Purification and Recovery of Hot-Dip Galvanizing Slag via Supergravity-Induced Cake-Mode Filtration

Abstract

The elimination and retrieval of slag produced during the hot-dip galvanizing process are crucial in reducing plating defects and enhancing economic efficiency. Hot-dip galvanizing slag can be separated and purified efficiently by using graphite carbon felt filtration in a supergravity field. The effects of the gravity coefficient (G), separation temperature (T), and separation time (t) on the separation efficiency were investigated. Under the optimal conditions as G = 300, T = 460 °C, and t = 120 s, these conditions yielded filtered zinc with 0.022 wt% Fe and 1.097 wt% Al. The separation efficiencies achieved were 87% for the acquisition ratio of filtered zinc (A_Zn), 93.67% for the recovery ratio of zinc (R_Zn), and 96.01% for the loss ratio of iron (L_Fe). Based on these laboratory findings, an amplified centrifugal separation apparatus was conceptually designed for future online separation and recycle of zinc slag on an engineering scale. The filtered zinc obtained from this apparatus contained 0.027 wt% Fe and 1.844 wt% Al, while the recovery ratio of zinc (R_Zn) and the loss ratio of iron (L_Fe) achieved 85.97% and 95.47%, respectively.

1. Introduction

Hot-dip galvanizing is a common corrosion-preventive treatment for metals [1,2,3]. It is widely used for its strong corrosion protection, low maintenance, high aesthetics, and malleability, and hot-dip galvanizing accounts for nearly half of all zinc used [4,5,6]. However, during continuous hot-dip galvanizing process, the zinc bath is often saturated with iron and hot-dip galvanizing slag inevitably precipitates. This slag is composed of Fe-Zn, Fe-Al, or Fe-Al-Zn intermetallic compounds formed at high temperatures between the dissolved Fe and molten Zn in the zinc bath or with the addition of Al to the zinc bath [7,8,9,10]. For the batch hot-dip galvanizing process, because its aluminum content is much lower, the Fe-Al phase slag will not be formed in the galvanizing bath, and only the hard zinc of Fe-Zn intermetallic compounds will be produced. Therefore, this study only focuses on the continuous hot-dip galvanizing process. In production, hot-dip galvanizing slag is generally removed by manual or mechanical methods since it harms the surface quality of the galvanizing layer [9,10,11,12]. Approximately 12% to 15% of metal zinc is inevitably carried in the process of fetching slag, resulting in a significant increase in galvanizing costs. Apparently, hot-dip galvanizing slag with a metal zinc content of more than 85% is a secondary resource with significant economic recovery value [13]. Recovery and utilization of zinc slag can extend the service life of zinc resources, reduce production energy consumption, reduce environmental pollution, and increase the economic benefits of plants.

Currently, there are several methods available for the recovery and treatment of galvanizing slag, such as electrolysis [14,15], vacuum distillation [16,17,18], chemical [19,20,21], and Welz methods [22,23]. Although these methods can recover hot-dip galvanizing slag, they still have several drawbacks that hinder their industrial application. For example, the electrolysis method requires frequent purification of the electrolyte due to the presence of iron impurities, which significantly increases costs. Vacuum distillation is a widely used method for separating zinc from zinc slag, but its production costs are high and productivity is low. The chemical and Welz methods involve a complex process of dissolving zinc slag in concentrated acid or ammonia, which is not suitable for mass production. Therefore, it is crucial to find a highly cost-effective and convenient way to recover zinc slag.

