Study on a New Process and its Kinetics of Iron Recovery and Glass-ceramics Preparation from Desulfurization Slag

The melting point and phase of slag was calculated with Factsage thermodynamics software, and base on this, taking desulfurization slag as the main raw material, which is leached by ammonium chloride as pretreatment. The composition of target slag system was adjusted with high aluminum powder coal ash and glass cullet, and then the reducing slag and metallic iron were separated by high temperature carbon thermal reaction. The prepared glass-ceramics with main crystal phases of diopside and nepheline were obtained by heat treatment, which shows that the new process is feasible. The crystallization activation energy was calculated by using Kissinger, Ozawa and Augis-Bennett method base on the differential heat curve. The results show that the crystallization activation energy is relatively high, and the crystal growth index n are all less than 3, which means that the crystallization capacity of the glass-ceramics is low. At the heating rate of 5 K/min, the n value of sample No. 3 is the largest, which is 2.7, and the mode of volumetric crystallization changes from two-dimensional to one-dimensional with the increase of heating rate, therefore, nucleating agent is needed in the preparation of glass-ceramics.


Introduction
The accumulation of industrial waste has become a serious problem in many countries, because it not only occupies a large amount of land, but also causes unforeseen environmental pollution, and waste reuse is an attractive disposal method for that it can increase resource conservation and reducing or even eliminating disposal costs and potential pollution problems [1] . Steelmaking desulfurization slag refers to the waste slag obtained from the pre-desulfurization treatment of molten iron before it enters the converter [2,3] . The main working principle is to add the desulfurizer composed of lime, limestone and uorite into the molten iron with high sulfur content, and make the desulfurizer and molten iron fully react through mechanical stirring to generate desulfurization products so as to achieve the purpose of desulfurization. Finally, the desulfurization products are extracted to obtain molten iron with low sulfur content and desulfurization slag. At present, the recycling of desulfurization slag is lack of due attention, the main treatment method is only simple use of electromagnetic crane for bulk magnetic separation as steel scrap recycling, the rest of which return to sintering, or takeaway for cement raw materials, road lling materials, etc. However, the bulk magnetic separation materials are often bonded together as slag and iron, which are directly adding into the converter will inevitably cause resulfurization, leading bad impact to the subsequent metallurgy process and production costs. And the small iron and slag particles after magnetic separation also wrapped in each other, which cannot be separated effectively. As the grade of iron is low and uctuates greatly, it not only reduces the sintering grade, but also has an adverse effect on the sintering strength. It is also not economical to take away the material directly [3,4] .
Compared with traditional ceramics and glass, glass-ceramics have many outstanding characteristics, including high mechanical strength, thermal shock resistance, wear resistance, chemical corrosion resistance and so on [5,6] . In recent years, many scholars have prepared glass-ceramics from different types of solid wastes, such as iron and steel smelting waste residue [6][7][8] , y ash [9,10] and waste glass [11] .
The composition of these solid wastes is very complex, containing many different kinds of oxides, and various oxides will affect the performance of glass-ceramics. Among them, SiO 2 Al 2 O 3 CaO and MgO are the most common and have the greatest in uence on material properties. Acidic oxide SiO 2 is essential as the forming body of the silicate network, while CaO, MgO and other oxides contribute to the phase separation and crystallization of the basic glass. Al 2 O 3 is an amphoteric oxide that can form a tetrahedral glass frame [6,7] . In addition, metallurgical solid waste may also contain TiO 2 , CaF 2 and Cr 2 O 3 , which are conducive to nucleation, and it is conducive to reducing the addition of external nucleating agents. However, as the high content of iron in desulfurization slag, it is often di cult to prepare glassceramics directly, or a lot of texturizine agent should be added. Therefore, the recovery of valuable metal iron through carbothermal reduction reaction can not only be directly recycled as raw materials for iron and steel smelting, but also eliminate its in uence on glass-ceramics. Besides containing iron, desulfurization slag also contains a large amount of SiO 2 , Al 2 O 3 , CaO and MgO, which can be used as raw material for CMAS glass-ceramics, however, desulfurization slag containing large amounts of calcium and its basicity is exorbitant in order to meet the requirements of molten iron desulfurization, and it need to add a lot of silica for conditioning to the preparation of glass-ceramics, therefore, it is necessary to pretreat desulfurization slag. Most of the calcium in the desulfurization slag exists in the form of calcium oxide and silicate, which can be leaching by ammonium chloride solution with high selectivity for calcium, and the basicity of steel slag can decrease obviously [12,13] , therefore, it will also reduce the addition of texturizine agent. Our preliminary experiments [14,15] showed that the ltrate of slag leaching by NH 4 Cl is a high purity system of CaCl 2 -NH 4 Cl-NH 3 -H 2 O, and high purity and commercial calcium carbonate can be prepared by blowing carbon dioxide gas into the solution. The process was spontaneous, and ammonium chloride could be recycled during the leaching-carbonization process.
Based on this, this paper creatively proposes a new process for preparing glass-ceramics after recovering iron from desulfurization slag as shown in Figure 1. Desulfurization residue as the main raw material, which is pretreated by ammonium chloride leaching to obtain desulfurization residue leached by ammonium chloride (DRLAC) and ltrate. The ltrate is fed with carbon dioxide to prepare calcium carbonate, and DRLAC, which is adjusting the composition by adding high alumina coal ash and glass cullet, can be separated from slag and iron through high temperature carbon thermal reaction to recover the metalic iron, and then the reduced slag after iron recovery is directly homogenized and heat treated to prepare basic glass and glass-ceramics. The new process can use the desulfuration residue sequestered carbon dioxide and preparing pure calcium carbonate without a lot of energy input, and it also can decrease the desulfurization slag basicity, which signi cantly reduce the addition of texturizine agent for preparing glass-ceramics. At the same time, the process can directly use the equipment of steel production, therefore, the process has many advantages, such as low investment and production cost, green environmental protection and turning waste into wealth. Since the preparation of calcium carbonate in CaCl 2 -NH 4 Cl-NH 3 -H 2 O system has been studied in the early stage, the focus of this paper is to use desulfurization slag with high aluminum coal ash and glass cullet to recover metal iron and prepare glass-ceramics.

