Preparation of Oxidized Pellets from Sulfuric Acid Residue Containing Zinc and Lead by Chlorination Roasting and Its Mechanism of Dezincing and Lead Removal

Abstract

The utilization of sulfur acid residue is an urgent problem confronting sulfuric acid production enterprises, especially the application of sulfur acid residue (SAR) containing lead and zinc. A method combining chlorination roasting using CaCl₂ with pelletizing for processing SAR containing lead and zinc was used in this study, and the effect of calcium chloride on pelletizing performance was studied; in addition, the removal behavior of lead and zinc was also studied by using polarized light microscopy (Zeiss double Axioskop 40A), X ray diffraction, SEM and EDS. The results showed that CaCl₂ migrated to the surface of the pellets during drying, and this phenomenon resulted in a lower removal rate of lead and zinc inside the pellets than outside the pellets during the preheating phase. When the roasting temperature was 1220 °C, with an increase in the basicity of pellets, the silicate minerals in the pellets gradually decomposed, the hematite particles were gradually refined, and more lead or zinc minerals were exposed, which further increased the removal rate of lead and zinc in the pellets. Finally, the SAR pellets with Pb and Zn removal rates up to 91.33 and 97.88%, and a compressive strength of 2789 N, could be obtained, which is very beneficial to the sustainable development of sulfuric acid mills.

1. Introduction

As a necessity for industrial development, about 37% of all sulfuric acid is produced from pyrite [1,2,3]. The sulfur acid residue, estimated to be more than 2000 tons in 2021, is a byproduct of roasting pyrite to obtain SO₂ [2]. Sulfur acid residue with an iron content of 50%~60% is an important secondary iron-containing resource [4]. Therefore, the efficient recovery of sulfur acid residue for acid production mills could undoubtedly make economic sense.

Non-ferrous metals associated with pyrite, such as PbS, ZnS, CuS, etc., exist as an oxide in sulfur acid residue and cannot be directly separated easily [5,6]. At present, sulfuric acid residue is mainly used as pigments or raw materials for ironmaking [6,7,8]. The latter accounts for over 80% of its large processing capacity [9,10]. Unfortunately, Pb and Zn deteriorate the quality of hot metal and the process of hot metal production [11]. Therefore, it is very necessary to remove lead and zinc before using sulfur acid residue as a blast furnace burden. It has been proposed in the literature that sulfur acid residue can be directly reduced to metal pellets by mixing coke breeze, and lead and zinc can also be reduced and volatilized [12]. Some studies have also proposed that non-ferrous metals in sulfur acid residue could be removed by high-temperature chlorination [13]. Taking energy consumption and its long reaction time into account, the chlorination method is more efficient than the direct reduction method. However, there are few studies on the migration behavior of chlorinated agents in the drying process and the influence of chlorinated agents on the microstructure and composition of pellets, which is not conducive to the application of the chlorinated roasting method in industrial production.

The separation of non-ferrous metals mainly depends on the reaction between chlorinated gases produced by calcium chloride and non-ferrous metals, forming metal chloride with a low boiling point and eventually becoming flue gas [14,15,16]. In the meantime, the addition of calcium chloride directly affects the basicity of pellets. The increasing of the basicity of pellets is conducive to the formation of the liquid phase, so that the liquid phase can fully enclose the Fe₂O₃ grains, resulting in an increase of the quality of pellets [17]. However, further increasing the basicity would reduce the iron content of pellets, while the bound phase in the pellet changes into calcium ferrite [16,17,18,19]. In this research, the effect of calcium chloride on the removal behavior of zinc and lead from sulfuric acid slag was studied, including the migration behavior of calcium chloride during the drying process, and its effect on pellet quality was also studied.

2. Materials and Methods

2.1. Materials

The chemical composition of sulfur acid residue from the pyrite roasting process is shown in

Table 1. Chemical composition of ferrous materials and bentonite/percentage.

