calcium from steel slag and performing secondary magnetic separation of iron according to the present invention;

[0049] FIG2 is a process flow chart of another method for selectively extracting calcium from steel slag and performing secondary magnetic separation of iron according to the present invention;

[0050] FIG3 is a hysteresis curve of steel slag, leached residue and magnetic separation powder;

[0051] Figure 4 shows the XRD patterns of steel slag, leached residue and magnetic separation powder. DETAILED DESCRIPTION

[0052] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present application belongs.

[0053] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular also includes the plural. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0054] The following will clearly and completely describe the technical solutions of the present invention in conjunction with the embodiments. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0055] Example 1

[0056] Hot-steamed steel slag was selected as the raw material. Its main composition is shown in Table 1. The free calcium oxide content was measured to be 9.2%. The treatment process is shown in Figure 1.

[0057] Table 1 Main components of hot stuffy steel slag

[0058] S1, the hot stuffy steel slag is crushed and sieved to 200 mesh, 33.2 kg is taken and added to a stirred tank through the steel slag inlet, the stirred tank is composed of a heating unit, a steel slag inlet, a leaching agent inlet, an oxidant inlet, a leaching slurry outlet, a gas outlet and a condensation reflux unit connected to the gas outlet, etc., 1522 L of a buffer solution containing ammonium chloride and ammonia with a mass concentration of 10% ammonium chloride is added through the leaching agent inlet, ammonium chloride is supplemented through the leaching agent inlet or ammonia generated in the reflux steam of the condensation reflux unit is controlled to have a pH of 5.5, and the temperature of the leaching system is controlled at 105 ° C by the heating unit;

[0059] S2. Continuously add oxidant hydrogen peroxide through the oxidant inlet, the amount of hydrogen peroxide added (in terms of H2O2) is 0.32 kg, and after leaching for 60 minutes, the slurry is discharged from the leaching slurry outlet;

[0060] S3. After the leaching slurry was filtered, the leaching residue and the calcium-containing solution were obtained. The leaching residue was dried and weighed to 21.14 kg. It was sent to the magnetic separation system for secondary magnetic separation. The magnetic field strength was controlled to 65 kA / m to obtain 3.2 kg of magnetic separation powder with a total iron content (TFe) of 40.2%. The mass of the remaining tail powder was measured to be 17.94 kg, of which the content of iron oxide was 40.1%.

[0061] Tests showed that the calcium ion concentration (calculated as calcium chloride) in the calcium-containing solution was 15.87 g / L, and the iron content in the extract was less than 0.00005 g / L. The calculated calcium extraction rate from the slag was 90.2%, of which the calcium extraction rate from the silicate phase was 87.34%, and the calcium selective extraction rate (mass fraction of calcium ions in the calcium-containing solution) was 98.74%. The magnetic separation rate of the residue after leaching (magnetic separation rate = mass of magnetic separation powder / total mass of the residue after leaching * 100%) was 15.14%.

[0062] The hysteresis curves of the above-mentioned hot-stifled steel slag raw material, the residue after leaching, and the magnetic separation powder were measured. As shown in Figure 3, the saturation magnetic induction intensity of the hot-stifled steel slag is 1.29emu / g, the saturation magnetic induction intensity of the residue after leaching is 1.80emu / g, and the saturation magnetic induction intensity of the magnetic separation powder is 4.62emu / g. This shows that after the leaching process, the magnetism of the residue after leaching is 40% higher than that of the original steel slag.

[0063] The above-mentioned hot-stifled steel slag raw material, leaching residue, and magnetic separation powder were taken for X-ray diffraction (XRD) crystal phase composition analysis. The XRD curves in Figure 4 represent the raw steel slag, leaching residue, and magnetic separation powder from bottom to top. As shown in Figure 4, the hot-stifled steel slag contains multiple crystal phases such as calcium ferrite (C2F), dicalcium silicate, and monocalcium silicate. After the leaching process, the crystal phase of calcium ferrite in the leaching residue is basically completely destroyed and exists in the form of free iron oxide, which enhances the paramagnetism of the leaching residue. No waste slag or new solid waste is generated during the entire process.

[0064] Example 2

[0065] Desulfurized steel slag was selected as the reaction raw material. Its main composition is shown in Table 2. The free calcium oxide content was measured to be 12%. The treatment process is shown in Figure 1.

