Increasing of the unit capacity of MF process at Miike Smelter

MF (Mitsui Furnace) is a half shaft blast furnace developed by Mitsui Miike Smelter in the 1960’s to treat vertical retort residue. Currently MF mainly treats various secondary wastes (EAF dust, Secondary Fly Ash, etc.) and recovers zinc and lead as crude zinc oxide, copper and silver as matte, meanwhile other elements such as FeO, CaO, SiO2, Al2O3 are concentrated in slag. There are two steps in the MF process. The first step is pretreatment. In this process, various waste materials are blended in a certain proportion with reductant and dried adding binder in a rotary dryer, then grinded in a rod mill. The grinded materials are made into an oval briquette by briquetting machine. The second step is treatment process. The briquettes fed into the MF are smelted and reduced by blowing air through the tuyeres. By enriching the tuyere air with oxygen generated by the VSA (Vacuum Swing Adsorption) oxygen generator in 2005 and 2012, we have improved the throughput of wastes. By the improvement of oxygen enrichment in the MF, the unit capacity increased significantly. However shortage of the drying capacity in the pretreatment process became a problem. To ensure sufficient drying capacity, we installed one more rotary drier in series in 2016. Additionally in 2018, we installed a granulator after the rod mill to improve the bulk density in briquette. As a result of these efforts, the treatment amount in the whole MF process has increased. The MF can treat various kinds of materials containing various non-ferrous metal-valuables. Through these process improvements, we promote the treatment of wastes containing heavy metals and contributes significantly to the recycling of metal resources.


Features of MF Process
2.2.1.First step is Pretreatment process.Today the two main raw materials which MF is treating are EAF dust and secondary fly ash.Table 1 shows the typical construction of MF materials.Figure 1 shows the flow sheet of MF Process.In MF process, lump materials that are difficult to pulverize in a rod mill are fed straight to the MF furnace, and powder materials after being briquette.In the briquetting process, such materials as EAF dust and secondary fly ash, silica as flux and coal are constantly fed and transmitted to the rotary dryer, where blended with the materials, and the moisture is regulated.These are milled and mixed in the rod mill, granulated in the granulator, and then sulfite liquor as binder is added in another rotary dryer.This mixture is briquetted and constantly fed to the furnace.
2.2.2.Second step is MF process.In the MF furnace, zinc and lead oxide dust, matte and slag are produced.The oxide dust, carried out from the furnace with the exhaust gas, is collected in the boiler, cooler and the bag filter.The exhaust gas, after the dust has been collected, contains Hg and SOx, which is decreased by the exhaust gas washing facility and emitted into the air.The oxide dust, after being washed, filtered in order to remove chlorine and fluorine, is transmitted to Zinc Smelter, as crude zinc oxide.From the MF furnace slag and matte are taken out constantly; the matte, separated and recovered in the settler by its specific gravity, is sold as the material for copper.This slag, stable and non-toxic, is sold as the material for cement.Table 2 shows the product amount of MF process.
Figure 2 shows the conceptual figure of MF furnace.The briquettes fed from the feeding shoot, after being dried and preheated in the coking zone, enter the smelting zone.Zinc and lead are deoxidized there, and after deoxidization turned into oxide dust; copper and precious metals are concentrated in the matte.It is an important feature of MF furnace that these metals can be recovered in it at the same time.Fed into the furnace, 85-90% of the zinc, 95-97% of the lead are recovered as oxide dust, and the copper and precious metals as matte.In addition, the furnace and the waste heat boiler are constructed in one unit so that efficient recovery of heat is possible.

Comparison of MF process with other process
Currently the main method used for EAF dust treatment is the Waelz process.The Waelz process utilizes a rotary kiln, where carbon material is added to EAF dust.The mixture is heated to around 1300℃ using heavy oil burners etc., zinc is vapored after zinc oxide is reducted.As the generated zinc vapor is reoxidized by atmospheric O2, zinc is finally recovered in crude zinc oxide.On the other hand, the residue left after zinc extraction, called as clinker, is discharged from the kiln.While a portion of it is recycled as electric furnace feedstock, the majority is processed for use as roadbed material or cement raw material [1].The clinker is not fully melted in the kiln, it exists in semi-melted state.Therefore, it cannot be separated into slag and matte.Consequently, if precious metals are present in the waste, they would be lost in the clinker.Whereas, in the MF process, the raw materials are completely melted in the furnace, so it can be separated into slag and matte.Therefore, valuable metals such as gold and silver contained in materials can be recovered as matte, which is highly economical.Furthermore, the MF furnace has characteristics such as the furnace temperature exceeds 1300℃, the ability to recover heavy metals.Taking advantage of them characteristics, Miike Smelting Co, Ltd. has started the detoxification treatment for low-concentration PCBs (polychlorinated biphenyls) waste in 2015.Because of the characteristics of PCBs, such as low solubility, high boiling point, resistance to thermal decomposition, non-flammability, and high electrical insulation properties, it was extensively produced in Japan during the 1960s to 1970s [2].PCBs were primarily used as electrical insulating fluid in capacitors and transformers, as well as in hydraulic systems, heat transfer systems, and lubricants.PCBs were also blended with other chemicals, finding applications in various products like sealants, adhesives, and carbonless copy paper [3].However, due to the recognized environmental and human health hazards posed by PCBs, their production and importation have been prohibited.Incinerating products containing PCBs or PCBs waste at inadequate temperatures and residence times can lead to the generation of dioxins and the dispersion of PCBs into the environment.
At Miike Smelting Co, Ltd., certification for the harmless disposal facility for low-concentration PCBs waste was obtained in 2015, enabling the treatment of waste with PCBs concentration below 0.5%.For advantage in heavy metal recovery techniques in MF furnace, the company has become capable of addressing the challenging needs of processing PCBs waste, containing lead.Additionally, in 2021, certification was acquired for waste treatment with PCBs concentration below 10% or less, handling a wider range of PCBs waste.Figure 3 shows the flow sheet of MF process at the point of initiating treatment of PCBs waste.PCBs waste that transferred to a small-sealed plastic container feed from the side of MF furnace.

