Synthesis of Cylon materials with gold tailings

Tailings are wastes from mining industry. At present, there are many problems in tailings treatment: occupying a large amount of land, causing huge waste of mineral resources and seriously affecting the ecological environment. In the secondary utilization process of tailings, there are also disadvantages of low value-added products and lack of market competitiveness. Gold Mine tailings are complex and intractable resources, the pollution of the environment is very prominent, and very large emissions, our only Henan Lingbao Gold Corporation currently had a gold tailings dumps more than 15 million t. Therefore, the study of the comprehensive utilization technology of gold and gold tailings is of great significance for the rational and rational development and utilization of mineral resources.

SiAlON is a solid solution based on Si ₃ N ₄ and composed of Si, Al, O and N. It has good high temperature oxidation resistance, thermal shock resistance and corrosion resistance, and has a promising future. Ca-α-SiAlON is a pentad-based celite material of solid solution alkaline earth metal, which has unique properties such as high hardness, good wear resistance and corrosion resistance. This study explores the use of Lingbao gold tailings as the main raw material to synthesize Ca-α-SiAlON/SiC powder by carbothermal reduction nitriding method in order to obtain high value-added gold tailings products, thus high-efficiency synthesis of gold tailings. Use to open up a feasible way.

First, the experimental principle

J. W. T. In 1995, Van Rutten et al. used carbon powder in CaO or CaSiO ₃ , SiO ₂ and Al ₂ O ₃ raw material systems to study the reaction mechanism of Ca-α-SiAlON synthesis by carbothermal reduction nitridation. They generally accepted their theoretical explanations. They found that the formation temperature of Ca-α-SiAlON is above 1450 °C. Single-phase Ca-α-SiAlON can be further synthesized by holding at 1500 ° C for 65 h; at 1350 ° C, the main products are SiO ₂ and Si ₂ N ₂ O; at 1450 ° C, α-SiAlON and β-SiAlON are mainly obtained; When the temperature is higher than 1650 ° C, the main product is SiC, not Ca-α-SiAlON. Research indicates that the entire reaction process can be summarized in two steps:

(1) Forming a low Z value of β-SiAlON:

4.6SiO ₂ +0.7Al ₂ O ₃ +9.9C=Si _4.6 Al _1.4 O _1.4 N _6.6 (1)

(2) Solid solution Ca and more N:

0.8CaO+2Si _4.6 Al _1.4 O _1.4 N _6.6 +2.4C+0.8N ₂ =Ca _0.8 Si _9.2 Al _2.8 O _1.2 N _14.8 (2)

Second, the experimental materials

The main raw material of the experiment is the tailings of Lingbao Gold Mine in Henan Province, which is blended with appropriate amount of silica sand and analytically pure CaO to adjust the raw material components. The chemical composition of Lingbao gold tailings and silica sand is shown in Table 1.

Table 1 Chemical composition of Lingbao gold tailings and silica sand

Third, the experimental method

The tailings, sand, activated charcoal and CaO analytically pure ethanol as medium alumina ball mill jar wet mixing 24h, into an oven and the slurry was sufficiently dried at 60 ℃, in an alumina ball mill jar then dry blended 4h Ensure that the raw materials are thoroughly mixed and then press-formed under a pressure of 40 MPa. The green body is immersed in BN powder and placed in a nitrogen furnace for atmospheric pressure sintering. The flow rate of high purity nitrogen (including N2>99.999%) is controlled at 1.0 L/min. The fired sample was kept at a constant temperature of 800 ° C for 6 hours to remove residual free carbon. After the sample was prepared, its phase composition was analyzed by X-ray diffraction (XRD), and its morphology was observed by scanning electron microscopy (SEM).

In this study, the amount of fixed silica sand added was SiO _{2 to} meet the stoichiometry, the amount of activated carbon added was 1.3 times of the theoretical carbon content, and the sintering holding time was 5 h. The two factors of CaO content and temperature were focused on the synthesis of Ca-α- The impact of SiAlON. Based on Ca _0.8 Si _9.2 Al _2.8 O _1.2 N _14.8 . It can be calculated that the CaO content in the raw material should be 4.2% according to the stoichiometric amount. In this experiment, the reaction of CaO in the five different levels of high temperature was carried out according to the stoichiometric incorporation (4.2%) and the excessive incorporation (6.3%). The two-factor five-level orthogonal optimization experimental scheme is shown in Table 2.