Supergravity is defined as significantly higher than the normal gravitational acceleration on the ground (9.81 m/s²). This technique utilizes a rotating centrifuge operating at high speed to generate supergravity, enhancing mass transfer and migration of non-homogeneous phases [24]. The relative velocity of the two phases is mainly determined by the buoyancy factor Δρg and the fluid viscosity, where Δρ represents the density difference and g denotes the gravitational acceleration. Supergravity can accelerate the separation between different phases by increasing, even if the density difference between the two phases is small. This technique has been successfully applied to remove impurities from alloys or metals. Under the supergravity field, by using graphite carbon felt (CFF) as the filtration medium, Sun Ningjie successfully recovered aluminum matrix alloys and SiC particles from scrap aluminum matrix composites (AMCs) [25], and Meng Long used supergravity to recover valuable metals (Pb, Sn, Zn, Cu) from granular computer printed circuit boards [26]. Meanwhile, by using Al₂O₃ Ceramic Filter Plate (Al₂O₃ CFP) as the filter medium, Sun Ningjie successfully removed the impurity particles from galvanizing zinc liquid [27] by super gravity-induced filtration. Sun Bolin used the supergravity induction technology to purify the scrap 1060 aluminum alloy containing iron and silicon through the Al₂O₃ CFp [28]. In summary, Supergravity can quickly and efficiently separate valuable components, and has the advantages of high efficiency, simple process, and low emission. Related studies have been performed in our laboratory [10], but no optimization of the parameters and no studies for industrial applications have been performed. Different galvanizing plants have different galvanizing slag, and the optimal conditions applicable to the production line are different.

This study aims to establish a method for the recovery and treatment of hot-dip galvanizing slag under the supergravity field. It is investigated through two dimensions: laboratory exploratory experiments and engineering experiments. The effects of process parameters on the laboratory separation effect were mainly investigated, and the purification mechanism was analyzed. Based on this, engineering-scale equipment for industrial applications was designed and developed, and validation tests were performed in the laboratory.

2. Experimental

2.1. Raw Material

The raw material investigated in this study is hot-dip galvanizing surface slag acquired from China Baowu Iron & Steel Co. Ltd. (Wuhan, China). To ensure uniform composition, the raw material is melted in a muffle furnace with proper stirring and then cooled in the furnace. For the subsequent supergravity separation test, the raw material was processed into 20 mm diameter samples using wire-electrode cutting, as depicted in Figure 1.

Table 1. Chemical composition of raw hot-dip galvanizing slag (wt%).

Zn	Al	Fe
91.84	7.68	0.48

illustrates the chemical composition of the hot-dip galvanizing slag, where the oxygen content in the raw material is insignificant and therefore disregarded. The contents of the other three principal elements, namely Zn, Al, and Fe, are normalized. Zn is the primary component with a content of 91.84 wt%, followed by Al and Fe with a content of 7.68 wt% and 0.48 wt%, respectively.

2.2. Apparatus

Schematic and physical diagrams of the centrifugal apparatus are shown in Figure 2, with a resistance heating furnace and counterweights symmetrically fixed on the horizontal rotor. This apparatus has several advantages, such as small volume, wide applicability, convenient operation and maintenance, and low energy consumption. It has two centrifugal tanks, with one used to heat and centrifuge the sample (separation tank) and the other used to balance the equipment and maintain the centrifugal process’s stability (counterweight tank). The resistance furnace is composed of Cr₂₇A_l7Mo₂ resistance wire for heating and alumina fiber used as the insulating layer. The counterweight tank must have the same weight and center of gravity as the separation tank. The temperature is controlled by a programmed controller with a B-type thermocouple, and the observation accuracy range is ±3 °C.

The gravity coefficient (G), which is defined as the ratio between centrifugal and normal gravity acceleration, was calculated using Equation (1).

$$G=\frac{\sqrt{g^2+{({\omega }^{2}r)}^2}}{g}=\frac{\sqrt{g^2+{(\frac{N^2{\pi }^{2}r}{900})}^2}}{g}$$

(1)

where g is normal gravity acceleration equal to 9.8 m/s²; w is the angular velocity (in rad/s); r is the distance from the axis to the sample center (equal to 0.25 m in this work); and N is a rotating speed (in r/min).

2.3. Experimental Procedures

To conduct the supergravity separation experiment, approximately 35 g of the sample was placed in the bottom of the upper crucible with graphite carbon felt, as depicted in Figure 1. The graphite crucible was then heated to the target separation temperature (T) in the furnace and held for 30 min before the centrifuge was started and operated at the specified speed (N = 0–1892 r/min, G = 1–1000) for the corresponding separation time (T). Once the centrifuge was switched off, the graphite crucible was immediately removed and cooled in air. The filtered zinc and residue were then separated into the lower and upper crucibles, respectively, in preparation for further characterization.