Sample	TFe	FeO	SiO2	Al2O3	CaO	MgO	S	P
Magnetite	66.17	27.42	7.31	0.57	0.35	0.37	0.04	0.02
SAR	56.92	14.30	8.59	1.35	1.30	1.71	1.05	0.11
Bentonite	6.20		54.22	14.91	4.22	2.96	0.29	0.09
Sample	K2O	Na2O	Au	Cu	Ag	Pb	Zn	LOI
Magnetite	0.11	0.12	0.10 × 10−4	0.13 × 10−2	5.30 × 10−4	0.01	0.02	−2.68
SAR	0.51	0.13	0.10 × 10−4	0.06	52.52 × 10−4	0.24	1.01	1.05
Bentonite								7.39

. It can be seen that the iron grade of sulfur acid residue was only 56.92%, and the SiO₂ content was up to 8.59%. The sulfur content was as high as 1.05%, and the lead and zinc content were up to 0.24% and 1.01%, respectively. In order to increase the iron content of pellets, magnetite with an iron content up to 66.17% was mixed with the sulfur acid residue in proportion, and the proportion of magnetite concentrate in mixed ore was 55%.

The distribution diagram of some elements in the SAR is shown in Figure 1, which shows that the sulfuric acid slag contained a certain amount of quartz. In addition, the distribution of Zn and S elements had a high coincidence, suggesting the existence of ZnS minerals. The XRD analysis of the SAR in Figure 2 also shows the presence of sphalerite (ZnS) in SAR. However, due to the low content of the lead element, its existent form was analyzed by the chemical dissolution method [20,21]. The mineral phases of zinc and lead in the SAR are listed in

Table 2. Distribution and composition of zinc in sulfur acid residue/percentage.

Minerals	ZnO	ZnS	ZnFe2O4	Total
Zn content/%	0.16	0.46	0.38	1.0
Distributions/%	16.0	46.0	38.0	100

and

Table 3. Distribution and composition of lead in sulfur acid residue/%.

Minerals	PbO	PbS	PbSiO3	Total
Pb content/%	0.13	0.08	0.03	0.24
distributions/%	54.17	33.33	12.5	100

, respectively. Zinc was mainly distributed in ZnO, ZnS, and ZnFe₂O₄, while lead was mainly distributed in PbO, PbS, and PbSiO₃.

The particle size composition of iron-bearing raw materials is shown in

Table 4. Particle size distribution of iron-containing raw materials/%.

Particle Size Composition	+0.15 mm	0.1~0.15 mm	0.074~0.1 mm	0.037~0.074 mm	−0.037 mm
Magnetite	1.91	9.74	10.56	30.18	47.61
SAR	5.93	13.32	6.34	31.89	43.86

. The particle size composition of the iron-bearing raw materials is fine, and the content of −0.074 mm in magnetite and SAR is 77.79% and 74.41%, respectively.

2.2. Methods

The schematic diagram of the test process in this manuscript is shown in Figure 3.

2.2.1. Preparation of Green Balls

The mixing ratio of raw materials is shown in

Table 5. The ratio of raw materials under different basicity.

Basicity	Magnetite	SAR	Bentonite	CaCl2
Nature basicity (0.10)	54.45	44.55	1.0	0
0.15	53.90	44.10	1.0	1.0
0.25	53.08	43.43	1.0	2.5
0.35	52.14	42.66	1.0	4.2
0.45	51.32	41.98	1.0	5.7
0.45	50.49	41.31	1.0	7.2

, and CaCl₂ (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) is added to the pellets. Pelleting was carried out in a disc pelleting machine (diameter 1000 mm, edge height 150 mm). Considering the burning loss of the pellets, the pellets with diameter of 10~18 mm were selected as qualified green balls. When the pelletizing time is 12 min, and the pelletizing moisture is 10%, the drop strength of green balls is guaranteed to be more than 4 times/0.5 m, and the compressive strength is more than 10 N/piece.