Enhancement in pretreatment process
For increase of the treatment capacity at the MF furnace, the oxygen production plant (VSA: Vacuum Swing Adsorption) with the capacity of 1,700 Nm 3 /h and 90% O2 was introduced in 2005, which improved the melting speed inside the MF furnace.Furthermore, in 2012 (1,700 Nm 3 /h, 90% O2), additional oxygen enrichment was installed, and two VSA units were operated [4].Currently, high oxygen concentration air with the preheating temperature of 200℃ and O2 concentration: 27% is blown into the furnace at the rate of 30,000 Nm 3 /h.As a result, the reaction speed inside the MF furnace improved, leading to the 30% increase in the treatment capacity.
With the aforementioned improvements, the MF treatment capacity, which was around 100,000 tons in the early 2000s, gradually increased, and reached 140,000 tons after the introduction of VSA in 2012.Figure 4 shows the trend in treatment amount of waste material from 2002 to 2022.However, increasing the treatment capacity, the drying capacity in the pretreatment process became insufficient apparently.Therefore, in 2016, the additional rotary dryer was installed, and the existing line, which was previously operated with one unit, was upgraded to operate with two units in series, thus increasing the drying capacity.Table 3 shows the specifications of the drying equipment.Previously, granulated EAF dust accounted for a significant portion of the treatment materials in Miike Smelting Co,.Ltd..However, since 2002, the proportion of powdered secondary fly ash has gradually increased.When the larger proportion of powdered waste materials is fed into the briquetting Treated Waste Materials (t/y) Year machine, the bulk density decreases, resulting in the decrease for briquette strength.To solve this problem, the granulator with water injection was installed before the second rotary dryer to increase the proportion of granulated waste materials fed into the briquetting machine and increase the bulk density of the briquette ore. Figure 5 shows the change in waste material density before and after granulation.The increase of the bulk density of the raw material, making it possible to suppress volume reduction during briquetting compared to only powdered material.Therefore, it is thought that harder briquetting ore can be produced.

Flexibilty of MF Treatment
The treatment of secondary fly ash has continued since 2002 at Miike Smelter, as a result, the composition of slag has gradually changed, with increases in calcium content.Table 4 shows the transition of slag composition up to the present.Since SiO2 content in the slag is relatively decreased and the viscosity of the slag is lowered, it is thought that the fluidity of the slag can be improved.Figure 6 shows the transition of slag composition in the FeO-SiO2-CaO phase diagram.In this figure, it is confirmed that the melting point of the slag gradually increased, the melting point of the slag exceeded 1100℃ in 2018.If the melting point of slag rises, the risk of clogging the slag port increases when cooling around the slag port or insufficient heat in the furnace occurs.However, it can be operated without problems since the temperature inside MF furnace is over 1300℃.In this way, MF furnace is able to perform stable raw material processing even with changes in the slag composition, allowing for flexible adaptation to a wide range of materials.Since 2016, approximately 150,000 tons of waste materials have consistently treated.

Enhancement in exhaust gas washing facility
With the increase in the treatment amount of waste material in the MF furnace, the absolute amount of sulfur content also increased, prompting the enhancement of the desulfurization plant.Sodium hydroxide is used as the alkaline agent in the desulfurization plant, and the exhaust gas is subjected to countercurrent washing using spray.To reduce the exhaust gas velocity during washing, the tower diameter was expanded from 5.1 m to 10 m, allowing for increased exhaust gas volume.The bottom liquid of the desulfurization plant is mostly discharged, but the portion is circulated within the tower to effectively utilize the chemical liquid.
Furthermore, to comply with the mercury emission regulations implemented in 2018, the demercuration plant was installed after the desulfurization plant.Table 5 shows the details of the washing plant.In the demercuration plant, an oxidizing agent is used to collect mercury from the gas into the liquid, and countercurrent washing has performed in the similar method.

Figure 3 .
Figure 3. Flowsheet of MF Process at the point of Initiating Treatment of PCB waste.

Figure 4 .
Figure 4. Transition of the Treating Amount in MF process.

Figure 5 .
Figure 5.Comparison of material density before and after granulation.

Table 2 .
The Product Amount of MF Process (t/yeat).

Table 3 .
Changes in Rotary Dryer Equipment specifications.

Table 4 .
Transition of slag composition.

Table 5 .
Specifications of desulfurization and demercuration plant.