Recovery of elemental sulfur from zinc concentrate direct leaching residue using atmospheric distillation: A pilot-scale experimental study

Abstract

Recovery of elemental sulfur from zinc concentrate direct leaching residue (DLR) using atmospheric distillation was systematically investigated on a pilot-scale system for the first time. Batch operating mode was suggested for recovery of elemental sulfur from water-rich DLR using atmospheric distillation. Elemental sulfur with purity higher than 99% was obtained under certain conditions in batch operating mode. With an appropriate feed amount of 1,200 kg, batch experiment conducted at 460°C resulted in sulfur purity of 96.22% and a recovery rate higher than 85%. Only 0.59 and 1.24 kWh power was needed to handle 1.0 kg DLR and produce 1.0 kg elemental sulfur, respectively. The results suggest that recovery of elemental sulfur from zinc concentrate DLR using atmospheric distillation is technologically and economically feasible. Moreover, other metal elements such as zinc were enriched in the distillation concentrate, which could be used for metal refining. Technologies could effectively lower the moisture content of DLR, and lowering the distillation temperature would be of great value for recovery of elemental sulfur from DLR using a distillation method.

Implications:

Distillation is a promising solution for recovery of elemental sulfur from DLRs. This work revealed the possibility of separation of elemental sulfur from zinc concentrate DLR using atmospheric distillation. Such knowledge is of fundamental importance in developing field-scale separation and purification technologies and devices in which simultaneous sulfur recovery and precious metal enrichment are possible. Important tasks for follow-up research are also suggested.

Introduction

Historically, zinc sulfide ores were treated by pyrometallurgical methods of smelting and converting processes to obtain the zinc metal (Halfyard and Hawboldt, 2011). However, this high-temperature route has always been plagued with environmental issues concerning pollution not only with SO₂ gas emissions but also with emissions of toxic compounds of arsenic, mercury, and other heavy metals (Padilla et al., 2007). As an alternative to the high-temperature process, a hydrometallurgical process called the Sherritt zinc pressure leach process was successfully commercialized in early 1981 (Ozberk and Jankola et al., 1995). The direct leaching process offers several advantages, such as lower operating cost, complete elimination of atmospheric pollution due to sulfur dioxide, more precise control, and higher level of zinc recovery (Gu et al., 2010). Moreover, the direct leaching process recovers sulfur in its elemental form, which is readily storable and saleable. As environmental awareness and regulations increase, sulfur dioxide emissions from metal production will be strictly regulated. The direct leaching process in which sulfur is discharged as solid elemental sulfur is considered to be highly attractive.

After the first commercialization by Cominco Limited in 1981 at their Trail, British Columbia, zinc refinery, more than four zinc pressure leach plants were commissioned in the last decades of 20th century (Krysa, 1995; Ozberk et al., 1995). In recent years, direct leaching processes have been successfully adopted by many other refineries to produce zinc and other metals (McDonald and Muir, 2007). However, there are still some shortages where improvements can be made; for example, low elemental sulfur recovery rates. Recovery of elemental sulfur from direct leaching residue (DLR) has only been conducted in few large-scale refineries; for example, the Trail refinery adopted a hot filtration method to recover elemental sulfur from DLR (Ozberk et al., 1995). Hot filtration requires raw material containing more than 70% elemental sulfur to ensure a satisfactory performance (Halfyard and Hawboldt, 2011), and the elemental sulfur content in DLR is usually lower than 70%. Therefore, flotation of DLR is usually integrated with the hot filtration process, which gives rise to significant cost increases in DLR disposal and sulfur recovery. In most other zinc refineries, especially those in China, DLR is generally dumped as solid waste. The sulfur component of DLR reacts with oxygen and water to form thiosalts and sulfuric acid, which are harmful to the environment and human health (Halfyard and Hawboldt, 2011). Furthermore, abundant hazardous metal elements existing in DLR pose disposal challenges. Therefore, it is urgent to develop effective technologies for proper handling of DLR.

