Low Carbon and High Efficient Utilization of Zinc-bearing Dust in Iron & Steel Industry by Rotary Kiln Process

More than 50 million tons per year of zinc-bearing dust with large amounts of zinc and iron resources is produced by iron and steel smelting in China, which is classified as hazardous solid waste. The zinc-bearing dust is an important secondary resource, in which the zinc and iron can be recycled as raw materials for the iron and zinc industries after separating them. Rotary kiln reduction is one of the main methods for separating zinc and iron from zinc-bearing dust. In this study, the 5 mm composite pellets containing zinc-bearing dust is prepared by forced disturbed pelleting as reduction charge. The rotary kiln system with controllable temperature field and atmosphere field by multi-point quantitative air suppling is used as the main equipment of reduction. This technology can realize the treatment of raw materials with high iron and zinc content in rotary kiln, but also can effectively improve the production efficiency and the quality of zinc oxide powder, reduce the kiln-ringing. Compared with the conventional rotary kiln method, the processing capacity of zinc-bearing dust increase by 30%. The zinc removal ratio reaches 95% and the ZnO content of zinc oxide powder is 53.93 wt.%. The TFe content and ZnO content of reduction product are 60.56 wt.% and 0.25%, respectively. The annual operation ratio of zinc-iron separation by rotary kiln with zinc-bearing dust is 92%. The metallization rate of reduction product is controllable. The reduction product with metallization rate of 65.97% can be used as a sintering raw material, and the comprehensive energy consumption is 187kgce/t. The reduction product with metallization rate of 88% can 1 be used as a converter steelmaking raw material, and the comprehensive energy consumption is 253.1kgce/t.


Introduction
The iron and steel production process produces a large number of solid wastes, most of which can be used as building materials for overall utilization [1][2][3].The other solid wastes cannot be directly utilized due to the high content of heavy metals and alkali metals [4,5].The requirements on transportation, storage and disposal of solid wastes with heavy metals and alkali metals are much higher than those of ordinary solid wastes [6,7].The difficulty and high cost of complex solid waste disposal are the key factors that restrict whether iron and steel enterprises can realize "solid waste not leaving the factory" [8].Zinc-bearing dust is the main solid waste containing heavy metals produced in iron and steel process [9].The output of zinc-bearing dust is about 3~5% of the output of crude steel.The output of zinc-bearing dust in iron and steel enterprises in China is about 30 ~50 million t/a.Zinc-bearing dust rich in iron, zinc and other valuable resources, has high recovery value [10,11].
The reserves of zinc and iron are insufficient in China [12].Therefore, the recovery of valuable metal from the zinc-bearing dust is an important way to solve the shortage of zinc and iron resources.However, zinc-bearing dust disposal process has great environmental risk because of its high pollution, which is also the key and difficult point of solid waste disposal in iron and steel plant.
At present, zinc-bearing dust are generally disposed by rotary hearth furnace or rotary kiln process [13,14].Rotary hearth furnace process has high primary investment, high energy consumption and poor product uniformity [15,16].In comparison, the rotary kiln method has wide adaptability of raw materials [17].The material rolls forward in the kiln and is evenly heated.The uniformity of physical mixing and chemical reaction is better.So, the stability and technical economy of rotary kiln products are better than those of rotary hearth furnace [18,19].However, the problems of easy ring forming and low metallization ratio in traditional rotary kiln process need to be solved urgently [19,20].It is of great significance to develop a low-carbon and efficient recycling technology of zinc-bearing dust in green iron and steel process.
In this paper, the new technology and equipment for efficient cleaning and separation of valuable components in zinc-bearing dust are investigated to reduce energy consumption, extend the service life of equipment and improve product value.Finally, the full quantitative recovery of zincbearing dust can be achieved by sintering and other metallurgical processes.The XRD pattern of zinc-bearing dust are shown in Figure 1.The major phases in the zincbearing dust A are hemihydrate gypsum and hematite.The major phases in the zinc-bearing dust B are magnetite, zinc ferrite, metallic iron, and a small amount of zinc oxide and calcium carbonate.
The major phases in the zinc-bearing dust C are complex, mainly containing hematite, magnetite and a small amount of manganese-zinc ferrite with crystalline water, whose crystal structure is similar to magnetite.The other phases also include carbon, a small amount of quartz, calcium carbonate and calcium ferrite.IOP Publishing doi:10.1088/1742-6596/2738/1/0120244 machine (Φ1000 × H200 mm) and forced disturbed pelletizing machine (Φ1000 × H500 mm).The reduction experiments were carried out in a muffle furnace.The high-temperature reduction products were cooled to room temperature in a nitrogen atmosphere.The reduction production were carried out in rotary kilns with a diameter of 2.5m and a length of 38m.The reduction products were prepared for analyzing the elemental composition, mineral phases and microstructure.