Material
Desulfurization slag is KR desulfurizing slag without any processing and it is shown in Fig. 2, which coming from Wuhan iron and steel group in this study. It can be seen in Table 1 that there is a lot of chemical element iron mixed in the slag, which is di cult to separate, and the calcium content in desulfurization slag is as high as 42.33% while the silicon content is only 6.2%, in other words, the binary alkalinity is as high as 6.83. In addition, the slag also contains more aluminum, this is related to the fact that the furnace slag on the surface of molten iron was not completely cleaned before desulfurization process. The desulfurization slag also contains certain magnesium, titanium and sulfur.
The pretreatment of desulfurization slag is to grind the desulfurization slag to less than 60 mesh, then leach it with 4 mol/L ammonium chloride solution for 24 h according to the ratio of steel slag mass to solution volume 1:10 (g/mL). After ltration, the lter slag is washed with ultra-pure water for 3 times.
After drying, DRLAC is obtained. High alumina coal ash comes from a factory in Inner Mongolia, China, and its composition is shown in Table 1 below. The glass cullet is ordinary glass on the market, ammonium chloride is analytical reagent and the water used is ultra-pure water.

Design of glass composition
In order to effectively separate the slag and iron from the desulfurization slag, the melting point of the reduced slag should be low except for the reduced metal iron, and the reduced slag after the recovery of iron will be directly used to prepare basic glass and glass-ceramics, the system of which is a typical CaO-MgO-SiO 2 -MgO (CMAS) system. Factage7.2 software was used to calculate the melting point and phases of the system as shown in Fig. 3 below, it can be seen from the dotted line in Fig. 3 that the melting point of the reduced slag is all below 1300℃, which is conducive to the slag-iron separation. Meanwhile, the phase of the reduced slag in this region is diopside. In view of the mechanical strength, wear resistance and chemical stability of diopside glass-ceramics, it is selected as the target main crystal phase in this paper. In addition, because of the glass cullet containing a certain amount of sodium, a nepheline is easy form during crystallization, which is bene cial to keep the molten state of vitreous body, and the two phases (diopside and nepheline) can better play the advantages of glass-ceramics [16] . Therefore, diopside and nepheline are determined as the main crystal phases of the glass-ceramics in this paper. In order to make full use of DRLAC and combine with its component characteristics, the DRLAC, glass cullet and high aluminum coal ash were mixed as shown in Fig. 3. There were four experimental samples in total and the content of carbon was 3% of the total weight.