2.4. Selection of Filter Medium

From the above introduction, it can be seen that in the super gravity field, different filtration medium are required for different samples to achieve efficient separation and purification. So the selection of filter medium plays a crucial role in thermal filtration. At present, there are two main filtration principles that are suitable for different impurity content: deep-bed filtration for low impurity content (<0.1%) and cake-mode filtration for high impurity content (>1%), as depicted in Figure 3. Al₂O₃ ceramic filter plates are often used as deep-bed filter media (Figure 3a), while carbon fiber felt (CFF) is commonly used as cake-mode filter media in the laboratory (Figure 3b). The principle of deep-bed filtration is that the impurities smaller than the media void can enter the media inside and be trapped by the long and tortuous pore channels and eventually adhere to the medium. Deep-bed filtration has no filter cake formation. Cake-mode filtration refers to the gradual deposition and bridging of impurity particles on the filter media, eventually forming a filter cake. What really plays the role of the filter medium is the filter cake itself, through which the subsequent molten metal is filtered, which also prevents a large number of impurity particles from blocking the pores of the filter medium.

Hot-dip galvanizing slag belongs to the latter raw material with high content of impurities. In order to avoid the problems of low separation efficiency caused by the blockage of pores of the ceramic filter plate, this study proposes to adopt cake-mode filtration in order to achieve better impurity removal, and CFF is selected as the filter medium to form the filter cake. In this experiment, a 5 mm thick layer of carbon fiber felt (CFF) was utilized as the filter medium. The CFF was procured from Jing Hang Te Tan Ltd. (Neimenggu) in China and possessed the following fundamental properties: bulk density of 0.171 g/cm³, real density of 1.82 g/cm³, and an average fiber diameter of 17 μm.

2.5. Characterization

The raw material, filtered zinc, and residue are each longitudinally divided into two parts along the central axis. To investigate the distribution, composition, and morphology of the impurity phase, one part is polished and subjected to analysis using scanning electron microscopy (SEM; MLA250, FEI Quanta, Hillsboro, OR, USA) and X-ray diffraction (XRD, Smartlab, Rigaku, Tokyo, Japan). The second part is subjected to analysis of element content using inductively coupled plasma emission spectrometry (ICP-OES, Optima 7000DV, Perkin Elmer, Waltham, MA, USA) for both the filtered zinc and residue. Image J software (1.53k version) is utilized for pore analysis of residue and statistical analysis of the average size of inclusions on obtained electron microscopy images. By comparing the results with those of the raw material, the effect of supergravity separation and purification can be elucidated.

To evaluate the recovery of filtered zinc and removal of iron from hot-dip galvanizing slag under different experiment conditions, the acquisition ratio of filtered zinc (A_Zn), the recovery ratio of zinc (R_Zn), and the loss ratio of iron (L_Fe) were defined, and calculated as shown in Equations (2), (3) and (4), respectively.

$$A_{Zn}=\frac{m_f}{m_o}\times 100\%$$

(2)

$$R_{Zn}=\frac{m_f\times {\omega }_{f,Zn}}{m_o\times {\omega }_{o,Zn}}\times 100\%$$

(3)

$$L_{Fe}=1-\frac{m_f\times {\omega }_{f,Fe}}{m_o\times {\omega }_{o,Fe}}\times 100\%$$

(4)

where $m_f$ and $m_o$ are weights of the filtered zinc and raw material, respectively, ${\omega }_f$ and ${\omega }_{o }$ are mass fractions of Zn or Fe in the filtered zinc and raw material, respectively.