Analysis Methods
The chemical compositions of the samples were determined by Inductively coupled plasma spectrometer(ICP, Spectro blue sop).
The reduction samples were mounted into epoxy resin, mechanically ground and polished.Then the polished surface was characterized by an electron microscope for microstructure.

Effects of pelletizing methods on pellet formation behavior
The effect of cylindrical pelletizing and forced disturbed pelletizing on pellet formation was simulated by EDEM, and the results were shown in Fig. 2 and Fig. 3.Under the same conditions, the agglomeration obtained by forced disturbed pelletizing is larger and the adhesion of fine particles is more.The strength of aggregate force chain increases rapidly with time and then decreases gradually in the process of forced disturbed pelletizing.At the early stage of the stirring paddle movement, the contact force of mixture increases rapidly, so that the strength of aggregate force chain increase.And then, the mixture accumulated under the feed port gradually tends to be distributed smoothly in the action of stirring force.The mixture acted by the stirring paddle decreases and tends to be stable, making the contact force gradually decrease and tends to be stable, and the aggregate further increases.The small contact force between the fine mineral powder attached to the surface will gradually reduce the strength of the right-hand chain.
The growth of particle aggregates can be divided into three stages: rapid growth stage, slow growth stage and steady change stage.When the agglomerates no longer grow, the strength of the particle agglomerates obtained from the forced disturbed pelletizing is greater than that of cylindrical

Effects of pelletizing methods on pellet size
The zinc-bearing dust A, B, C were mixed at 30%, 45%, 25% to pellet.The effect of forced disturbed pelletizing and cylindrical pelletizing of the pellet size under different moisture is shown in Table 2.When the moisture is 8.25 wt.%, the content of +3mm pellets by forced disturbed pelletizing is 10.4% more than that by cylindrical pelletizing.The average pellet size of pellets by forced disturbed pelletizing is 5.08 mm.When the proportion of +3mm pellets is at the same level, the moisture of forced disturbed pellets is 0.5% lower than that of cylinder pellets, and the proportion of 3-5mm pellets is 59.28%, which is 12.71% higher than that of cylinder pellets.The particle size of forced disturbed pellets are more uniform than that of cylinder pellet.The effect of different pelletizing methods on the volatilization of zinc and the reduction of iron is not significant.However, the moisture of the forced disturbed pellets is lower and the strength is higher, which is beneficial to improve the strength of the green pellet drying process and reduce the drying energy consumption.
The pellet produced by forced disturbed pelletizing has high strength and produces less powder during the reduction process, which can alleviate the agglomeration in the kiln and prevent the reoxidation of reduction products.

Effect of temperature and time on reduction
The effect of temperature and time on zinc removal and iron reduction is shown in Fig. 4 and Fig. 5.The zinc removal ratio and metallization ratio increased gradually with the increase of reduction temperature.When the reduction temperature is 1150 ℃ , the zinc removal ratio and metallization ratio reach the highest value.After the reduction time exceeds 35min, the zinc removal ratio and metallization ratio do not increase.The experimental results show that the metallization ratio of pellets can reach more than 96% and the zinc removal ratio can reach more than 99% as the reduction temperature is 1150°C and the reduction time is 35min.The microstructure of composite pellets and reduced pellets reduced at 1150℃ for 35min is shown in Fig. 6.The mineral particles in the composite pellets are dispersed and almost have no pore structure.The iron element in reduction pellets mainly exists in the form of metallic iron.The number of metallic iron particles in reduced pellets were well developed.The metal iron particles are connected with each other, and some iron particles enriches and grows to about 100 μm.At the same time, the carbon in the composite pellets is consumed by the reduction reaction, resulting in the increase of pore structure and pore size.The pellet internal structure is relatively

