Production increase of high-grade crude zinc oxide pellets at Shisaka Smelting Co., Ltd.

Shisaka Smelting Co., Ltd. has recovered crude zinc oxide pellets from an electric arc furnace dust (EAF dust) by the Waelz kiln process since 1977. The EAF dust processing line consists of a reduction roasting process, a Waelz oxide washing process, and a drying/calcination process. In late years, about 90,000 tons of EAF dust is processed annually at Shisaka. The recovered zinc is sold to zinc smelters as crude zinc oxide pellets, and the amount of production of the pellets is about 40,000 tons per year. Two types of zinc oxide pellets are produced, and the product for electro-refinery requires high quality, especially in fluorine reduction. Most of fluorine are distributed to the clinker in a reduction roasting process, and others are distributed to the Waelz oxide. Conventionally, high-grade crude zinc oxide pellets were produced by washing the Waelz oxide and by volatilizing and separating fluorine under high temperature operation in a drying rotary kiln. However, formation of accretion in the kiln and damage of the shell and refractories of the kiln often occurred under the influence of high temperature operation, and stable consecutive operation of the kiln was difficult. To improve this situation, the fluorine input was reduced by compounding management of EAF dust in the reduction roasting process, and the material residence time in the drying rotary kiln was extended by reducing rotary speed. These improvements resulted in stable consecutive operation under low temperature conditions and increased production of high-grade crude zinc oxide pellets. To improve this situation, the fluorine input was reduced by compounding management of EAF dust in the reduction roasting process, and the material residence time in the drying rotary kiln was extended by reducing rotary speed. These improvements resulted in stable consecutive operation under low temperature conditions and increased production of high-grade crude zinc oxide pellets.


Introduction
Recently, climate change due to global warming has been recognized and carbon neutrality which reduces greenhouse gas emissions to zero, has been attracting attention to prevent from the global warming.In this situation, steel production of electric arc furnace (EAF) makes it advantageous for carbon neutrality compared to the blast furnace (BF), therefore, shift from BF to EAF is in full swing in Japan.Steel scrap is melted in EAF, and generated dust is called EAF dust, which contains 10 to 30% Zn because steel scrap includes galvanized steel sheets.
EAF dust contains lead and cadmium which recognized as hazardous metal, so its treatment and disposal are strictly regulated.However, many countries and regions still do not have sufficient facilities to treat EAF dust, and the dust is landfilled or accumulated without treatment for zinc recovery and recycling.Therefore, it is necessary to promote stable treatment of EAF dust by developing treatment technologies [1] .
EAF dust treatment process is divided into pyrometallurgical and hydrometallurgical processes.The Waelz process using a rotary kiln is the main pyrometallurgical process, and another main process is rotary heath furnace (RHF) process.The ammonium chloride leaching process is used as a hydrometallurgical process [2,3] .
In 1977, Shisaka Smelting Co., Ltd. of the Sumitomo Metal Mining (SMM) Group started zinc and lead recycling operations using the Waelz process to recover zinc and lead from dust generated in EAF dust as crude zinc oxide (pellets) for use in an ISP smelter.Figure 1 shows the EAF dust process chart at our plant.This process consists of a reduction, washing and dewatering, and drying process.Figure 2 shows the changes in amount of treated EAF dust at our plant.We started operations in 1977 using a system with one Reduction Rotary Kiln (RRK), washing facilities, and one Drying Rotary Kiln (DRK).At that time, the EAF dust treatment capacity was 50,000 tons per year [4] .And its capacity was increased to 120,000 tons per year by installing another RRK in 1992 [5] .In 2004, we modified the washing facilities to improve crude zinc oxide pellets production capacity to 50,000 tons per year, which enabled EAF dust treatment capacity of 120,000 tons per year.Recently, approximately 90,000 tons of EAF dust is treated per year.

Production of high-grade crude zinc oxide pellets
The recovered zinc from EAF dust is sold to zinc smelters as crude zinc oxide pellets, and there are two types of crude zinc oxide pellets marketed: one type is for ISP and the other type is for electrolytic smelting.The latter is called high-grade crude zinc oxide pellets, which are high valued as containing less than one-tenth of fluorine and chlorine compared with crude zinc oxide pellets for ISP.Therefore, increased production of high-grade crude zinc oxide pellets leads to an expansion of sales channels and improvement in profit margins.
In order to reduce the fluorine content of high-grade crude zinc pellets less than one-tenth of crude zinc oxide pellets for ISP, additional operation control is implemented as below.・Controlling fluorine content by blending EAF dust ・Increasing in volatilization rate of fluorine due to high temperature operation of the DRK However, there were two problems for production increase.One is that fluorine in the crude zinc oxide pellets frequently exceeds the standard value.Another is that operation rate is decreased due to formation of accretion and damage of refractories in high temperature operation.This report describes the improvement of those problems by reforming the blending method and lowering temperature of the DRK, which were implemented to stably increase the production of high-grade crude zinc oxide pellets.