[0066] Table 2 Main components of desulfurized steel slag

[0067] S1. The above-mentioned desulfurized steel slag was crushed and sieved to 200 mesh, and 33.2 kg was added to the bubble column reactor through the steel slag inlet. The bubble column reactor was composed of a temperature control coil, a steel slag inlet, a leaching agent inlet, an oxidant inlet, a leaching slurry outlet, a gas outlet, and a condensation reflux unit connected to the gas outlet. 1015 L of a buffer solution containing ammonium chloride and ammonia with a mass concentration of 15% was added through the leaching agent inlet. Ammonium chloride was supplemented through the leaching agent inlet or ammonia partially generated in the reflux steam of the condensation reflux unit to control the pH of the leachate to 6.5. The temperature of the leaching system was controlled at 95 ° C by the temperature control coil;

[0068] S2, oxidant air is continuously introduced from the oxidant inlet, with an air flow rate of 0.8m 3 / h, the amount of air added (in terms of O2) is 0.11kg, and after leaching for 30min, the slurry is discharged from the leaching slurry outlet;

[0069] S3. After the leaching slurry is filtered, the leaching residue and the calcium-containing solution are obtained. The leaching residue is dried and weighed to 21.24 kg. It is sent to the magnetic separation system for secondary magnetic separation. The magnetic field strength is controlled to 80 kA / m to obtain 5.6 kg of magnetic separation powder with a total iron content (TFe) of 40.3%. The mass of the remaining tail powder is measured to be 15.64 kg, of which the content of iron oxide is 40.1%. No waste residue or new solid waste is generated during the whole process.

[0070] Tests showed that the concentration of calcium ions (calculated as calcium chloride) in the calcium-containing solution was 23.63 g / L, and the iron content in the extract was less than 0.00004 g / L. The calculated calcium extraction rate from the slag was 83.2%, of which the calcium extraction rate from the silicate phase was 76.86%. The calcium selective extraction rate (mass fraction of calcium ions in the calcium-containing solution) was 99.32%. The magnetic separation rate of the residue after leaching (magnetic separation rate = mass of magnetic separation powder / total mass of the residue after leaching * 100%) was 26.37%.

[0071] Example 3

[0072] The hot-suffocated steel slag in Example 1 was used as the reaction raw material, and the material addition amount and operation were basically the same, except that the oxidant was added after the leaching was completed, and the addition amount was consistent with Example 1. The treatment process flow is shown in Figure 2.

[0073] Specifically:

[0074] After the leaching is completed, the leaching slurry is discharged, and 0.32 kg of hydrogen peroxide (calculated as H2O2) is added to the leaching slurry, stirred evenly, and allowed to stand for 10 minutes, and then filtered to obtain the leaching residue and calcium-containing solution;

[0075] The residue after leaching was dried and weighed to 21.24 kg. It was sent to the magnetic separation system for secondary magnetic separation. The magnetic field strength was controlled at 65 kA / m to obtain 2.9 kg of magnetic separation powder with a total iron content (TFe) of 40.2%. The mass of the remaining tail powder was measured to be 18.34 kg, of which the iron oxide content was 40.1%. No waste residue or new solid waste was generated during the entire process.

[0076] Tests showed that the calcium ion concentration (calculated as calcium chloride) in the calcium-containing solution was 15.75 g / L, and the iron content was less than 0.00004 g / L. The calculated calcium extraction rate of the steel slag was 89.5%, of which the calcium extraction rate in the silicate phase was 86.43%, and the calcium selective extraction rate (mass fraction of calcium ions in the calcium-containing solution) was 97.31%. The magnetic separation rate of the residue after leaching (magnetic separation rate = mass of magnetic separation powder / total mass of the residue after leaching * 100%) was 13.66%.

[0077] Comparative Example 1

[0078] The steel slag used in Example 1 was used as raw material, and the operating steps were basically the same as those in Example 1, except that no oxidant was added in this control example.

[0079] The residue after leaching was dried and weighed to 21.06 kg. It was sent to the magnetic separation system for secondary magnetic separation. The magnetic field strength was controlled at 65 kA / m to obtain 0.01 kg of magnetic separation powder with a total iron content (TFe) of 40.2%. The mass of the remaining tail powder was measured to be 21.05 kg, of which the iron oxide content was 40.1%.

[0080] Tests showed that the calcium ion concentration (calculated as calcium chloride) in the leaching solution was 15.18 g / L, and the iron content was 0.00014 g / L. The calculated calcium extraction rate from the slag was 86.3%, of which the calcium extraction rate from the silicate phase was 82.30%, and the calcium selective extraction rate (the mass fraction of calcium ions in the calcium-containing solution) was 81.12%. The magnetic separation rate of the residue after leaching (magnetic separation rate = mass of magnetic separation powder / total mass of the residue after leaching * 100%) was 0.047%.