A burst temperature furnace is used for blast drying of green balls. 300 g raw balls were taken each time, and the weight of the pellets was record after drying for 30 s. When the weight of the pellets no longer changes, it is considered that the drying is complete. The drying temperature and wind speed are set at 200 °C and 1.0 m/s respectively. Static drying of green balls is done in an oven at 200 °C.

2.2.2. Preheating and Firing

The preheating and roasting of pellets were carried out in a horizontal tube furnace (Hefei Kejing Material Technology Co., Ltd., Hefei, China) with air atmosphere and a flow rate of 5 L/min. Push the porcelain boat with pellets into the preheating zone 5 times, each time at an interval of 1 min. After a certain time of preheating, the porcelain boat is directly pushed into the roasting area. After roasting, the boat is pulled out from the left side and cooled. Among them, the preheating temperature is 800 °C~1000 °C, and the roasting temperature is 1160 °C~1280 °C. The compressive strength of the roasted pellets was measured by ZQYC intelligent compressive strength measuring machine (Jinan Times new light instrument Co., Ltd., Jinan, China), and the average value was taken.

The zinc and lead removal rates of roasted pellets were calculated according to the follow formula:

$$R_n=(M_0·m_0-M_t·m_t)\times 100/(M_0·m_0)$$

where R_n is Zn or Pb removal rate of the balls, %; M₀ is Zn or Pb content in dry balls, %; m₀ is mass of dry balls, g; M_t is Zn or Pb content of roasted pellets, %; m_t is mass of roasted balls, g.

2.2.3. Microstructure and Chemical Composition

In order to understand the changes of Pb and Zn content in pellet, the chemical composition of all samples was determined by X-ray fluorescence spectrometry (Bruker, Saarbrucken, Germany). Phase studies of the pellets were performed using a polarized light microscope (Axioskop 40 A Pol, Carl Zeiss, Oberkochen, Germany), and the microscopic composition and compositional analysis of the pellets were performed using Scanning electron microscopy (Nova NanoSEM 450, FEI, Hillsboro, OR, USA) and energy spectrometer (AZtec X-MaxN80, Oxford Instruments, Oxford, UK). The crystal composition of the pellets was determined by X-ray diffraction (SIMENS D500, Siemens, Munich, Germany).

3. Results and Discussion

3.1. Effects of Calcium Chloride on Green Pellets

The effect of calcium chloride on green pellets can be seen from the Figure 4a. When the basicity of SAR increased from natural basicity (R = 0.1) to 0.55, the wet crush strength of green pellets and its drop number was increased, while the thermal shock temperature exhibited the opposite rule. As the basicity of SAR increased from natural basicity (R = 0.1) to 0.55, the wet crush strength of green pellets and its drop number was increased from 12.4 N/P, 4.2 time/(0.5 m) to 13.6 N/P, 5.4 times/(0.5 m), respectively. Meanwhile, the thermal shock temperature decreased from 520 °C to 220 °C. This is because the water absorbency of calcium chloride crystals, the higher the basicity implies the more calcium chloride used in green pellets, thereby leading to the increasing of moisture level, the space between the particles is filled by water, and bridge liquid bond between particles is enhanced [22,23].

Generally, when the balling moisture is elevated, the water saturation of green balls increases and pores between the particles are filled with water which can hold the particles together. Meanwhile, capillary tube is squeezed and shrink when the balls is subject to mechanical action, resulting in the increase of capillary pressure. Bridging liquid bond is able to pull particles towards the spherical nucleus, holding the particles together provided cohesion and thus increase the strength of green balls. The water in the green balls can impart plasticity and improve shock absorption property of the balls, especially for the drop strength [24]. However, when the water exceeds a certain value, the water fills the pores between the particles, the capillary force in the green ball disappears, and the compressive strength will be reduced. At this time, the plasticity of pellets will still be guaranteed, so that the falling strength is still increasing [19].