Recovery of elemental sulfur and other metal elements from DLR is considered preferable to safe storage. Technologies used for separation and purification of elemental sulfur from DLR can be divided into three types: physical, chemical, and biotical methods (Zhou, 1997; Li, 2008). Physical methods with low operating costs and large disposal capacity are attractive methods that have received considerable attention from scientists and engineers. Because the boiling point of sulfur (444.6°C) is much lower other main components of DLR (Lide, 2009), elemental sulfur in DLR can be separated from other components by distillation methods. Mackiw et al. (1967) conducted a distillation experiment at 427°C under a nitrogen and water atmosphere. The results showed that for chalcopyrite leach residue with 38.02% elemental sulfur, the sulfur recovery rate was around 100% and the sulfur purity was higher than 99%. Lu et al. (2006) conducted a small-scale (20 g per batch) experiment at 440–480°C to separate sulfur from DLR and found that sulfur with purity higher than 97% was obtained after distillation of the DLR under atmospheric pressure. Recovery of sulfur from anode mud containing higher than 82% elemental sulfur using vacuum distillation (30–600 Pa) was investigated by Huang and He (2002). The results showed that for a small-scale (10 g) experiment, the sulfur recovery rate and sulfur purity were higher than 97% and 99%, respectively. As stated above, it is possible for elemental sulfur in DLR to be separated and purified by distillation methods. Distillation processes not only can recover sulfur from DLR but can concentrate zinc in the distillation concentrate (DC), which can be used as raw material for zinc metal production. However, no pilot- or field-scale studies on distillation for disposal of DLR have been reported to date.

In this work, recovery of elemental sulfur from zinc concentrate DLR using atmosphere distillation was studied using a pilot-scale distiller for the first time. Variations in temperature and power consumption during the entire distillation process were analyzed. The effect of distillation temperature and feed amount on the distillation process and economic parameters was systematically investigated. Moreover, a comparison study between batch experiments and continuous experiments was conducted. The enrichment of metal elements such as zinc along with sulfur distillation was investigated as well. The results of this study will improve our understanding of the entire distillation process for sulfur recovery from DLR and hence are critical in developing larger industrial systems for effective disposal of DLR, which will facilitate extensive application of the direct leaching process for metal production.

Experimental

Characterization of DLR, distillation concentrate, and sulfur product

DLR samples were collected from a zinc refinery where a direct oxygen-rich leaching process is adopted to produce zinc metal. The moisture content of the DLR samples was examined by an atmospheric drying method using a drying oven. An EDAX Eagle III X-ray fluorescence spectrometer (PANalitical, Netherlands) was used to determine the oxides of major elements, including SiO₂, Al₂O₃, CaO, Fe₂O₃, ZnO, PbO, and SO₃. Minerals in DLR and DC samples were identified by X-ray diffraction (XRD) using a PANalytical X’ Pert PRO diffractometer equipped with a graphite diffracted-beam monochromator. The XRD patterns were recorded over a 2θ interval of 5–85°, with a step size of 0.02° and a counting time of 10 sec per step. The morphological properties of DLR and DC samples were investigated by scanning electron microscopy (SEM) using an FEI Sirion 200 microscope. Energy-dispersive X-ray (EDX) analysis was carried out using an EDAX Genesis X-ray detector. Thermal analysis was conducted on a Netzsch STA 409C (Netzsch Geratebau, GmbH, Germany) thermobalance, using a furnace with an SiC heater and a High RT2 sample holder. A 10-mg sieved DLR sample was heated in pure N₂ (flow rate of 100 mL/min⁻¹) under atmospheric pressure at a heating rate of 10°C/min from ambient temperature to 500°C and then kept at 500°C for 1 hr to obtain the thermogravimetry (TG), differential thermogravimetry (DTG), and differential scanning calorimetry (DSC) curves simultaneously. Elemental sulfur content of the DLR, DC, and sulfur product was identified using a gravimetric method according to China National Standards (Inspection and Quarantine of the People's Republic of China, 2007). The zinc content of DLR and DC was analyzed using atomic absorption spectrometry according to China National Standards (Ministry of Environmental Protection of People's Republic of China 1995). All chemical reagents used in this study were of analytical grade.