3. Results

3.1. Characterization of the Raw Material

Figure 4 presents the XRD image of the hot-dip galvanized slag, and Figure 5 illustrates the SEM images and EDS point analysis results of the characteristic positions in the hot-dip galvanizing slag. Based on Figure 4 and Figure 5, it can be concluded that the main components inside the raw material are metallic zinc and Fe₂Al₅Zn_X inclusions. The low content of ZnO cannot be analyzed by XRD and can be observed in electron microscopy images. The dark contrasting Fe-Al-Zn particles are polygonal and uniformly distributed on the light contrasting metallic Zn substrate, and the ZnO inclusions are very small and serpentine on the substrate. Therefore, hot-dip galvanizing slag is a valuable secondary resource with a large amount of metallic zinc within it and has great potential for recovery.

Based on the SEM images shown in Figure 6, significant differences were observed among the three different positions (top, middle, and bottom) of the raw hot-dip galvanizing slag. The Fe-Al-Zn particles were mainly present in the top and middle positions and were observed to be large in size, while the particles in the bottom position were few and small in size. This phenomenon can be attributed to the fact that the density of Fe-Al-Zn particles (4.2 g/m³) [29] is lower than that of molten zinc (6.5 g/m³) [30], resulting in the particles moving towards the surface of the molten zinc and aggregating to form surface slag. The average diameter of Fe-Al-Zn particles at different positions was calculated using ImageJ software, and the results are presented in

Table 2. Average inclusion diameter in different positions of raw hot-dip galvanizing slag (μm).

Top	Middle	Bottom
46.83	42.26	14.67

. It was found that the average diameter of the top particles was slightly larger than that of the middle particles (46.83 μm and 42.26 μm, respectively), while the bottom particles were significantly smaller in size, with an average diameter of only 14.67 μm.

3.2. Efficiency of Centrifugal Separation

In this study, the separation and purification of hot-dip galvanizing slag were carried out under a super-gravity field with varying conditions, and the impact of the gravity coefficient (G), separation temperature (T), and separation time (t) on the elimination of Fe-Al-Zn particles from hot-dip galvanizing slag was investigated.

3.2.1. Effect of the Gravity Coefficient

Figure 7 illustrates the impact of G on the macroscopic separation at T = 460 °C and t = 180 s. As depicted in Figure 7a,b, the hot-dip galvanizing slag sample could not be filtered into the lower crucible at G = 1 or G = 30, as the centrifugal force was not strong enough to overcome the resistance of the CFF, impeding separation and purification. As the gravity coefficient increases to 80, molten zinc begins to flow through the CFF into the lower crucible, while some of the unfiltered residue is trapped in the upper crucible. When G exceeds 300, the volume of filtered zinc in the lower crucible no longer significantly increases, as indicated in Figure 7c–e.

Table 3. Effect of the gravity coefficient on the chemical composition of the filtered zinc and separation efficiency.

Gravity Coefficient	AZn	RZn	LFe	Chemical Composition of the Filtered Zinc (wt%)
				Zn	Fe	Al
80	77.41	82.96	96.29	98.426	0.023	1.551
100	83.19	89.36	96.19	98.655	0.022	1.323
200	86.52	93.01	96.21	98.723	0.021	1.254
300	87.77	94.31	95.79	98.686	0.023	1.291
400	87.70	94.22	95.98	98.662	0.022	1.314
500	87.64	94.40	95.62	98.928	0.024	1.048
800	87.68	94.42	95.80	98.900	0.023	1.077
1000	87.78	94.45	95.61	98.819	0.024	1.157

and Figure 8 illustrate the influence of the gravity coefficient on the chemical composition of the filtered zinc and separation efficiency (A_Zn, R_Zn and L_Fe) at T = 460 °C and t = 180 s. It can be observed that the gravity coefficient has a negligible impact on the chemical composition and L_Fe of the filtered zinc. The chemical composition of the filtered zinc remains highly consistent throughout the range of gravity coefficients (G = 80–1000), with average contents of Zn, Al, and Fe of approximately 98.725 wt%, 1.252 wt%, and 0.023 wt%, respectively. The variation in L_Fe is minor and consistently fluctuates between 96.29 wt% and 95.61 wt%. In contrast, A_Zn and R_Zn are significantly influenced by the gravity coefficient. It is evident that when G is only 80, A_Zn and R_Zn are 77.41% and 82.86%, respectively. As the gravity coefficient increases, A_Zn and R_Zn also increase markedly with a similar trend. At a gravity coefficient of 300, A_Zn and R_Zn reach 87.77 wt% and 94.31 wt%, respectively. Beyond a gravity coefficient of 300, there is no significant change in A_Zn and R_Zn, which stabilize at approximately 97% and 94%, respectively. Hence, the gravity coefficient of 300 is a more appropriate working parameter.