Blending F/Zn control
Three kinds of EAF dust are blended to keep zinc content constant and fed to RRK.The amount of the Waelz oxide produced from the RRK is proportional to the zinc content of blended EAF dust.
During the production campaign of high-grade crude zinc oxide pellets, we focused only on the blended fluorine content of EAF dust to satisfy the fluorine content of the pellets.However, when the blended zinc content of EAF dust was low, the proportion of fluorine content of Waelz oxide is increased.Although fluorine in the Waelz oxide is removed in the DRK, producing high fluorine content of the Waelz oxide leads to lowering product yield rate.Therefore, we set F/Zn ratio as a new parameter to keep the fluorine content of the Waelz oxide below to a fixed value stably.

Extension of residence time in the kiln
To prevent damage of the refractories and formation of accretion, which cause decreasing operation rate, we have developed an operation method that lowers the operating temperature of the DRK while keeping the amount of volatilized fluorine.
Low operating temperature causes the reaction rate of fluorine volatilization decreases and the volume of fluorine volatilization becomes low.Komori et al [6] showed that the volatilization behaviour of fluorine depended on the calcining temperature and the volatilization rate of fluorine declined by about from 15 to 30% as the temperature decreases by 100 ℃ in the range of from 950 to 1,150 ℃.Based on these findings, we investigated the residence time of kiln to keep the volatilization rate of fluorine.

Blending F/Zn control
Figure 3 shows the changes in fluorine content of the Waelz oxide and blending F/Zn ratios.The F/Zn ratio varied in the range of 0.02 to 0.03 before April 2021, because we controlled only fluorine content of EAF dust.Therefore, since June 2021, the F/Zn ratios have been controlled below to 0.013, and fluorine content of the Waelz oxide has been also below 0.2%.Figure 4 shows the fluorine content of the pellets before and after the change of control.After controlling blending F/Zn ratios, the fluorine content of the pellets has been decreased, and product yield rate has been improved.

Extension of residence time in the kiln
Lowering the operating temperature decreases the volatilization rate of fluorine and increases fluorine content of the pellets as formula (1) and Arrhenius formula (2).Previous study [6] showed that reducing the operating temperature by approximately 100 °C from 900-1,000 °C to 800-900 °C decreased the volatilization rate in the range of approximately 15-30 %.
To avoid the decrease of the volume of fluorine volatilization, the residence time in DRK needs to be extended.The residence time was obtained by Sullivan's formula (3) [7] and is inverse proportion to the rotary speed, so the residence time was extended by reducing the rotation speed.Under low temperature conditions, the volatilization rate of fluorine was reduced by up to 30%, the residence time was extended by 40% to keep a same volatilization rate.
k : the reaction rate coefficient, A : the pre-exponential factor, E : the activation energy, R : the gas constant, T : the absolute temperature.
T : the residence time, L : the length, β : the angle of repose, D : the diameter, S : the inclination, N : the rotation speed.
Figure 5 shows the relationship between the calculated residence time and fluorine volatilization rate at each temperature.By extending the residence time from 70-90 minutes (rotation speed: from 0.8 to 1.0 rpm) to 100-130 minutes (rotation speed: from 0.5 to 0.6 rpm), the fluorine volatilization rate could be maintained at 90-95%, which is equivalent to high-temperature operation.The operation campaign was extended from 4 days to 10 days or more without serious formation of accretion or damage of refractories.Table 1 shows the volatilization rate of fluorine.Lowering the operating temperature resulted in approximately 35% decrease, which close to expected value.

Production increase of high-grade zinc oxide pellets
Figure 6 shows the amount of high-grade crude zinc oxide pellets production.After 2021, annual production increased 4,000 tons per year from 2,000 to 6,000 tons per year without any formation of accretion and damages to refractories during campaign.Fig. 6 The amount of high-grade crude zinc oxide pellets production Table 2 shows the production increasing impact of high-grade crude zinc oxide pellets.By controlling the blending F/Zn ratio, the variation of the fluorine content of high-grade crude zinc oxide pellets was reduced and the product yield rate was improved.About 920 tons per year of high-grade crude zinc oxide pellets which was approximately 23 % of the production increase was achieved.On the other hand, extending the operation campaign by lowering operating temperature and extending the residence time in the kiln about 3,080 tons per year of high-grade crude zinc oxide pellets which was approximately 77 % of the production increase was achieved.

Conclusions
Shisaka Smelting has greatly improved the technology used in its zinc recycling over the 45 years since commencement of EAF dust treatment operation.The high-grade crude zinc oxide pellets production technology reported here, in particular, has been indispensable for continuing stable treatment of EAF dust, leading to increased production of high-grade crude zinc oxide pellets as well as expansion of sales channels and improvement in profit margins.In Japan, as moves are made toward carbon neutrality,

Fig. 2
Fig.2 Changes in amount of treated EAF dust

Fig. 3
Fig.3 Changes in fluorine content of the Waelz oxide and blending F/Zn ratios

Table 2 .
The production increasing impact of high-grade crude zinc oxide pellets

Table 1
Volatilization rate of fluorine