Kinetics of zinc removal by compound pellet reduction
In the reduction process, the carbon contained in the composite pellets will gasification and generate CO reducing gas, which will enhance the reducing atmosphere inside the pellets and  The reduction kinetics model of compound pellet conforms to the nucleation and growth models.
After the carbon inside the pellets undergoes gasification reaction, a high reduction atmosphere is formed inside the pellets.The reduction reaction of iron and zinc components occurs synchronously and uniformly inside the pellets.This avoids the low reduction rate caused by the reduction gas passing through the product layer from outside during the conventional pellet reduction process, improves the reduction efficiency.

Thermodynamic analysis of ring formation in rotary kiln
The isothermal phase diagram of CaO-FeO-SiO2-0.2%ZnO slag system is shown in Fig. 8.The liquid phase region gradually expands as the temperature increases.The liquid phase region is mainly distributed in areas with high silicon content and high ferrous content.When the temperature increases from 1200 ℃ to 1250 ℃, the liquid phase region in the system significantly expands.The liquid phase region expands as CaO/SiO2 decreases.Therefore, in order to achieve the reduction and separation of zinc while avoiding the melting and kilning of reaction materials, the reduction temperature should be controlled below 1200 ℃, and CaO/SiO2 should be increased as much as possible.

Bonding between zinc-bearing dust powder and refractory materials
The zinc-bearing dust powder will react with the refractory material of rotary kiln during the reduction process.The effect of reduction temperature on the bonding between zinc-bearing dust powder and refractory materials is shown in Fig. 9.When the reduction temperature increases from 1050 ℃ to 1100 ℃, there is less reduced zinc-bearing dust powder adhered to the refractory material.
During the process of increasing the reduction temperature from 1100 ℃ to 1200 ℃, the reduced zinc-bearing dust powder adhered to the refractory material increases sharply.The adhesion ratio increased to 45.9% when the reduction temperature was 1150 ℃.When the reduction temperature reaches 1200 ° C and 1250 ° C, the bonding phenomenon between the reduced zinc-bearing dust powder and refractory material is severe, and the adhesion ratio is close to 100%.Strictly controlling the high-temperature range of the reduction process will be beneficial for reducing the erosion of refractory materials by reduced zinc-bearing dust.Extending the high-temperature reduction time at 1100 ℃ can achieve efficient reduction and dezincification of zinc-bearing dust composite pellets, while avoiding ring formation in the rotary kiln.

Industrial application of zinc-bearing dust reduction by rotary kiln method
The low-temperature direct reduction pilot production line that includes forced disturbed pelleting, rotary kiln system with controllable temperature field and atmosphere field has been built by Zhongye Changtian International Engineering Co., Ltd., which is shown in Fig. 10.The traditional rotary kiln method for treating zinc-bearing dust involves blowing all the air from the kiln head, which will form a high-temperature and oxygen rich zone inside the kiln.High temperature and oxygen rich areas will generate local high temperatures, exacerbating the tendency of rotary kilns to form rings and the re-oxidation of reduced products.The rotary kiln of this technology adopts multipoint air intake on the kiln body, reducing the oxygen intake at the kiln head, alleviating the reoxidation of metallic iron.At the same time, controlling the multi-point air intake of the kiln body achieves precise temperature control inside the kiln, eliminates local high temperature points, and greatly alleviates the ring formation in rotary kiln.The pilot study on the reduction and dezincification of zinc-bearing dust was carried on the lowtemperature direct reduction pilot production line of Zhongye Changtian International Engineering Co., Ltd..The 5 mm composite pellets containing zinc-bearing dust is prepared by forced disturbed pelleting as reduction charge.The φ1m10m rotary kiln system with controllable temperature field and atmosphere field by multi-point quantitative air suppling is used as the main equipment of reduction.When the reduction temperature is 1100 ℃ and the material stays at high temperature for 30 min, the dezincification rate reaches 99.09%.The ZnO content of the reduction product is 0.14%, which is only half of that when the multi-point air supply is not turned on.The metallization rate of the reduction product is 79.15%.The ZnO content in the zinc oxide powder reached 77.19%.There is no obvious ring formation phenomenon during the stable operation of the rotary kiln.The reduction product and collected dust is shown in Fig. 11.

