Introduction

Textile dyeing sludge is the solid waste produced by textile and dyeing factories after treatment of waste waters. This kind of waste product is produced broadly in China (Xie et al. 2019; Wang et al. 2019), India (Sarvajitha et al, 2018), South Korea (Oh and Shin 2017), France (Djafer et al. 2017), Iran (Khorram and Fallah 2018), Malaysia (Wong et al. 2018), and Tunisia (Haddad et al. 2018). The sludge contained various harmful materials, such as organo-chlorine compounds, polycyclic aromatic hydrocarbons (Man et al. 2018a, b), phosphorus compounds, metal-complexing compounds, and heavy metal compounds (Liu et al. 2018; Ghosh et al. 2017); all of these were hard to deal with and can cause bad environmental ruin. Owing to the multiple environment problems posed by this waste, comprehensive treatment and harmless development of this resource were necessary. The pyrolysis features of textile printing and dyeing sludge under microwave radiation at different temperatures were studied by Zhang et al. (2018). The distribution traits and migration of heavy metals and their latent ecological ruin were examined. Different heavy metals (Pb, Ni, Cr, Zn) had different enrichment characteristics, and a large proportion in heavy metal remains in sludge carbon. The heavy metals content in the sludge char was acceptable according to the CJ/T 362–2011 Chinese National Standard.

Textile dyeing sludge was characterized by its finely disseminated grain size of valuable elements, complex structures, and intricate mineralogical characteristics, which caused difficulties in its use. Magnetizing roasting–magnetic separation processes were very efficient for recovering refractory iron resources, such as red mud (Samouhos et al. 2017), low-grade siderite and limonite (Zhang et al. 2017), complex iron tailings (Li et al. 2010; Su et al. 2016), mixed iron ores (Chun et al. 2015), and pyrite cinder (Fan et al. 2015) that could not be handled by traditional physical mineral processing technologies. This technology provided an effective approach to convert the non-magnetic iron component into magnetic iron and enable efficient utilization of textile dyeing sludge. The beneficiation of iron ore from red mud based on magnetizing roasting using hydrogen followed by magnetic separation was studied by Samouhos et al. (2017): the maximum conversion rate of hematite to magnetite (87%) was obtained at 480 °C, and in the meanwhile, the grade of iron concentrate was 54%.

There had been little research on tailings produced by magnetizing roasting–magnetic separation of textile dyeing sludge. The major elements of tailings were silicon, iron, aluminum and calcium. Combined with the fineness, these features suggest that potential uses of nonmetallic building materials, for instance, ceramics (Li et al. 2016; Niu et al. 2016), fly ash (Wu et al. 2019; Detcheva et al. 2015), silica (Li et al. 2010), road-base building material (Manjarrez and Zhang 2018), concrete admixture (Gayana and Chandar 2018), together with cement slurry backfill material (Chen et al. 2017). Textile dyeing sludge might also be used to produce ceramic filter. Ceramic filter had a wide range of uses in water treatment, such as drinking water pretreatment in rural areas (Zeng et al. 2018; Puttaiah et al. 2018), removing Escherichia coli from water (Huang et al. 2018), recycling sand filter backwash water (Tung et al. 2018), removal of humic acids (Mosier-boss et al., 2017), and detection of bacteria (Man et al. 2018a, b). Man et al. conducted the loading of Fe–Ti/Fe(II) on ceramic filter materials for residual chlorine filter from water which drank (Shafiquzzaman et al. 2018). The experiments demonstrated that Fe–Ti/Fe(II) had a low preparation cost and high hypochlorite sorption capacity, achieving 88% removal within 1 h. The hydroxyl groups and Fe–O–Ti bonds on the surface of the material provided the active center for adsorption and formed Fe–O–Ti–Cl bonds after adsorption. The improved material was effective and economical for the removal of residual chlorine in drinking water.