Distillation experiments

To separate and recover elemental sulfur from zinc concentrate DLR, a pilot-scale experimental setup was built as shown in Figure 1. In each test, a certain amount of DLR was fed into the distiller, which was heated by temperature-controlled silicon carbide rods to control the reaction temperature. Due to the design of the distiller, it is difficult to place thermocouples in the distiller. Therefore, the temperatures reported in this work were represented by temperatures on the wall of the distiller. A condenser downstream of the distiller condensed sulfur vapor into the liquid phase while water vapor exited. The gas stream at the outlet of the condenser still contained an abundance of sulfur particles. To prevent blocking and corrosion, before proceeding to the centrifugal fan, the gas flow passed through a cyclone separator and a scrubber, where particulate matter and acid gases were removed, respectively. The DC was discharged by a screw placed in the distiller. The mass of DLR, sulfur product, and distillation concentrate was weighed by an electronic balance (HT-12, Hengping, Wuxi, China) with a scale interval of 0.1 kg. The mass of sulfur product was recorded every 30 min after the start of each experiment to determine the sulfur yield rate.

Figure 1. Schematic diagram of the experimental system.

Four sets of experiments were conducted and the experimental conditions are summarized in Table 1. Several preliminary experiments were conducted to confirm the balance of elemental sulfur. The results showed that an acceptable element balance can be achieved. Experiments in Set I and II were batch experiments, in which a fixed amount of DLR was loaded into the distiller within a few minutes at the beginning of the distillation process. In Set I, experiments were conducted at 490°C to investigate the effect of feed amount. Set II experiments were designed to identify the effect of distillation temperature using a fixed feed amount of about 1,200 kg, which was adopted as the optimal feed amount for this experimental system. In both Set I and Set II experiments, temperature, power consumption, and the weight of sulfur product were recorded continuously until the distillation process had reached its terminal point, which was defined as a sulfur yield rate of less than 0.0005 kg/(min.kg DLR). To determine the effect of operating mode, a semibatch experiment and another continuous experiment were conducted in Sets III and IV, respectively. For both Sets III and IV, the distillation processes were longer than 24 hr. In Set III, a semibatch experiment was conducted at 460°C with a DLR feed rate of 250 kg/h (feeding of 250 kg DLR within 5 min at the beginning of each hour). The Set IV experiment aimed to confirm the feasibility of continuous distillation and was conducted at 460°C with a DLR feed rate of 4 kg/min (feeding DLR uniformly during the entire experiment period) controlled by a screw feeder.

Table 1. Experimental conditions

Experiments	Temperature (°C)	Feed amount	Operating mode
Set I	490	400, 810, 1,185, 1,606, 2,032 kg/batch	Batch
Set II	430, 460, 490	About 1,200 kg/batch	Batch
Set III	460	250 kg/h	Semibatch
Set IV	460	4 kg/min	Continuous

At the beginning of each experiment, DLR was precisely balanced to obtain the feed weight of DLR (W _DLR). The actual weight of elemental sulfur in the feed DLR (F _sul) was obtained by deducting the weight of water from W _DLR and then multiplying by the percentage composition of elemental sulfur. Then, DLR was distilled to obtain the sulfur product and distillation concentrate. After the distillation process was completed, sulfur obtained from the condenser and cyclone separator was weighed to obtain the rough yield, which was then multiplied by purity to get the actual sulfur yield (Y _sul). The definition of elemental sulfur recovery rate (R _sul) is as follows:

(1)

Obviously, R _sul is related to sulfur evaporation in the distiller and sulfur collection in the condenser and cyclone separator.

Results and Discussion

Characterization of direct leaching residue, distillation concentrate, and sulfur product

The moisture content of the DLR samples was between 9 and 20%. Pictures of DLR with moisture contents of 9.42 and 19.85% are shown in Figure 2. As can be seen, the DLR was black, existing in powder form when the moisture content was low and in mud form when the moisture content was high. The chemical composition of the DLR sample is listed in Table 2. The primary chemical components of the DLR were SiO₂, Fe₂O₃, ZnO, and SO₃. The content of SO₃ in DLR was 74.75%, which is much higher than the SO₃ content of sphalerite, indicating that sulfur was enriched in the direct leaching process. The ZnO content of DLR was 6.64%. In addition to X-ray fluorescence spectrometry analysis, a gravimetric method according to China National Standards (Inspection and Quarantine of the People's Republic of China, 2007) was adopted to identify the elemental sulfur content in dry DLR. The results showed that for all DLR samples the elemental sulfur content was between 45 and 65%.

Figure 2. Pictures of DLR with (a) 9.42% and (b) 19.85% moisture content.