[0081] Comparative Example 2

[0082] The steel slag used in Example 1 was used as raw material, and the operating steps were basically the same as those in Example 1, except that the pH of the leaching system was controlled to be less than 4 in this comparative example.

[0083] The residue after leaching was dried and weighed 18.61 kg. It was sent to the magnetic separation system for secondary magnetic separation. The magnetic field strength was controlled at 80 kA / m to obtain 1.25 kg of magnetic separation powder with a total iron content (TFe) of 40.2%. The mass of the remaining tail powder was measured to be 17.36 kg, of which the iron oxide content was 40.1%.

[0084] After testing, the calcium ion concentration (calculated as calcium chloride) in the leaching solution was 16.59 g / L, the iron content was 0.00031 g / L, and the aluminum content was 0.00011 g / L. The calculated calcium extraction rate in the steel slag was 94.3%, of which the calcium extraction rate in the silicate phase was 92.64%. The calcium selective extraction rate (mass fraction of calcium ions in the calcium-containing solution) was 70.39%. The magnetic separation rate of the residue after leaching (magnetic separation rate = mass of magnetic separation powder / total mass of the residue after leaching * 100%) was 6.72%.

[0085] Comparative Example 3

[0086] The steel slag used in Example 1 was used as raw material, and the operating steps were basically the same as those in Example 1, except that ammonium chloride was used as the leaching agent to leach calcium in the steel slag, the concentration of ammonium chloride in the leaching system was 10%, and the pH of the leaching system was 4.6.

[0087] The residue after leaching was dried and weighed 19.71 kg. It was sent to the magnetic separation system for secondary magnetic separation. The magnetic field strength was controlled at 80 kA / m to obtain 1.46 kg of magnetic separation powder with a total iron content (TFe) of 40.2. The mass of the remaining tail powder was measured to be 18.25 kg, of which the iron oxide content was 40.1%.

[0088] Tests showed that the calcium ion concentration (calculated as calcium chloride) in the leaching solution was 16.15 g / L, the iron content was 0.0002 g / L, and the aluminum content was 0.00006 g / L. The calculated calcium extraction rate in the steel slag was 91.8%, of which the calcium extraction rate in the silicate phase was 89.41%, and the calcium selective extraction rate (mass fraction of calcium ions in the calcium-containing solution) was 75.47%. The magnetic separation rate of the residue after leaching (magnetic separation rate = mass of magnetic separation powder / total mass of the residue after leaching * 100%) was 7.41%.

[0089] Comparative Example 4

[0090] The steel slag used in Example 1 was used as the raw material, and the operating steps were basically the same as in Example 1. The difference was that different silicate phase calcium extraction rates were obtained by changing the pH value of the leaching system and the amount of ammonium chloride added. The effects of different silicate phase calcium extraction rates on the magnetic separation rate of the leached residue are shown in Table 3.

[0091] Table 3 Effect of different silicate phase calcium extraction rates on the magnetic separation rate of the residue after leaching

[0092] Table 4 Treatment effects of steel slag in Examples 1-3 and Comparative Examples 1-3

[0093] As can be seen from Table 4, by the method of selectively extracting calcium from steel slag and performing secondary magnetic separation of iron according to the present invention, a leaching agent and an oxidant are added to the steel slag. By controlling the process conditions and process parameters, not only is efficient and selective extraction of calcium from the steel slag achieved, but also, due to the synergistic effect of the oxidant and the change in the mineral phase of the raw steel slag after the calcium in the steel slag is extracted, the content of ferrosoferric oxide in the steel slag is increased, thereby enhancing the magnetism of the residue after leaching, creating conditions for secondary magnetic separation of iron from the steel slag. As a result, magnetic separation powder with a particle size of no more than 8 mm and a total iron content of more than 40% can be obtained and returned to the sintering section of the steel plant, and the tail powder with an iron oxide content of more than 40% can be used as a cement iron corrector, etc., and no new solid waste is generated in the process, thereby achieving 100% utilization of the steel slag and having high economic value.

[0094] In Comparative Example 1, although the mineral phase of the raw steel slag was changed due to the extraction of calcium from the steel slag, no oxidant was added to oxidize the divalent iron in the mineral phase into magnetic ferroferric oxide. Therefore, its magnetic separation rate was only 0.047%.

[0095] In Comparative Example 2, the pH of the leaching system was less than 4, causing a large amount of Al and Fe to be dissolved from the steel slag, thereby increasing the iron and aluminum contents in the leaching solution and reducing the iron content in the residue after leaching, resulting in a magnetic separation rate of only 6.72%.