Table 2 Two-factor five-level orthogonal optimization experimental scheme

Fourth, experimental results and discussion

(1) Effect of CaO content on product formation

In the process of preparing α-SiAlON in the past, a rare earth additive is often selected as a sintering aid. CaO is used as a sintering aid, which is cheaper than rare earth additives and has a wider application prospect. The CaO content has an important influence on the product formed. The phase analysis results of the products formed under different experimental conditions in the temperature region where Ca-α-SiAlON can be generated are shown in Table 3. The mass fraction of the Ca-α-SiAlON and SiC phases in the product produced in the table is calculated by the following formula: W _{Ca - α - SiAlON} / W _Sic :

(3)

Where I _{α ( 102 )} and I _{α ( 210 )} are the integrated intensity of the X-ray diffraction peaks of the (102) and (210) planes of Ca-α-SiAlON, respectively; I _{sic ( 111 )} , I _{sic ( 111 )} is SiC The integrated intensity of the X-ray diffraction peaks at the (111) and (220) planes.

Table 3 Experimental conditions and product phase analysis results

The experimental results show that when the sintering temperature is 1350 °C and 1450 °C, no Ca-α-SiAlON phase is formed. At 1500 °C, when the CaO content is stoichiometric (4.2%) and 6.3%, a small amount of Ca-α-SiAlON is formed. At 1550 °C, with the increase of CaO content, the content of Ca-α-SiAlON in the product increased, and the content of SiC decreased relatively. At 1600 °C, the most Ca-α- was formed at the stoichiometric point of CaO (4.2%). SiAlON, and the Ca-α-SiAlON ratio decreases when the CaO content is 6.3%. From this, it can be seen that when CaO is excessively added, the effect of the increase in temperature on the relative ratio of Ca-α-SiAlON is weakened (0.97 at 1550 ° C and 1.05 at 1600 ° C). However, at higher synthesis temperatures, the excessive addition of CaO reduces the proportion of Ca-α-SiAlON in the product. Therefore, it is necessary to comprehensively consider the two conditions of temperature and CaO addition amount in the synthesis process. This is also difficult J. W. T. The theory of Van Rutten et al., that is, when the temperature is low, only CaO is excessive to dissolve more Ca into the phase to form Ca-α-SiAlON, and conversely, CaO is excessively added, and Ca ^{2 + is} more solid. Dissolving into the phase causes Ca-α-SiAlON to form at lower temperatures. At higher temperatures, Ca ^{2 +} activity increased, more solid solution phase composition, if at this time is added in excess CaO, Ca ^{2 +} into the more stable silicon-oxygen tetrahedra of the silicate network texture, reducing the O ^{2 -} into the [SiN _4] ^{8 -} tetrahedron opportunity, thus easily generating a Ca-α-SiAlON.

(2) Effect of temperature on product formation

The theoretical generation temperature of Ca-α-SiAlON is 1450 °C. At 1350 ° C, most of the experimental products were free carbon and glass phases, but no Ca-α-SiAlON phase was found, indicating that it is difficult to generate Ca-α-SiAlON at low temperatures. Figure 1 is an XRD pattern of the product formed at different temperatures when the CaO content is stoichiometric (4.2%).

Fig.1 XRD pattern of samples prepared at different temperatures when CaO content is 4.2%

â–²-C; â—†-β-SiC; â–¡-α-Si ₂ N ₄ ; â– -β-Si ₃ N ₄ ; ●-Ca-α-SiAlON

By comparing the XRD patterns of the products formed at different temperatures, it can be concluded that the formation of Ca-α-SiAlON undergoes the following process as the temperature increases:

1. There is almost no carbothermal reduction of oxide at 1350 ° C. The product is mainly unreacted carbon powder and glass phase. The scattering peak formed by the diffraction of the glass is shown in Fig. 1(a), indicating that the liquid phase production process mainly occurs near this temperature.