Table 2. Major chemical composition of DLR sample (w/%)

	Composition
	SiO2	Al2O3	Fe2O3	CaO	MgO	K2O	SO3	MnO	ZnO	PbO
Content (%)	5.27	0.68	9.06	0.24	0.29	0.23	74.75	0.47	6.64	1.25

The XRD patterns of DLR and DC are shown in Figure 3. Crystalline phases were identified by comparison with ICDD files. As shown in Figure 3a, crystalline sulfur was observed to be the dominating phase of DLR. Only a few weak peaks from other crystalline matters such as ZnSO₄, ZnS, and FeS₂ were observed, indicating that the DLR was rich in elemental sulfur. This is in line with the results of chemical component analysis, which showed that sulfur is the most abundant element of DLR. XRD analysis of the DC sample was also conducted, and the results are shown in Figure 3b. The results showed that most sulfur peaks disappeared, and peaks attributed to ZnSO₄, ZnS, SiO₂, and FeS₂ dominated. This indicates that most sulfur was effectively separated from other components of the DLR during the atmospheric distillation process.

Figure 3. XRD profiles of (a) DLR and (b) DC.

The results of SEM and EDX analysis are shown in Figure 4. As shown in Figure 4a, most fine DLR particles joined together and existed as large irregular particles with a diameter larger than 5 μm. Most DLR particles were observed to be extremely smooth. Most DC particles were irregular in shape with edges and corners and were smaller than 2 μm in diameter and most had a rough surface. The EDX results showed that peaks attributed to zinc, oxygen, iron, and silicon became more remarkable after distillation, and the sulfur mass content decreased from higher than 70% to about 20%. These phenomena indicate that glossy elemental sulfur scattered on the surface of the DLR particles, and most of it evaporated during the distillation process. This is in accordance with the XRD results that most sulfur peaks disappeared after distillation.

Figure 4. SEM and EDX analysis of (a) DLR and (b) DC.

The TG and the DTG curves are shown in Figure 5. There are three obvious weight loss stages. The weight loss at temperatures ranging from 90 to 280°C was due to dehydration. The limited weight loss around 100°C indicates that most of the moisture in the DLR sample existed in the inner pores of the DLR or in the form of crystal water. The other two weight loss peaks in the temperature ranges of 280–360°C and 360–460°C were probably attributed to the evaporation of sulfur on the surface and in the inner part of the DLR particles, respectively. The total weight loss of DLR during the entire process was about 60%, which is close to the total water and elemental sulfur content of the DLR sample. As shown in Figure 6, the DLR distillation process was endothermic. The major endothermic peaks on the DSC curve were due to water evaporation, sulfur fusion, and sulfur evaporation, respectively. Limited power was needed after 60 min, when the temperature reached at 460°C, indicating that sulfur evaporation was completed. This is in line with the TG curve, which showed that negligible weight loss was observed when the temperature was higher than 460°C. By integrating the DSC curve, the power consumption per unit dry DLR dispose (kWh/kg) was estimated to be 0.47 kWh/kg.

Figure 5. TG and DTG curves of DLR.

Figure 6. TG and DSC curves of DLR.

Variations in temperature, power consumption, and sulfur yield during the distillation process

To understand the detailed distillation process, temperature, power consumption, and sulfur yield were recorded frequently for a batch experiment (460°C, 1,200 kg), and the results are shown in Figures 7 and 8. As shown in Figure 7, the entire distillation process could be divided into three periods: the water evaporation period, the temperature rise period, and the sulfur evaporation period. For the water evaporation period, the temperature decreased from 465 to 362°C within 1 hr after the beginning of the distillation process, when 1,200 kg DLR was added to the distiller. The relatively low temperature of DLR and evaporation of water in DLR were responsible for this dramatic temperature decrease. Then, the temperature increased gradually to the boiling point of sulfur and finally reached the set temperature of 460°C. This indicates that most water in the DLR evaporated during the water evaporation period. However, most power was consumed in the temperature rise period, because the water evaporation period was relatively shorter than the temperature rise period. After the process reached the sulfur evaporation period, power consumption was limited, and most sulfur was yielded at this stage (Figure 8). This is different from the TG curve, which showed that part of the sulfur evaporated before the boiling point of sulfur. Two major reasons were responsible for this discrepancy: (1) the actual temperature of the DLR in the distiller was much lower than the temperature of distiller wall, due to the high heat transfer resistance of DLR, and (2) the condenser and cyclone retained a certain amount of sulfur yielded during a previous period, because sulfur yield was recorded every 30 min in all experiments. The abundance of sulfur on the surface of the DLR particles evaporated around the boiling point of sulfur and hence resulted in a huge spike of sulfur yield. Similar to the TG experiments, sulfur obtained during the subsequent evaporation period was probably from the inner part of the DLR particles. It is harder for sulfur to evaporate from the inside of the particle than from the particle surface. Therefore, sulfur yield decreased gradually as the temperature increased from the sulfur boiling point to the set temperature of 460°C.