In the current study, we beneficiated valuable iron resources from textile dyeing sludge and developed a recycling method for the tailings generated by the magnetic separation process. Hence, we created a method which named zero-waste recycling for dealing with textile dyeing sludge. In this work, we optimized the magnetic roasting magnetic separation process together with improved tailing content in a ceramic filter material that could meet national water treatment standards. Based on these results, we developed a technology to beneficiate iron with comprehensive recovery of the tailings to solve an environmental problem associated with textile dyeing sludge resources.

Experiment and method

Materials

The textile dyeing sludge materials (drying first) used in this research were provided from Zhejiang Province, China. Because red iron oxide was used in the dyeing process, the iron content in this sludge was high. Chemical compositions of the two materials, as evaluated by X-ray fluorescence analysis, are presented in Table 1. The textile dyeing sludge was rich in iron (26.02%), silicon (11.47%), aluminum (8.93%), calcium (4.59%), and sulfur (2.14%). The magnetic separation tailings were produced by optimized magnetizing roasting–magnetic separation, which we used to prepare the ceramic filter. This typically had a low content of iron (16.47%) and high contents of silicon (12.44%), aluminum (9.28%), and calcium (4.26%). This material represented a qualified feedstock for ceramic filter production.

Table 1 Chemical compositions of textile dyeing sludge and tailings from magnetic separation (mass%)
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The X-ray diffraction (XRD; RIGAKU D/Max 2500, Rigaku, Japan. Radiation: Cu Ka; tube current 250 mA) models of the two materials are presented in Fig. 1. The main minerals of textile printing and dyeing sludge were hematite, quartz, and calcium sulfate, so the magnetizing roasting process was a suitable technology to convert hematite to magnetite. Hematite, quartz, and magnetite were the major minerals of the magnetic separation tailings. The magnetite originated from materials that were not collected by magnetic separation. As shown in Fig. 2, the particle morphologies of the two materials, as evaluated by scanning electron microscopy (SEM; JSM-6490LV, JEOL, Japan), were both loose together with porous, which were helpful for the magnetizing roasting and ceramic filter preparation, respectively.

Fig. 1
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X-ray diffraction patterns of two materials: a textile dyeing sludge; b magnetic separation tailings

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Fig. 2
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Particle morphologies of two materials: a textile dyeing sludge; b magnetic separation tailings

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The laser particle size analyses of the two materials are exhibited in Fig. 3. The materials were both fine-grained, and the main particle size distributions were concentrated in the 10.0–70.0 µm range. This fine particle size meant that subsequent magnetic separation and the ceramic filter preparation process did not require grinding of the materials. The pore volumes were determined to further illustrate the advantage of the two materials for magnetizing roasting and ceramic filter preparation, as shown in Fig. 4. The volumes of pore were 0.025 and 0.039 CC/g, separately. This pore scale was favorable for follow-up calcination because the gas was more easily diffused into the particles through these pores and fractures.

Fig. 3
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Laser particle size analysis of two materials: a textile dyeing sludge; b magnetic separation tailings

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Fig. 4
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Pore volume analysis of two materials: a textile dyeing sludge; b magnetic separation tailings

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Methods

The flowsheet for the experimental test sequence of this study is shown in Fig. 5. Magnetizing roasting was conducted in a muffle furnace. A mass of 500 g of textile dyeing sludge was blended with coal and poured into a cylindrical alumina crucible (diameter 100 mm; height 80 mm). In the high-temperature region of the furnace, there the crucible was placed. After a certain period of roasting, putting the crucible containing the sample directly into a bucket of water for cooling, taking a small part of roasted ore (about 10 g) for analysis, besides, conducting magnetic separation for the remaining materials.

Fig. 5
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Flowsheet for experimental sequence used in this study

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The roasted ore was placed in a CRIMM magnetic drum separator (diameter 400 mm; height 300 mm) operated at a magnetic field intensity of 0.15 T to recover the iron. The magnetic and non-magnetic samples were cleanly separated to obtain concentrate and tailings portions.