Fig. 11 The reduction product and collected dust
The key technologies of this study have been applied industrially in the production lines of 150000 t/a and 60000 t/a zinc-bearing dust reduction and dezincification.The 150000 t/a production line adopts a kiln head burner heating combined with carbon reduction inside the pellet.The 60000 t/a production line adopts the method of carbon distribution inside the pellet for heating and reduction.Compared with the conventional rotary kiln method, the processing capacity of zincbearing dust increase by 30%.The zinc removal ratio reaches 95% and the ZnO content of zinc oxide powder is 53.93 wt.%.The TFe content and ZnO content of reduction product are 60.56 wt.% and 0.25%, respectively.The annual operation ratio of zinc-iron separation by rotary kiln with zincbearing dust is 92%.The metallization rate of reduction product is controllable.The reduction product with metallization rate of 65.97% can be used as a sintering raw material, and the comprehensive energy consumption is 187kgce/t.The reduction product with metallization rate of 88% can be used as a converter steelmaking raw material, and the comprehensive energy consumption is 253.1kgce/t.

Conclusion
(1) The forced disturbed pelletizing can achieve efficient preparation of zinc-bearing dust composite pellets.The particle size of forced disturbed pellets are more uniform than that of cylinder pellet.The moisture of the forced disturbed pellets is lower and the strength is higher, which is beneficial to improve the strength of green pellet drying process and reduce the drying energy consumption.
(2) The composite pellets with carbon inside are conducive to the rapid reduction of iron and zinc.Lowering the reduction temperature and increasing the CaO/SiO2 ratio of composite pellets can effectively inhibit the adhesion of reduction products to refractory materials.Extending the hightemperature reduction time at 1100 ℃ can achieve efficient reduction and dezincification of zincbearing dust composite pellets, while avoiding ring formation in the rotary kiln.
(3) The industrial production of zinc-bearing dust disposal shows that efficient reduction and dezincification of zinc-bearing dust can be achieved by using forced disturbed pelletizing, multi field collaborative controllable rotary kiln reduction.Compared with the conventional rotary kiln method, the processing capacity of zinc-bearing dust increase by 30%.The zinc removal ratio reaches 95%.
The TFe content and ZnO content of reduction product are 60.56 wt.% and 0.25%, respectively.The annual operation ratio of zinc-iron separation by rotary kiln with zinc-bearing dust is 92%.The metallization rate of reduction product is controllable.

Fig. 1
Fig. 1 XRD pattern of the zinc-bearing dust

Fig. 4 Fig. 5
Fig. 4 Effect of temperature on zinc removal and iron reduction (reduction time of 30min)

Fig. 6
Fig.6 Microstructure of composite pellets (a) and reduced pellets reduced at 1150 ℃ for 35 min (b)

Fig. 7
Fig.7 Reduction kinetics analysis of 5 mm reduction pellets

Fig. 9
Fig. 9 Effect of temperature on the bonding between zinc-bearing dust powder and refractory materials

Fig. 10
Fig. 10 Low-temperature direct reduction pilot production line Three kinds of zinc-bearing dust were used as raw materials in this study.The chemical compositions of zinc-bearing dust are listed in Table1.The ZnO content of zinc-bearing dust A is the highest of 8.51 wt.%, and the Fe2O3 content is 16.11 wt.%.The ZnO content of zinc-bearing dust B is 4.73 wt.%, and the Fe2O3 content reaches 51.50 wt.%.The ZnO content of zinc-bearing dust C is 0.99.

Table 2
Effects of pelletizing methods on pellet size