Figure 7. Temperature variation and power consumption during the distillation process.

Figure 8. Sulfur recovery during the distillation process.

For semibatch and continuous experiments (results not shown), variations in temperature throughout the experiment were relatively small, and the temperatures were around the set temperature of 460°C. Therefore, the mean temperatures for semibatch and continuous experiments were higher than that for batch experiments, though the set temperature (460°C) was the same. For semibatch and continuous experiments, particularly continuous experiments, the distiller temperature was always around 460°C, which is higher than the boiling points of both water and sulfur. Moreover, well-distributed addition of DLR to the distiller during the continuous experiments resulted in a much better condition for heat transfer, which allowed the actual temperature of the DLR to be much closer to the distiller temperature. Therefore, water and sulfur evaporated simultaneously in the semibatch and continuous experiments.

Effect of feed amount

Five batch experiments were conducted at 490°C in Set I to study the effect of feed amount on the distillation process. As shown in Table 3, the balance of elemental sulfur for each experiment was around 89–97%, demonstrating the reliability of the experimental results. In all experiments the purity of the sulfur product was higher than 95%, with the highest purity of 99.26%. The feed amount exhibited no obvious effect on sulfur purity. For sulfur recovery rate, more than 85% sulfur was recovered in most experiments, and the highest recovery rate of 93.37% was obtained with a feed amount of 1,185 kg. No obvious variation in sulfur recovery rate was observed when the feed amount was increased from 400 to 2,032 kg. Elemental sulfur contents of distillation concentrates were lower than 11%. With the increase in feed amount from 400 to 2,032 kg, elemental sulfur contents of distillation concentrates increased from 8.08 to 10.57%. Higher feed amounts corresponded to greater DLR thickness in the distiller, which inhibited sulfur evaporation and hence resulted in higher elemental sulfur content in the distillation concentrate. The effect of feed amount on power consumption is also shown in Table 3. When the feed amount was 400 kg, 3.49 and 1.87 kWh power was needed to produce 1 kg of elemental sulfur and handle 1 kg of DLR, respectively. Increasing the DLR feed amount had a beneficial effect on power saving. Power consumption decreased dramatically as the feed amount was increased from 400 to 810 kg. A further increase in feed amount from 810 to 2,032 kg yielded further decrease in power consumption but no obvious decrease in power consumption. This indicates that an increase in the feed amount decreased power consumption. However, the feed amount cannot exceed the maximum capacity of the distiller, which was estimated to be around 2,000 kg. Moreover, with an increase in the feed amount, much more time was needed for sulfur evaporation. Therefore, the disposal capacity (kg DLR/day) of the pilot-scale experimental system decreased when the feed amount increased from 1,185 to 2,032 kg.

Table 3. Experimental results

	Set I					Set II		Set III	Set IV
Serial number	1	2	3	4	5	6	7	8	9
Set temperature (°C)	490	490	490	490	490	460	430	460	460
Feed amount (kg/batch)	400	810	1,185	1,606	2,032	1,200	1,198	250 (kg/h)	4 (kg/min)
Moisture content of DLR (%)	16.76	15.79	16.11	9.42	17.96	19.85	12.64	15.00	12.10
Dry basis elemental sulfur content of DLR (%)	60.35	55.35	55.52	45.53	53.98	55.78	54.38	61.37	63.27
Sulfur purity (%)	98.26	99.26	98.21	97.63	95.54	96.22	99.12	94.55	93.15
Actual sulfur yield (kg/batch)	178.8	311.7	481.2	571.1	779.0	456.1	468.8	74.7 (kg/h)	85.7 (kg/h)
Sulfur recovery rate (%)	89.00	82.55	93.37	86.23	86.57	85.01	82.38	51.32	55.80
Dry basis elemental sulfur content of DC (%)	8.08	8.35	8.15	9.02	10.57	9.37	12.53	9.95	10.23
Balance of elemental sulfur (%)	96.76	89.52	93.91	96.31	93.77	90.32	89.42	—	—
Power consumption per unit of sulfur yield for dry DLR (kwh/kg)	3.49	2.4	2.27	2.19	1.84	1.24	1.08	2.66	1.80
Power consumption per unit of dry DLR disposal (kwh/kg)	1.87	1.1	1.09	0.86	0.85	0.59	0.48	0.94	0.73
Distillation period (min)	210	240	300	420	540	300	360	—	—
Disposal capacity per day (kg DLR/day)	2,743	4,860	5,688	5,506	5,418	5,760	4,792	6,000	5,760