Preparation process
The mixed raw materials were put into a corundum crucible and heated in a mu e furnace to conduct iron recovery experiments, in which the temperature was heated to 1500 ℃ and holding for 2 h, then the sample were cooling to room temperature within mu e furnace. And after that, the crucible were taken out and opened to take macroscopic photos, weight metal iron after slag-iron separation, analyze the iron content and phase in the reduced slag.
The reducing slag after carbothermal reduction separation was put into the corundum crucible again and heated to 1500°C for 2 h in a mu e furnace to simulate the molten reduction slag after iron separation, and then the crucible was quickly taken out of the high-temperature furnace, and the molten reducing slag was poured onto a pre-heated steel plate. Subsequently, the steel plate was quickly moved into a mu e furnace (60°C for2 h) to remove the internal stress generated by the melt after rapid cooling at high temperature. A small amount of basic glass is ground into powder for comprehensive thermal analysis, and according to the differential thermal analysis, a controlled-crystallization method was used to obtain the glass-ceramics. Then the macroscopic photos and phases of glass-ceramics were test.

Characterization
The chemical compositions of raw material were analyzed by X-ray uorescence (XRF) (XRF-1800, Shimadzu, Japan). The reduced slag and iron were removed from the crucible for weighing and chemical analysis (GB YB/T 148-2009) to calculate the iron recovery rate and the total iron in the slag according to Eq. (1) and Eq. (2), Where X Fe is iron recovery rate in desulfurization slag, M Fe is recovered iron amount, M TFe is amount of total iron in desulfurization slag, M RTFe is amount of total iron remaining in reduced slag. The phases of the slag were determined using an X-ray diffraction (6100 lab instrument from Shimadzu) with Cu Kα radiation. The thermal behavior was determined by a simultaneous thermal analyzer (Beijing HengJiu Science Instrument Factory, HCT-3) in a corundum crucible under the protection of atmospheric argon.