Figure 9 presents the electron micrographs of filtered zinc obtained at G = 300. The micrographs reveal that, in comparison to the raw material (Figure 6), most of the Fe-Al-Zn particles were entirely eliminated through supergravity separation (Figure 9a) and are not easily discernible under low magnification electron microscopy. The number and size of inclusion particles were considerably reduced, leading to a significant enhancement in the purity of the filtered zinc. At high magnification, only a small quantity of fine Fe-Al-Zn particles, which are usually less than 5 μm in size, were found in the filtered zinc. The morphology of these particles is similar to that in the original sample, with a polygonal shape (Figure 9b).

The SEM images displayed in Figure 10 reveal that the residue obtained under different gravity coefficients contains a significant number of Fe-Al-Zn particles along with numerous pores resulting from the separation of molten zinc into the lower crucible. The porosity of the residue samples was determined by analyzing the volume fraction of pores in the SEM images using the ImageJ software, and the results are presented in

Table 4. Porosity at different gravity coefficients (%).

G = 80	G = 300	G = 1000
27.28	37.79	42.84

. The data show that the residue porosity is 27.28% at G = 80 and gradually increases to 37.79% and 42.84% as the gravity coefficient increases to 300 and 1000, respectively. Thus, it can be observed that an increase in the gravity coefficient leads to a corresponding increase in the residual porosity, which can be attributed to the separation of more molten zinc into the lower crucible at higher gravity coefficients, resulting in the formation of more pores in the residue.

In order to further investigate the phase composition and distribution in the residue, scanning electron microscopy and porosity calculation are performed on the position (A–C) of the residue shown in Figure 7, and the results are presented in Figure 11 and

Table 5. Porosity in different positions of the residue at G = 300 (%).

Top	Middle	Bottom
53.86	36.31	22.68

. It is evident that the top position of the residue contains minimal remaining metal Zn, and the escape of molten Zn leads to the formation of numerous pores between Fe-Al-Zn particles, as depicted in Figure 11a. The porosity at this position is measured to be 53.86% using the ImageJ software. Conversely, the porosity is reduced to 36.31% and the metal matrix is increased, filled with Fe-Al-Zn particles and meandering ZnO, as shown in Figure 11b. The bottom position of the residue (depicted in Figure 11c) contains the highest metal zinc matrix, with the fewest Fe-Al-Zn particles and the lowest porosity at 22.68%.

In summary, it can be seen that the porosity of the residue gradually increases from the lower to the upper, which is attributed to the fact that the metal zinc in the top position of the sample is first separated into the lower crucible during supergravity separation. As the separation proceeds, a large number of Fe-Al-Zn particles accumulate on the carbon felt, resulting in an increase in filtration resistance, and eventually causing the sample to collapse on the carbon felt. Hence, the highest porosity is found in the top position of the residue, while the lowest porosity is found in the bottom position of the residue near the CFF.

3.2.2. Effect of Separation Temperature

Table 6. Effect of separation temperature on the chemical composition of the filtered zinc and separation efficiency.