The magnetic separation tailings were the main raw material for preparation of the ceramic filter. Moderate amounts of fluxing additive and pore-forming additive were added. Elements affecting the performance of ceramic filters were studied, including the content of tailings, fluxing agent type and content, roasting temperature, and holding time. A 100 g mass of dry material was weighed for each scheme. After mixing, 30–40 ml starch solution was added to generate a slurry. The slurry was loaded into a plastic bag and aged for 72 h and then divided into three parts that were placed into cylindrical metal molds and made into three round blanks (diameter 45 mm; height 10 mm). These were dried in a drying oven. The roasting experiments were conducted in the muff furnace. The heating rate was 5 °C/min, and the holding time was 30 or 60 min. The quality of ceramic filter is assessed according to the JB/T 11098–2011 Chinese National Standard to assess the conclusion.

Results and discussion

Magnetizing roasting

Effects of reductant dosage

A reductant (coal) was necessary for the magnetizing roasting process to convert hematite to magnetite. The influence of the amount of reducing agent on the degree of magnetization roasting was researched. We chose a roasting time of 60 min and a temperature of 750 °C, while the reductant dosage was varied between 2.5% and 10.0%. Figure 6a suggests that the iron rank and the rate of recovery increased with the increase of reductant dosage and remained stable when the dosage exceeded 5.0%. These results showed that 5.0% reductant was adequate for magnetizing roasting under this environment, and the effect of additional dose increases was very small. These consequences showed that the relationship between the amount of reducing agent and the conversion of hematite to magnetite was simple and clear.

Fig. 6
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Effects of reductant dosage of magnetizing roasting on: a iron grade and recovery of magnetic concentrate; and b X-ray diffraction analyses of roasted ore

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The roasted ores with different reductant dosages were evaluated by XRD analysis. The results are shown in Fig. 6b. The peak strength of hematite was greatly reduced, and that of magnetite was correspondingly increased occurred when the reductant dosage increased from 2.5 to 5.0%. When the amount of reducing agent was 5.0%, the peak strength of hematite and magnetite was low, the situation manifested that the magnetizing roasting progress was almost complete and the contents of these two minerals remained stable at higher reductant dosages (7.5% and 10.0%). The minerals of the roasted ore with reductant dosages from 5.0 to 10.0% were essentially the same. Therefore, a 5.0% reductant dosage was considered adequate to achieve complete reaction of the magnetizing roast, and this value was selected for follow-up tests.

Effects of roasting temperature

The effect of roasting temperature on the transformation from hematite to magnetite was studied. The roasting time was 60 min, the amount of reducing agent was 5.0%, and the roasting temperature was 650–850 °C. Iron quality advanced with temperature, whereas the trend for the recovery increased at first and then decreased (Fig. 7a). These results showed that temperature was a significant factor influencing the generation of magnetite.

Fig. 7
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Effects of roasting temperature of magnetizing roasting on: a iron grade and recovery of magnetic concentrate; and b X-ray diffraction analyses of roasted ore

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Mineralogical XRD analysis of the tailings roasted at different temperatures is demonstrated in Fig. 7b. When the temperature increased from 650 to 750 °C, the content of magnetite increased and that of hematite decreased. We attributed this to insufficient magnetizing roasting under low temperatures, resulting in low conversion of hematite to magnetite. The content of magnetite was highest at 800 °C; however, while the temperature was increased to 850 °C, ferrous oxide was present in the roasted ore. We attributed these compositional differences to the over-reduction of hematite, resulting in the generation of FeO, rather than magnetite; the content of magnetite correspondingly decreased. Hence, we selected 800 °C for the magnetizing roasting of this textile dyeing sludge.

Effects of roasting time

The influence of roasting time on magnetization roasting is shown in Fig. 8a. The roasting temperature was 800 °C, the amount of reducing agent was 5.0%, and the roasting time was 30–90 min. The results indicated that with the increase of roasting time, the iron quality increased obviously, while the recovery increased first and then decreased. The results indicated that 45 min was adequate for completion of magnetizing roasting.