[0096] In Comparative Example 3, since ammonium chloride was used alone as the leaching agent, the initial pH value of the ammonium chloride solution was 4.6 at 105° C., and Al and Fe in the steel slag were partially dissolved, resulting in an increase in the iron and aluminum contents in the leachate, a decrease in the iron content in the residue after leaching, and ultimately a magnetic separation rate of only 7.41%.

[0097] However, control example 4 shows that only when the extraction rate of calcium in the silicate mineral phase of steel slag is greater than 30% (mass fraction) can the mineral phase of steel slag be changed, and then the secondary magnetic separation rate of iron can be significantly improved.

[0098] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for selectively extracting calcium from steel slag and secondary magnetic separation of iron, characterized in that: The following steps are involved: S1. Add leaching agent to steel slag, control the leaching temperature to 95-125°C, the pH of the leaching system to 5.5-8, and continue leaching calcium for 0.5-2h; S2. During or after the calcium leaching process, an oxidant is added to the reaction system to oxidize the non-magnetic ferrous oxide in the steel slag into magnetic ferrosoferric oxide; S3, after the oxidation treatment, the reaction system is subjected to solid-liquid separation to obtain a leached residue and a calcium-containing solution, and the leached residue is subjected to magnetic separation to obtain a magnetic separation powder with a total iron content greater than 40% and a tail powder with an iron oxide content greater than 40%; Wherein, in step S1, the leaching agent is an ammonium chloride-ammonia buffer solution, and during leaching, the extraction rate of calcium in the silicate mineral phase of the steel slag is controlled to be greater than 30%; In step S2, the oxidant includes any one or more of hydrogen peroxide, calcium peroxide, air and oxygen; In step S2, after the calcium is continuously leached for 0.5-1 h, an oxidant is added.

2. The method for selectively extracting calcium from steel slag and performing secondary magnetic separation of iron according to claim 1, characterized in that: In step S1, the method for controlling the pH of the leaching system includes: adding ammonium chloride to the leaching system or evaporating the ammonia generated during the leaching process.

3. The method for selectively extracting calcium from steel slag and performing secondary magnetic separation of iron according to claim 1, characterized in that: In step S1, the amount of leaching agent used per kg of steel slag is 18-76L, wherein the molar ratio of ammonium chloride in the leaching agent to calcium in the steel slag is (1.2-2.4):1, and the mass fraction of ammonium chloride in the leaching agent is 6-25%, and the amount of ammonia in the leaching agent is determined according to the pH range of the leaching system.

4. The method for selectively extracting calcium from steel slag and performing secondary magnetic separation of iron according to claim 1, characterized in that: In step S2, the amount of the oxidant is measured according to the amount of oxygen element contained in the oxidant, and the amount of the oxidant used is 1 / 6-1 / 3 of the amount of iron element contained in the steel slag.

5. The method for selectively extracting calcium from steel slag and performing secondary magnetic separation of iron according to claim 1, characterized in that: In step S3, during the magnetic separation, the maximum value of the magnetic field intensity when the iron oxide content in the remaining tail powder is greater than 40% is taken as the critical value to obtain magnetic separation powder with a total iron content greater than 40% and tail powder with an iron oxide content greater than 40%.

6. The method for selectively extracting calcium from steel slag and performing secondary magnetic separation of iron according to claim 1, characterized in that: The particle size of the steel slag is 100-300 meshes.

7. The method for selectively extracting calcium from steel slag and performing secondary magnetic separation of iron according to claim 1, characterized in that: The steel slag is steel slag after magnetic separation, including any one of converter steel slag, electric arc furnace steel slag and waste slag treated outside the furnace, wherein the treatment method of the waste slag treated outside the furnace includes any one of hot stuffing method, hot pouring method, water quenching method, drum method and air quenching method.

8. The method for selectively extracting calcium from steel slag and performing secondary magnetic separation of iron according to claim 1, characterized in that: The calcium leaching device used during leaching is a liquid-solid two-phase reactor or a gas-liquid-solid three-phase reactor.

9. According to the method for selectively extracting calcium from steel slag and secondary magnetic separation of iron as described in claim 8, the calcium leaching device comprises a reactor body, a liquid circulation unit, a heating unit and a condensation reflux unit, and the reactor body is provided with a steel slag inlet, a leaching agent inlet, an oxidant inlet, a leaching slurry outlet and a gas outlet, and the gas outlet is connected to the condensation reflux unit.

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