Separation Temperature (°C)	AZn	RZn	LFe	Chemical Composition of the Filtered Zinc (wt%)
				Zn	Fe	Al
430	83.22	89.49	96.53	98.755	0.020	1.225
440	86.42	92.90	96.22	98.723	0.021	1.256
460	87.77	94.31	95.79	98.686	0.023	1.291
480	88.08	94.56	95.60	98.602	0.024	1.374
500	88.15	94.79	95.23	98.763	0.026	1.211
550	88.44	95.06	94.29	98.718	0.031	1.251
600	88.49	95.09	93.92	98.690	0.033	1.277

and Figure 12 demonstrated the effect of separation temperature on the separation efficiency and chemical composition in the hot-dip galvanizing slag under G = 300 and t = 180 s. Overall, A_Zn and R_Zn varied in a similar trend with increasing temperature, with values increasing from 83.22%, 89.49% at 430 °C to 87.77% and 94.31% at 460 °C, respectively. After reaching 460 °C, the values of A_Zn and R_Zn tend to stable and no longer vary with the increase of temperature. Conversely, η_iron exhibits an opposite trend. As the temperature increases from 430 °C to 600 °C, L_Fe gradually declined from 96.53% to 93.92% and the Fe content in filtered zinc increased from 0.020 wt% to 0.033 wt%, which is mainly attributed to the fact that more Fe is centrifuged to the lower crucible as the solubility of Fe in the molten zinc elevated with the rise in temperature. Considering the temperature of the zinc bath and the online slag removal technology, 460 °C is regarded as the optimal separation temperature for the online treatment of hot-dip galvanizing slag by supergravity.

Figure 13 shows the SEM images of different positions in the filtered zinc at different separation temperatures. It can be observed that the number of Fe-Al-Zn particles tends to decrease from the top to bottom region of samples, and the amount and average diameter of Fe-Al-Zn particles in each position decreases with reducing temperature. More Fe-Al-Zn particles are present in the top position of filtered zinc compared to the other position, which is owing to the fact that the density of Fe-Al-Zn particles is smaller than the density of molten zinc and hence migrates towards the top of the filtered zinc.

Table 7. Average inclusion diameter in different positions of filtered zinc at different separation temperatures (μm).

Position	Raw Material	600 °C	550 °C	460 °C
Top	46.83	19.84	11.30	3.5
Middle	42.26	14.16	9.19	3.3
Bottom	14.67	13.91	7.10	3.0

and Figure 14 show the average diameter of the inclusions at different positions of the filtered zinc under different temperatures. Compared to the raw material, a significant reduction in the inclusions size was observed in all three positions after super gravity. At the same temperature, the number and size of inclusions in each sample presents a gradient decreasing distribution from top to bottom. The size of the inclusions at the same position diminishes with decreasing temperature, which also coincides with the trend of L_FE in Figure 12. When the separation temperature was 550 °C and 600 °C, the inclusions in the upper and middle part decreased from 45 μm to approximately 15 μm. the inclusions in the top and middle position were only approximately 3 μm. Therefore, 460 °C is a more desirable separation temperature.

3.2.3. Effect of Separation Time

At G = 300 and T = 460 °C, the effect of the separation time on the chemical composition of the filtered zinc and separation efficiency was studied, and the results are shown in

Table 8. Effect of separation time on the chemical composition of the filtered zinc and separation efficiency.

Separation Time (s)		AZn	RZn	LFe	Chemical Composition of the Filtered Zinc (wt%)
					Zn	Fe	Al
10	85.12		91.27	96.63	98.475	0.019	1.506
60	86.00		92.39	96.24	98.664	0.021	1.315
120	87.00		93.67	96.01	98.881	0.022	1.097
180	87.77		94.31	95.79	98.686	0.023	1.291
300	88.00		94.47	95.60	98.592	0.024	1.384

and Figure 15. It can be observed that the separation time has a slight effect on the chemical composition of filtered zinc and separation efficiency. The elemental content of Zn, Fe and Al in filtered zinc fluctuated in the interval 98.475%~98.881%, 0.019 wt%~0.024 wt% and 1.097 wt%~1.506 wt%, respectively, with small variations as the time increases.

For the separation efficiency, A_Zn and R_Zn increased slowly from 85.12% and 91.27% to 88.00% and 94.47%, respectively, as the separation time increased from 10 s to 300 s, while there was a slight decrease in L_Fe, from 96.63% to 95.60%. In summary, it can be concluded that efficient acquisition ratio of filtered zinc can be achieved in the supergravity field with only 10 s, which proves the efficiency of super gravity technology is efficient in enhancing multiphase separation.