3.2. Calcium Chloride Migration Behavior

Due to the strong water solubility of the calcium chloride, the migration behavior of calcium chloride inside the pellet was observed during the process of green balls being dried, as shown in the Figure 4b. Moreover, the migration behavior of the chloride agent is more obvious in blast drying than that of static drying. The calcium chloride crystals obviously precipitate out the surface of the blast drying, when the CaCl₂ content in the inner, middle and outer layers were 1.44%, 3.23% and 4.23%, respectively. The SEM images of the pellets (Figure 5 and Figure 6) also confirmed its authenticity. During static drying process, the distribution of Ca and Cl elements were relatively uniform in the statically dry pellets, but it turned out an increasing gradient distribution from inside to outside in blast drying, which indicated the migration behavior of chlorinated agent did occur during drying process.

During the preheating process, it could be seen from Figure 7 that the distribution of chlorine element in the pellet sparser significantly, and the distribution of chlorine element decreased from inside to outside, indicating that the chlorinating agent on the outer layer of the pellets has started to decompose during the preheating stage. Therefore, the migration behavior of the chlorinated agent in the pellet could be indirectly inferred from the electron microscope scanning pictures (Figure 5, Figure 6 and Figure 7) and Figure 4c. When the pellet was dried by air blast, the chlorinated agent inside the pellet would migrate to the outer layer with the process of water evaporation for its strong solubility. However, the outer chlorinated agent of the pellet would decompose first in the preheating stage, resulting the removal of lead and zinc was also carried out from the outside to the inside.

3.3. Zinc and Lead Removal Behavior

3.3.1. Thermodynamics

During the roasting process, CaCl₂ can decompose into HCl and Cl₂ by chlorination reaction with water vapor [25]. Moreover, the presence of Fe₃O₄, Fe₂O₃ and SiO₂ can effectively promote the decomposition of CaCl₂ [26].

The onset of the chlorinating agent’s decomposition also marks the beginning of the removal of lead and zinc during the preheating process. The zinc in sulfur acid residue mainly exists in the form of ZnO, ZnS and ZnFe₂O₄, while lead mainly occurs in the form of PbO, PbS and PbSiO₃, as shown in Table 2 and Table 3. Therefore, thermodynamic calculations were performed for the chlorination of these minerals, and the reaction equations for zinc-containing minerals and lead-containing minerals are shown in

Table 6. The reaction equation for lead-containing minerals with chloride.

Chemical Formula	ΔGTθ/J·mol−1	T/K
PbO + 2HCl = PbCl2 + H2O	ΔGTθ = −197,493.2 + 111.02T	298 < T < 768	1-1
	ΔGTθ = −166,615 + 70.64T	768 < T < 1158
	ΔGTθ = −196,973.8 + 96.72T	1158 < T < 1600
PbO + Cl2 = PbCl2 + 1/2O2	ΔGTθ = −139,605.5 + 44.57T	298 < T < 768	1-2
	ΔGTθ = −109,820 + 5.28T	768 < T < 1158
	ΔGTθ = 876,204.2 − 816.36T	1158 < T < 1600
PbS + 2HCl = PbCl2 + H2S	ΔGTθ = −97,339.88 + 120.66T	298 < T < 768	1-3
	ΔGTθ = −62,838.72 + 75.84T	1158 < T < 1158
	ΔGTθ = 70,787 − 33.24T	1005 < T < 1600
PbS + Cl2 = PbCl2 + 1/2S	ΔGTθ = −1,196,311.3 + 59.3T	298 < T < 768	1-4
	ΔGTθ = −161,548.34 + 13.92T	768 < T < 1158
	ΔGTθ = −669,471.02 − 65.33T	1158 < T < 1600
PbSiO3 + 2HCl = PbCl2 + SiO2 + H2O	ΔGTθ = −180,559.1 + 115.5T	298 < T < 768	1-5
	ΔGTθ = −147,441.36 + 72.17T	768 < T < 1307
	ΔGTθ = −173,597.25 + 97.19T	1307 < T < 1600
PbSiO3 +Cl2 = ZnCl2 + SiO2 + 1/2O2	ΔGTθ = −122,671.34 + 49.06T	298 < T < 768	1-6
	ΔGTθ = −89,657.57 + 5.37T	768 < T < 1307
	ΔGTθ = −114,400.03 + 29.1T	1307 < T < 1600
PbS + 3O2 = 2PbO + 2SO2	ΔGTθ = −219,140 + 101.15T	298 < T < 1158	1-7
	ΔGTθ = −185,100 + 72.03T	1158 < T < 1600