As stated above, when the feed amount was higher than 1,200 kg, a further increase decreased the power consumption to a certain extent. However, elemental sulfur content in DC increased, and the disposal capacity decreased. Therefore, 1,200 kg was selected as the optimal feed amount to study the effects of temperature and operating mode.

Effect of distillation temperature

Temperature is one of the most important parameters for the atmospheric distillation process, because matter can only evaporate dramatically under atmospheric pressure when the temperature is around or higher than its boiling point (Perry, 1984). The experiments in Set II and number 3 in Set I were designed to study the effect of distillation temperature on sulfur recovery rate, elemental sulfur content of DC, power consumption, and disposal capacity of the system. As shown in Table 3, a temperature of 430°C resulted in a sulfur recovery rate of 82.38%. An increase in temperature from 430 to 490°C yielded higher sulfur recovery, and a recovery rate higher than 90% was observed when the temperature was 490°C. Higher temperature facilitated sulfur evaporation and hence resulted in more sulfur extraction from the DLR, accordingly lowering the residual sulfur in the DC. This is in accordance with the chemical analysis of the DC (results shown in Table 3), which showed that the sulfur content of DCs decreased gradually as the temperature increased from 430 to 490°C. Only 1.08 kWh power was needed to produce 1.0 kg elemental sulfur at 430°C, corresponding to 0.48 kWh power to dispose of 1.0 kg dry DLR, which was almost equal to the theoretical power consumption estimated from integration of the DSC curve. It should be noted that the DSC test was conducted from room temperature to 500°C, which is higher than the temperature used in this experiment (430°C). A lower terminal temperature would lower the theoretical power consumption calculated from the DSC curve. Moreover, the lower gas flow adopted in the DSC test could also reduce the theoretical power consumption calculated from the DSC curve. Therefore, there is still room to improve the operating conditions in order to reduce power consumption. Power consumption increased slightly when the temperature increased from 430 to 460°C and increased dramatically as the temperature increased further from 460 to 490°C. At 490°C, producing 1.0 kg elemental sulfur consumed 2.27 kWh power, which is more than two times higher than that at 430°C. This was probably due to the large amount of energy lost during the distillation process by heat dissipation, deslagging, and water evaporation. At 430°C, 4,792 kg DLR can be disposed of in one day (24 hr), corresponding to a distillation period of 360 min. The distillation period decreased as the temperature increased from 430 to 460°C. When the temperature was lower than the boiling point of sulfur, a higher temperature accelerated the separation of elemental sulfur from the DLR and hence reduced the time needed for complete distillation. Because 460°C is higher than the boiling point of sulfur, a further increase in temperature to 490°C resulted in no obvious beneficial effect on sulfur distillation; that is, the distillation period for one batch was almost the same as that at 460°C, which was about 300 min. An increase in temperature from 460 to 490°C slightly promoted sulfur recovery and lowered the sulfur content in the DC. However, an increase in temperature from 460 to 490°C resulted in greater power consumption and did not increase the disposal capacity of the system. Therefore, 460°C was adopted for the experiments in the next stage.

In addition to sulfur recovery, some metal elements such as zinc were enriched simultaneously in the distillation process. For batch experiments conducted at 460°C with a feed amount of 1,200 kg, zinc content in solid matter increased from 6.45 to 21.50% after distillation. The zinc concentrated solid matter could be reused as raw material for zinc metal production.