As time increases, A_Zn and R_Zn have similar trends and slowly increase from 85.12% and 91.27% to 88.00% and 94.47%, respectively, while L_Fe has a slight decrease from 96.63% to 95.60%. when t = 10 s, A_Zn and R_Zn are 85.12% and 96.63%, respectively. As the time continues to increase, A_Zn, R_Zn and L_Fe tend to level off. The experimental results show that the separation of metallic zinc in hot-dip galvanizing slag can be completed in 10 s time by using the supergravity technique, which demonstrates the high efficiency of the supergravity technique in enhancing multiphase separation.

3.3. Centrifugal Separation on an Engineering Scale

To realize the practical application of separation and purification of hot-dip galvanizing slag under supergravity, engineering-scale equipment was designed and developed to verify the practical application of supergravity separation, as shown in Figure 16. The engineering supergravity separation of hot-dip galvanizing slag is mainly divided into four procedures: (a) dipping into the zinc bath, (b) absorbing slag online, (c) lifting out of zinc bath, and (d) discharging slag, as illustrated in Figure 17. Engineering separation experiments of hot-galvanizing zinc dross was proceeded at G = 50 with the T = 460 °C and t = 120 s, after which the filtered zinc and residue were identified. The filtered zinc contained 0.027 wt% Fe and 1.844 wt% Al while the value of R_Zn and L_Fe, respectively, achieved 85.97% and 95.47%. There is a deviation existing between engineering separation experiments and laboratory exploratory experiments. Compared to the raw material (Figure 6), the amount of inclusions in the filtered zinc is drastically reduced and the size is small. The filtered zinc is relatively pure (Figure 18a), containing only 0.027% iron, which can be returned directly to the galvanizing bath for processing. The loose and porous residue is filled with a significant number of Al-Fe-Zn particles and a little metal zinc (Figure 18b), which can be applied as a composite Aluminum-zinc-iron (PAZF) coagulant for the purification of wastewater. Such coagulants are very effective in removing turbidity and organic matter [31]. Filtered zinc and residue from hot-dip galvanizing slag after ultra-gravity separation can all be utilized in a highly efficient manner.

4. Discussion

To ensure that the surface slag can complete the separation process in the supergravity field, the relationship between the filtration resistance (P_r), centrifugal pressure (P_c) and the gravity coefficient G also needs to be analyzed. the centrifugal pressure generated by the supergravity field is proportional to the mass of the hot-dip galvanizing slag on the surface of the graphite carbon felt [10,32]. In order to obtain clean filtered zinc, the filtration resistance generated on the surface of the graphite carbon felt should be lower than P_c, which is calculated as follows:

$$P_c=\frac{F_c}{s_i}=\frac{m\times g\times G}{s}$$

(5)

where F_c denotes the centrifugal force, m denotes the weight of hot-dip galvanizing slag in the crucible and s_i denotes the internal surface area of graphite crucible. P_r can be defined by on the capillary law:

$$P_r=-\frac{4\sigma \mathit{cos}\theta }{d}$$

(6)

where σ denotes the surface tension of molten zinc (0.753 N/m at 460 °C [33]); θ is the wetting angle of zinc liquid on graphite carbon felt (137° at 460 °C [34]); and d is the average pore size of the filter medium, 17 μm. Based on the capillary law, the equation for calculating the P_r of permeation of liquids zinc through graphite carbon felt was introduced [35]:

$$P_r=-4\sigma \cos \theta \frac{V_f}{\left(1-V_f\right)D}$$

(7)

where D denotes the average fiber diameter (17 μm [30]); V_f is the fiber volume fraction of graphite carbon felt (0.094 [10]).