and

Table 7. The reaction equation for zinc-containing minerals with chloride.

Chemical Formula	ΔGTθ/J·mol−1	T/°C
ZnO + 2HCl = ZnCl2 + H2O	ΔGTθ = −125,349.77 + 119.85T	298 < T < 591	2-1
	ΔGTθ = −109,604.93 + 93.49T	591 < T < 1005
	ΔGTθ = −11,740.47 − 27.72T	1005 < T < 1600
ZnO + Cl2 = ZnCl2 + O2	ΔGTθ = −67,648.02 + 53.89T	298 < T < 591	2-2
	ΔGTθ = −51,109.53 + 26.09T	591 < T < 1005
	ΔGTθ = 71,478.51 − 96.24T	1005 < T < 1455
ZnS + 2HCl = ZnCl2 + H2S	ΔGTθ = −50,930.02 + 118.53T	298 < T < 591	2-3
	ΔGTθ = −35,573.57 + 92.77T	591 < T < 1005
	ΔGTθ = 89,300.4 − 31.63T	1005 < T < 1293
	ΔGTθ = 71,475.2 − 17.81T	1293 < T < 1455
ZnS+ Cl2 = ZnCl2 + 1/2S	ΔGTθ = −150,089.46 + 57.66T	298 < T < 591	2-4
	ΔGTθ = −133,677.05 + 30.25T	591 < T < 1005
	ΔGTθ = −15,100.71 − 88.51T	1005 < T < 1455
ZnFe2O4 + 2HCl = ZnCl2 + Fe2O3 + H2O	ΔGTθ = −120,179.66 + 123.22T	298 < T < 591	2-5
	ΔGTθ = −100,078.94 + 89.76T	591 < T < 1005
	ΔGTθ = 21,112.90 − 331.37T	1005 < T < 1455
ZnFe2O4 +Cl2 = ZnCl2 + Fe2O3 + 1/2O2	ΔGTθ = −62,477.90 + 57.26T	298 < T < 591	2-6
	ΔGTθ = −41,674.55 + 22.45T	591 < T < 1005
	ΔGTθ = 80,820.33 − 99.86T	1005 < T < 1455
ZnS+ 3O2 = 2ZnO + 2SO2	ΔGTθ = −445,246.66 + 78.62T	298 < T < 1455	2-7

. as shown in Figure 8a,b, the chlorination reaction of zinc and lead is easy to react in the forward direction at 300 K~1600 K, indicated Zn and Pb is available to remove. Meantime, the lower activation energy of the chlorination reaction of PbO also implied the more likely reaction of PbO than that of ZnO. It’s obviously known that ZnS and PbS are more likely to undergo oxidation reaction first under oxidizing atmosphere, oxidation producing ZnO and PbO.

3.3.2. Preheating and Firing

When the basicity of the pellet increased from the natural basicity (R = 0.1) to 0.55, an increasing of the removal rate of lead and zinc and also improving the mechanical strength of pellets happened. When the preheating temperature was 900 °C, the roasting temperature was 1250 °C, the preheating and roasting times were both 12 min, and the basicity of the pellet was increased from the natural basicity to 0.55, the compressive strength, lead and zinc removal rate of the pellet increased from 2207 N, 8.47% and 10.77% to 4398 N, 93.33% and 98.85%, respectively. As shown in Figure 9, the recommended optimum basicity is 0.45, while the compressive strength of the pellet is 4289 N, the removal rate of zinc is 98.85%, and the removal rate of lead is 91.99%.