Effect of operating mode

In addition to feed amount and distillation temperature, operating mode affected the atmospheric distillation process and its economic and technical norms. To study the effect of operating mode and confirm the feasibility of continuous distillation of sulfur, a semibatch and a continuous experiment were conducted in Sets III and IV, respectively. In these two experiments, the feed amounts were 250 kg/hr and 4 kg/min, respectively. These two feed amounts were equivalent to the feed amount of 1,200 kg used in the batch experiment and the corresponding total disposal capacities (kg DLR/day) were about the same. The purity of sulfur produced from semibatch and continuous experiments was lower than that from the batch experiment. Simultaneous evaporation of water and sulfur under the semibatch and continuous operating modes resulted in higher gas flow during the sulfur evaporation period, which carried more fine DLR particles to the condenser and cyclone. DLR particles were collected by liquid sulfur and liquid sulfur drops in the condenser and cyclone and hence lowered the purity of the sulfur. For semibatch and continuous operating modes, sulfur recovery rate were lower than 60%, which was much lower than that observed in batch experiment (85.01%). This was primarily attributed to poor sulfur collection performance in the condenser and the cyclone separator, because the sulfur content in the DC was slightly higher than that for the batch experiment, demonstrating that sulfur evaporation in the distiller was only slightly inhibited in semibatch and continuous operating modes. For most of the distillation period during semibatch and continuous experiments, pressure in the distiller was slightly higher than in the batch experiment. This was due to continuous evaporation of moisture from the DLR in the semibatch and continuous experiments. In the batch experiment, most of the moisture evaporated in the initial stage of the distillation process, which resulted in lower pressure for the subsequent distillation process and hence facilitated sulfur evaporation. However, the sulfur evaporation was limited because the pressure difference was minimal. Power consumption in both semibatch and continuous experiments was much higher than in the batch experiment. The mean temperature for semibatch and continuous operating modes was higher than that for the batch experiments, though the set temperature (460°C) was the same. Higher temperature facilitated heat dissipation from the system. Moreover, more heat was discharged from the distiller due to the higher temperature water vapor. Therefore, a higher mean temperature in semibatch and continuous operating modes was probably responsible for the increased power consumption.

Conclusion

Recovery of elemental sulfur from zinc concentrate DLR using atmospheric distillation was systematically investigated in a pilot-scale system for the first time. Continuous atmospheric distillation of elemental sulfur from DLR was demonstrated to be feasible to a certain extent. However, simultaneous evaporation of water and sulfur under continuous operating mode lowered the purity of the sulfur. The sulfur recovery rate was observed to be lower than 60%, which was attributed to the poor sulfur collection performance in the condenser and cyclone separator under continuous operating mode. Moreover, a higher mean temperature in continuous operating mode resulted in more power consumption for DLR disposal and sulfur production. Therefore, if the same sulfur distillation configuration is adopted for further application, batch operating mode is suggested. A temperature of 460°C was selected as the optimal temperature among temperatures investigated in this work. With an appropriate feed amount of 1,200 kg, the batch experiment conducted at 460°C resulted in sulfur purity of 96.22% and recovery rate higher than 85%. Only 0.59 and 1.24 kWh power was needed to handle 1.0 kg dry DLR and produce 1.0 kg elemental sulfur, respectively. This indicates that recovery of elemental sulfur from zinc concentrate DLR using atmospheric distillation is technologically and economically feasible. Moreover, metal elements such as zinc were enriched in the distillation concentrate, which could be used for metal refining.

This study revealed the possibility of separation of elemental sulfur from zinc concentrate DLR using atmospheric distillation. Such knowledge is of fundamental importance in developing field-scale separation and purification technologies and devices in which simultaneous sulfur recovery and precious metal enrichment are possible. Lowering the moisture content of DLR effectively to minimize the adverse effect of water vapor and improve the overall performance of continuous distillation is an important task for follow-up research. To save power consumption, future work should be conducted under a vacuum atmosphere to investigate the feasibility of recovery of elemental sulfur from DLR using vacuum distillation.

Funding

This project was supported by the Natural Science Foundation of China (51206192), the China Postdoctoral Science Foundation (No. 2012M521549), the Postdoctoral Science Foundation of Central South University (No. 113997), and the Science-Technology Foundation of Hunan Province, China (No. 2012FJ4084).