In order to complete the solid–liquid separation of the impurities from the hot-dip galvanizing slag, the centrifugal pressure P_c on the zinc liquid needs to be higher than P_r of 13.443 kPa (calculated by the Formula (7)). As the separation proceeds, the amount of hot-dip galvanizing slag above the CFF decreases, lowering the P_c. The variation in the residue in the upper part of the crucible in relation to the P_c at different gravity coefficients is illustrated in Figure 19. When the gravity coefficient is greater than 80, the zinc liquid can flow through the graphite carbon felt. As the separation proceeds, the remaining amount of zinc liquid in the upper part of the graphite carbon felt also decreases. When the centrifugal pressure on the residue is less than 13.443 kPa, the zinc liquid cannot pass through the graphite carbon felt. The theoretical percentages of the residue and filtered zinc at different gravity coefficients are listed in

Table 9. The theoretical percentages of the residue and filtered zinc at different gravity coefficients.

Gravity Coefficients	30	80	200	300	500	800	1000
The theoretical percentages of the residue (%)	100	39	16	10	6	4	3
The theoretical percentages of the filtered zinc (%)	0	61	84	90	94	96	97

. The proportion of residue becomes smaller as the gravity coefficient increases, while the percentage of filtered zinc becomes larger. With the increase of the gravity coefficient from 50 to 300, the amount of residue drops steeply from 100% to 17% while the percentage of filtered zinc increased sharply from 0 to 83%. With increasing gravity factor to 800, the amount of residue in the upper part of the carbon felt continues to decrease slowly to 6%, while the percentage of filtered zinc gradually increases to 94%. It is apparent that the larger the gravity coefficient, the better the supergravity separation of hot-dip galvanizing slag, which echoes the previous conclusion.

In the current factory, hot-dip galvanizing slag is mainly fished out manually or by robots, and there is still a large amount of metallic zinc in the slag they fished out, which will cause a serious waste of zinc resources, and will also increase the operating costs of the factory. At the same time, this method also requires labor cost and the purchase and maintenance cost of the robot, which is also very expensive. In this study, an industrialized design has been carried out based on laboratory work and scaled-up experiments have been performed for engineering. There are expectations to address the problem of wasted zinc resources and high costs in galvanizing plants. The cost of this experimental equipment is low, and does not require manual maintenance, and the zinc slag fished out contains only a small amount of metallic zinc, which can greatly reduce the loss of metallic zinc, saving the cost of the entire production line. Therefore, from the perspective of the whole galvanizing market, this experimental study is still very meaningful and economical.

5. Conclusions

In this study, the hot-dip galvanizing slag was subjected to a detailed characterization. Supergravity was applied to separate and reduce the impurity content of the hot-dip galvanizing slag to improve the metal zinc purity so that it can be returned online directly to the zinc bath for galvanizing. The effects of separation temperature, separation time, and the gravity coefficient were systematically investigated. The formation process of filter cake and the relationship between the gravity coefficient and filtration resistance were analyzed and clarified. The main findings are as follows.

(1)

Supergravity has been verified as a very efficient and economical means of separating and recovering hot-dip galvanizing slag. The optimal conditions for laboratory conditions are G = 300, T = 460 °C and t = 120 s, under yielded the filtered zinc containing 0.022 wt% Fe and 1.097 wt% Al and separation efficiencies reached 87% for A_Zn, 93.67% for R_Zn and 96.01% for L_Fe.

(2)

With the increase in the gravity coefficient and separation temperature, the acquisition ratio A_Zn and R_Zn increased. L_Fe is minimally affected by the gravity coefficient, but decreases significantly as the separation temperature increases. The separation time has little effect on both the filter zinc yield, and the iron removal rate.

(3)

Cake-mode filtration is the most suitable filtration mechanism for hot-dip galvanizing slag with high impurity content. The filtration parameters and the relationship between filtration resistance and centrifugal pressure were analyzed.

(4)

Based on the separation conditions obtained from exploratory laboratory experiments, the industrial scale-up equipment was designed and fabricated, and industrial scale-up experiments were performed. It was concluded that the filtered zinc contained 0.027 wt% Fe and 1.844 wt% Al, while the values of R_Zn and L_Fe reached 85.97% and 95.47%, respectively. A fully utilized process route was also designed to maximize the benefits of the hot-dip galvanizing slag.