2. At 1450 ° C, SiO ₂ begins a carbothermal reduction reaction to form a SiC phase. At this point, the nitridation process has not yet occurred, the main product is SiC, and there are still scattering peaks in the XRD pattern. Although Ca-α-SiAlON can be formed theoretically at 1450 ° C, for the complex raw material system with high impurity content in the experiment, the SiAlON phase cannot be produced at this temperature, and a higher reaction temperature is required.

3. At 1500 ° C, the nitridation process begins, and the main phases of the product are SiC, α-Si ₃ N ₄ and β-Si ₃ N ₄ . High temperature, high impurity content of the raw material system to produce more liquid than the feed system a low impurity content, and in the presence of a large amount of liquid, Al ^{3 +} ions is easier and the Si-O tetrahedral Si ^{4 +} Interchange and enter the tetrahedron to form a stable structure. Only when the reaction temperature is high enough, Al ^{3 +} can obtain sufficient energy to be released from the Si-O skeleton, and recombine with Si, O, and N to form SiAlON.

4. At 1550 ° C, α-Si ₃ N ₄ and β-Si ₃ N ₄ gradually disappear, producing a small amount of Ca-α-SiAlON, and the phase of the product is Ca-α-SiAlON and SiC, of ​​which SiC is Main phase.

At 1660 °C, the amount of SiC in the system is relatively reduced, and the amount of Ca-α-SiAlON is obviously increased. At this time, the phase of the product is Ca-α-SiAlON and SiC, and Ca-α-SiAlON is dominant.

In summary, as the temperature increases, the reaction products are in turn SiC, α-Si ₃ N ₄ , β-Si ₃ N ₄ and Ca-α-SiAlON. At 1600 ° C, Ca-α-SiAlON is formed in large quantities and α-Si ₃ N ₄ and β-Si ₃ N ₄ disappear, indicating that α-Si ₃ N ₄ and β-Si ₃ N ₄ are only intermediates in the reaction process. .

(III) Selection of process conditions for synthesis of Ca-α-SiAlON

When the CaO content is stoichiometric (4.2%) and the sintering temperature is 1600 °C, the obtained product (product No. 9 listed in Table 3) has the highest W _{Ca - α - SiAlON} / W _Sic value, which can be calculated by formula (3). The relative content of Ca-α-SiAlON in the crystalline phase of the product reached 72%. The product was subjected to electron microscopic scanning to confirm the morphology of Ca-α-SiAlON, and the results are shown in Fig. 2.

Figure 2 SEM photo of the product No. 9.

The results of microscopic scanning showed that the product No. 9 mainly existed as a columnar crystal. The XRD analysis of Fig. 1(e) shows that the main crystal phase is Ca-α-SiAlON at this time, so it can be inferred that the columnar crystal is a Ca-α-SiAlON phase. According to the crystal structure theory, the unit cell parameter α/Si ₃ N ₄ of α-SiAlON is c/a=0.38. During the sintering process, the c-axis direction is its preferred growth direction, so the product is mainly columnar crystal. The temperature rises again, such as J. W. T. As pointed out by Van Rutten et al., the main product will be SiC instead of Ca-α-SiAlON. Accordingly, it is determined that the process conditions corresponding to the product No. 9 are suitable conditions for synthesizing Ca-α-SiAlON/SiC.

V. Conclusion

(1) Within a certain temperature range, increasing the temperature is conducive to the synthesis of Ca-α-SiAlON phase. As the reaction temperature increases, the reaction products are SiC, α-Si ₃ N ₄ , β-Si ₃ N ₄ and Ca-α-SiAlON, and α-Si ₃ N ₄ , β-Si ₃ N ₄ and SiC are synthetic Ca. An intermediate of -α-SiAlON.

(2) For the raw material system, the suitable conditions for synthesizing Ca-α-SiAlON are sintering temperature 1600 ° C, heat preservation for 5 h, and CaO is compounded by stoichiometry (4.2%). The product was mainly Ca-α-SiAlON, with a small amount of SiC, and the morphology of Ca-α-SiAlON was columnar six crystal.