During the preheating process, zinc and lead minerals began to react with chlorinating agent. As shown in Figure 9a,b, with the increase of preheating temperature and time, the compressive strength of the pellet, the dezincification rate and lead removal rate all show an increasing trend. When the preheating temperature is 900 °C, the preheating time is increased from 6 min to 18 min, the compressive strength, lead and zinc removal rate of sulfur acid residue pellet increase from 344 N, 56.91% and 60.71% to 521 N, 74.41% and 79.19%, respectively. Meantime, the influence of preheating temperature on the quality of pellet is also obvious. When the preheating time is 15 min and the preheating temperature increases from 800 °C to 1000 °C, the compressive strength, lead and zinc removal rate of sulfur acid residue pellet increase from 333 N, 20.77% and 69.93% to 533 N, 77.01% and 81.9%, respectively. Considering the removal rate of zinc and lead, the recommended preheating temperature is 950 °C, and the preheating time is 15 min. while the compressive strength, lead and zinc removal rate of the pellets is 517 N, 74.28% and 80.20%, respectively.

A small amount of zinc and lead still needs to be removed during the roasting stage, and the strength of the pellet would also be enhanced. Under the recommended preheating regime, the roasting temperature was 1220 °C and the roasting time was increased from 5 min to 25 min, the compressive strength, the lead and zinc removal rate of pellets increased from 1847 N, 87.54% and 89.3% to 3077 N, 92.03% and 93.33%, respectively. When the roasting time was 15 min, the roasting temperature increased from 1160 °C to 1280 °C, the compressive strength, lead and zinc removal rate of acid slag pellets increased from 996 N, 97.31% and 97.13% to 4498 N, 98.47% and 98.21%, respectively. It could be seen that when the recommended preheating temperature is 950 °C, the preheating time is 15 min, the roasting temperature is 1220 °C, the roasting time is 15 min, and the basicity is 0.45, the lead and zinc removal rates of the pellets could up to 91.33%, 97.88%, respectively, and the compressive strength at this time was up to 2789 N, which could meet the requirements of raw materials for blast furnace iron making [5,27].

The sulfur element in SAR will be gradually removed during the oxidation roasting process, which will reduce the phenomenon of excessive sulfur content and deterioration of the quality of hot metal [7,13]. The chemical composition of the roasting ball of SAR under the recommended system is shown in

Table 8. Chemical composition of mixed iron ore/%.

Sample	TFe	SiO2	Al2O3	CaO	MgO	S	Pb	Zn	P
SAR fired pellets	61.57	6.97	0.92	3.14	0.97	0.055	0.0091	0.0095	0.06

, and the sulfur content is reduced to 0.055%. Referring to the Chinese standard for acid iron ore pellets for blast furnace use [28,29,30], as shown in

Table 9. Chemical composition and metallurgical properties.

Project	Grade	Chemical Composition (wt,%)				Metallurgical Performance (wt,%)
		TFe	SiO2	S	P	RSI	RI	RDI+3.15
Product Grade	I	≥65.0	≤3.50	≤0.02	≤0.03	≤15.0	≥75.0	≥75.0
	II	≥62.0	≤5.50	≤0.06	≤0.06	≤20.0	≥70.0	≥70.0
	III	≥60.0	≤7.00	≤0.10	≤0.10	≤22.0	≥65.0	≥65.0
SAR fired pellets		61.57	6.97	0.055	0.06	11.87	66.97	93.54

, the chemical composition and metallurgical properties of SAR fired pellets under the recommended system have nearly met the requirements of blast furnace burden.

Interestingly, it can be seen from Figure 9e that the rate of zinc removal from pellets is higher than that of lead, contrary to thermodynamic calculation results. This because zinc content in sulfur acid residue is significantly higher than lead content, Zinc bearing minerals are more likely to contact with calcium chloride [31]. Moreover, some lead bearing minerals were embedded in iron-containing mineral particles or silicate minerals, resulting its slower reaction rate. As shown in Figure 10, Some lead-containing minerals occurred in iron-containing minerals or silicate minerals, while zinc-containing minerals had no obvious aggregation.