Results And Analysis
It can be seen in Fig. 4 that the separation of slag and iron after carbon thermal reduction reaction is very good, and a relatively regular cake shaped iron block was obtained, the testing and calculation of iron also indicates that the recovery rate of iron in desulfurization slag is higher than 99%. XRD spectra of reducing slag in Fig. 5 show that the main crystalline phase is diopside and the secondary crystalline phase is nepheline. The reducing slag was heated to molten state and then homogenized to prepare basic glass, simulating the high-temperature melting slag directly preparing basic glass and glassceramics, the macroscopic photos are shown in Fig. 6. The glass-ceramic process of basic glass usually includes two processes, namely the formation of crystal nucleus (endothermic process) and the growth of crystal (exothermic process). Therefore, endothermic peaks and exothermic peaks appear in the differential temperatures in DSC curve. As can be seen from the DSC curve in Fig. 7, sample 1 has an obvious endothermic peak around 985 K, and two obvious exothermic peaks around 1040 K and 1155 K. In comparison, the endothermic peak are not obvious in sample 2, 3, and 4, in additional, the temperatures of former exothermic peak drops to about 1030 K and the intensity of the peaks also becomes remarkably smooth, even indistinguishable, while the latter exothermic peak also drops to around 1130 K and the intensity of the peaks was enhanced, the DSC curve indicated that two crystal phases might precipitate in each of the four samples, and the crystal phase near 1130 K is obviously more than that near 1030 K for sample 2, 3, and 4 .
With the increase of heating rate, both the initial crystallization temperature and the peak temperature of the four samples increased gradually, which was mainly caused by the heat cannot be supplied in time with the increase of heating rate [17] . From the position of endothermic and exothermal peak, these two temperatures are far lower for that the glass-ceramic's nucleation and crystallization temperatures of metallurgical slag-based are usually about 900 and 1100 ℃ [16] , respectively. It can be seen that the formation of nepheline crystals is conducive to reducing the heat treatment temperature and reducing the process energy consumption.
Based on the DSC curves of the four samples, it was determined that the nucleation was carried out at 1033 K for 2 h, and the crystallization was carried out at 1138 K for 2 h, with a heating rate of 5 ℃/min, and the glass-ceramics obtained after the heat treatment are shown in Fig. 6. XRD analysis of the glassceramics in Fig. 5 shows that the crystalline phases are diopside and nepheline, which is consistent with the XRD spectra of the reduction slag and the results of the two exothermic peaks in the DSC curve.
However, from crystal phase intensity of diopside and nepheline, the difference between the four samples was small, which seemed to be not consistent with the characteristics of DSC curves. In fact, 1033 K was not only the formation point of crystal nucleus, but also the growth point of nepheline near 1030 K. In other words, after holding for 2 h at 1033 K in heat treatment, it also promotes the crystallization of nepheline.
The glass-ceramics is obtained by the controlled crystallization of the basic glass after heat treatment, which is a non-uniform nucleation process. At present, classical theories on the crystallization behavior of glass-ceramics mainly include Kissinger equation (Eq. 3), Ozawa equation (Eq. 4) and Augis-Bennett equation (Eq. 5). In this article, these three formulas were used to calculate the crystallization kinetics of glass-ceramics under non-isothermal conditions. The crystallization activation energy E was used to represent the potential barrier needed to overcome the structural rearrangement during the transition from glassy state to crystal state, that is, the di cult degree of crystallization, and the crystal growth index n was used to judge the growth form of crystal.
According to the DSC curves of the four samples at different heating rates, combined with the Kissinger equation, Ozawa equation and Augis-Bennett equation, the linear tting was carried out with 1/T p as the X-axis and ln(T p 2 /α), ln(1/α) and ln(T p /α) as the Y-axis, respectively. The linear tting of dynamics was obtained in Fig. 8. Then T p , α and R are substituted into the equation to obtain the crystallization activation energy E, as shown in Table 2. By comparing and analyzing the crystallization activation energy obtained by different methods, it can be found that there is little difference between results obtained by the three methods. The value of Ozawa method is relatively high, while the value of Kissinger method is relatively low. Compared with the four samples, the crystallization activation energy of sample No. 3 is the smallest, that is, sample No. 3 has the lowest potential barrier needed to overcome the structural rearrangement and is easier to crystallization. The crystal growth index was calculated by using the Augis-Bennett equation (Eq. 6). By substituting the E values of different crystallization activation energies calculated in Table 2 into the equation, and combining with the DSC curve, the crystal growth index n shown in Table 3 can be obtained. It can be seen that the crystal growth index is not very high, which may be due to the fact that there are few oxides with nucleation function, such as TiO 2 , in the raw material under the condition of not adding nucleating agent in this paper, and two kinds of crystals are precipitated during heat treatment, which results in mutual interference in the initial nucleation stage and weakened crystallization ability of glass. Among them, the n value of sample No. 3 is the largest at the heating rate of 5 K/min. The crystallization mode of the glass-ceramics is changed from two-dimensional to one-dimensional volume crystallization [17] , indicating that the preparation of glass-ceramics in this paper needs to add nucleating agent. A new process of iron recovery and glass-ceramics preparation from desulfurization slag is designed in this paper. Taking desulfurization slag as the main raw material, which is leached by ammonium chloride as pretreatment and the composition of target slag system was adjusted with high aluminum powder coal ash and broken glass. The carbon thermal reduction experiment can make the iron recovery rate over 99%, and the reducing slag can be transformed into glass-ceramics whose main crystalline phases are diopside and nepheline by homogenization and heat treatment, which show that the new process is feasible.
2) The kinetic study of glass-ceramics shows that the crystallization activation energy ranges from 383.8 to 616.8 kJ /mol, which is relatively high, and the crystal growth index is less than 3, therefore, the preparation of glass-ceramics requires the addition of nucleating agent. Under the heating rate of 5   The separation macrograph of slag and iron after carbon thermal reduction reaction The macrograph of basic glass and glass-ceramics The DSC curve of different basic glass sample