3.4. Microstructure and Composition

With the roasting process of pellets in oxidizing atmosphere, the mineral phase of pellets changed. As can be seen in Figure 11, the main mineral phase in the pellets is changed from magnetite to hematite. The hematite crystals are gradually refined, the crystals are changed from irregular sheet and block to granular, and the interlaced distribution between the crystals is more uniform as the roasting time increases. Moreover, the silicate structure in the pellet was gradually destroyed, and the low melting point compound was formed by the reaction with Ca²⁺ to fill the hematite crystal particles, which strengthened the connection between the particles and made up for the cracks in the pellet, and was conducive to the improvement of the compressive strength of the Pellet.

The changes of the mineral phases in the pellets of SAR, especially the refinement and homogenization of hematite particles, are significantly conducive to the increase of the compressive strength of the pellets [32]. Extending the roasting time is beneficial for the recrystallization process of hematite crystals and the strengthening of hematite crystals, which is the main reason for mechanism of agglomerated of pellets [33,34]. As shown in Figure 12, the silicate phase gradually disappeared with the progress of roasting. The diffraction peaks of hematite at diffraction angles of 40.8° and 49.5° enhanced with the increase of roasting time, and the diffraction peaks of hematite at diffraction angles of 57.1° and 72.5° disappeared first and then reappeared, which fully reflected the recrystalization process of hematite in pellets.

When the pellet entering the roasting section, the internal consolidation reaction of the pellet is gradually strengthened. Due to the addition of CaCl₂, the basicity of the pellets increased from the 0.15 to 0.55, which would promote the reaction of silicaluminate with Ca²⁺ to form low melting point compound, destroy the silicate structure, And the rate of solid solution reaction between Ca²⁺, Al³⁺, Mg²⁺ and hematite increase, indicating More Ca²⁺, Al³⁺ and Mg²⁺ elements would be dissolved into hematite. Meantime, the hematite crystal in the pellet will undergo high temperature recrystalization at the recommended roasting temperature, which makes the crystal form of hematite crystal in the pellet gradually change from irregular block and sheet to granular, resulting that it’s more conducive to the exposure of zinc and lead elements in hematite or silicate, thus promoting the further removal of lead and zinc elements. The removal rate of zinc and lead increased to 97.88% and 91.33%, respectively, when the preheated pellets were roasted at 1220 °C for 15 min.

The microstructure of pellets changed with the increase of calcium chloride dosage (

Table 10. EDS analysis results for the areas in Figure 12.

Spot	Elemental Compositions/(wt, %)						Mineral Phases
	O	Mg	Al	Si	Ca	Fe
a1	23.13	0.06	0.08		0.11	76.61	Fe2O3
a2	41.99	0.04	19.07	24.51	12.91	1.47	CaO∙Al2O3∙FeO∙SiO2
a3	20.76	1.35	0.49	2.38	1.73	72.99	Fe2O3
a4	45.99		15.40	36.98		1.58	Al2O3∙FeO∙SiO2
b1	27.53	0.50	0.44	2.73	16.19	52.50	Fe2O3
b2	23.52	0.07	0.20	0.01	0.27	75.80	Fe2O3
b3	39.94	8.97	1.09	26.05	14.85	9.04	CaO∙MgO∙Al2O3∙FeO∙SiO2
b4	44.08	0.00	0.04	54.72	0.00	1.02	FeO∙SiO2
b5	12.15	0.18	0.73	0.38	0.49	86.04	Fe2O3
c1	24.39	0.00	0.00	0.00		75.51	Fe2O3
c2	39.12	4.86	3.06	28.45	13.61	10.11	CaO∙MgO∙Al2O3∙FeO∙SiO2
c3	26.13		0.54	0.35	0.24	72.72	Fe2O3
c4	30.75	3.19	2.15	18.0	5.71	39.50	CaO∙MgO∙Al2O3∙FeO∙SiO2
d1	25.99	0.13	0.00	0.09	0.18	73.31	Fe2O3
d2	26.67	0.75	0.00	1.65	0.00	70.92	Fe2O3
d3	10.69	0.00	0.11	0.41	0.77	87.99	Fe2O3
d4	37.94	4.59	3.20	26.82	11.81	15.51	CaO∙MgO∙Al2O3∙FeO∙SiO2
e1	15.95	0.05	0.47	0.05	0.25	83.05	Fe2O3
e2	27.72	0.03	0.54	0.06	0.05	71.60	Fe2O3
e3	21.33	1.31	1.0	6.64	2.17	67.28	Fe2O3
e4	35.18	3.88	3.36	25.90	11.10	20.24	CaO∙MgO∙Al2O3∙FeO∙SiO2
f1	26.58	2.50	1.98	13.26	7.27	48.28	CaO∙MgO∙Al2O3∙FeO∙SiO2
f2	25.14		0.65	0.00	0.03	72.14	Fe2O3
f3	33.46	4.03	2.19	23.22	15.19	21.74	CaO∙MgO∙Al2O3∙FeO∙SiO2
f4	18.16	0.08	0.42	0.79	0.80	79.49	Fe2O3

). After the sulfur acid residue pellets with different calcium chloride contents were roasted under the same conditions, the silical-aluminate structure inside the pellets gradually formed glassy texture, which was filled with hematite particles and strengthened the connection between particles, as can be seen in Figure 13b, the edge of iron olivine (FeO∙SiO₂) gradually changed to glassy (CaO∙MgO∙Al₂O₃∙FeO∙SiO₂). At the same time, in terms of chemical composition, the iron content in silicon aluminate gradually increases, which will enhance the strength of glass phase, and improve the metallurgical properties of pellets [35,36,37]. The composition of hematite also changes gradually with the increase of basicity, the content of Ca²⁺, Al³⁺ and Mg²⁺ increases in hematite, the hematite particles are further refined, and the distribution part is more uniform, which will greatly improve the performance of pellet products [38,39].

4. Conclusions

In order to remove lead and zinc from sulfur acid residue, the sulfur acid residue was roasted by chlorination. The conclusions are drawn as follows:

• Calcium chloride has the property of moisture absorption, and its addition is beneficial to improve the drop number of green balls, but will reduce its thermal shock temperature. At the same time, during the drying process of green balls, calcium chloride has a certain migration and will migrate to the surface of pellets with water, which is one of the important reasons for the low removal rate of lead and zinc in the inner layer of pellets in the preheating stage.
• The removal process of lead and zinc in sulfuric acid slag is divided into two steps. In the preheating stage, the chlorination agent is decomposed and reacts with lead-zinc containing minerals, and most of the zinc and lead minerals can be removed. Some lead and zinc minerals embedded in iron ore or silicate minerals need to be removed at the roasting stage. With the extension of roasting time, the hematite particles undergo recrystallization process, and the crystals are gradually refined. At the same time, the silicate structure is destroyed, and the lead and zinc minerals in the particles are exposed, so as to achieve further removal of lead and zinc elements in the pellets.
• the increase in the amount of calcium chloride is conducive to destroying the silicate structure, promoting the crystal of hematite changes from irregular sheet and block to granular, and improving the strength of pellets. Finally, under recommended conditions, the compressive strength of pellets is 2789 N, the zinc removal rate is 97.88%, and the lead removal rate is 91.33%, which has an important guiding significance for the industrial application of sulfur acid residue pellets.

5. Highlights

• Recovery of harmful elements of lead and zinc and preparation of pellets were realized by “one step method”;
• The influence of migration behavior of chlorinated agent on pellet drying was investigated;
• The effect of chlorination agent on microstructure and composition of pellets was investigated.