Fig. 8
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Effects of roasting time of magnetizing roasting on: a iron grade and recovery of magnetic concentrate; and b X-ray diffraction analyses of roasted ore

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The mineralogical changes of roasted ore were analyzed by XRD (Fig. 8b). For 30–90-min-baked samples, XRD was used to analyze changes in the mineralogy of the roasted ore (Fig. 8b). For specimen roasted from 30 to 90 min, the XRD showed that the magnetite peak intensities increased slightly when the roasting time was below 45 min and remained essentially stable between 45 and 60 min. Longer roasting time had an adverse effect on the hematite-to-magnetite conversion; further increasing the roasting time (75 and 90 min) promoted the formation of ferrous oxide, resulting in a decrease of magnetite content. Hence, a 45-min roasting time was selected for magnetizing roasting of these tailings.

In conclusion, magnetizing roasting–magnetic separation was an efficient way for recovering iron resources from printing and dyeing sludge. Under conditions of 5.0% reductant dosage, 800 °C, and 45 min reaction time, a concentrate with 68.11% iron recovery, 57.85% iron content of the product, and satisfactory mineralogy was obtained.

Ceramic filter production

Effects of content of tailings

The study aimed to improve the content of magnetic separation tailings in ceramic filters element; therefore, the relationship between tailings content and ceramic filter performance was studied. Only silica and a pore-forming agent (charcoal) were blended with the tailings. The batch compositions and results of the preparation of the ceramic filter are presented in Table 2. The holding time and roasting temperature were set as 30 min and 1100 °C, respectively.

Table 2 Compositions of ceramic filter batches with different tailings contents and results for the preparation of agglomerates
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The tailings content had a strong influence on the formation of agglomerates of the ceramic filter. When the tailings contents were 45% and 50%, the agglomerates were scattered because of the shortage of fusion elements, such as Si, Ca, Mg, and Al, which were provided by the tailings. As the tailings content increased to 55% and 60%, the preparation of ceramic filter was successful due to suitable element collocation of the blended materials; however, when the tailings content increased to 65%, the agglomerate was easily liquated. This was mainly due to the excess of low-melting-point materials from the tailings, causing the materials to fuse at lower temperature. Hence, a tailings content of 55% was selected for subsequent experiments.

Effects of fluxing agent type (fly ash or zeolite) and content

The selection of a proper fluxing agent in the composition of the ceramic filter feed can reduce the roasting temperature and enlarge the range of roasting temperatures and can be beneficial to the chemical stability and strength of the ceramic filter. In this study, we chose fly ash and zeolite as fluxing agents, making use of their multiple functions in solubilization, pore-forming, and adjustment of the Si/Al ratio of the material. The batch experimental program considering the fluxing agent type and content is presented in Table 3, the roasting temperature procedure is demonstrated in Table 4, and the agglomerate preparation results for the different batch compositions are shown in Table 5.

Table 3 Compositions of fluxing agent type and content/%
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Table 4 Results for preparation of agglomerates from different batch compositions
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Table 5 Effects of fluxing agent type and content on ceramic filter properties
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The agglomerates prepared by adding fly ash as the fluxing agent were better in the temperature range of 1100 to 1150 °C, while those that added zeolite as the fluxing agent were better in the range of 1050 to 1075 °C. This was due to the different mineral compositions of fly ash and zeolite: the Al2O3 content of fly ash was much higher, requiring a higher roasting temperature.

The effects of fluxing agent type and content on the properties of the ceramic filter are demonstrated in Table 6. When using fly ash as the fluxing agent, the properties of the ceramic filter with 15% fly ash were obviously better than that with 20% fly ash. A higher content of fly ash would create more liquid, blocking the pores and decreasing water absorption and porosity.

Table 6 Properties of ceramic filter compared with JB/T 11098–2011 National Standard
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When using zeolite as the fluxing agent, the properties of the ceramic filter were worse than when adding fly ash. The Al2O3 content and melting point of zeolite were lower: it was completely fused at about 1000 °C. The melting points of the tailings and silica were much higher, and these components could not react completely at this temperature, so that there was a mismatch between the ingredients. When the tailings reached their melting point, the structure of the zeolite itself had been destroyed and the intention of using zeolite to improve the porosity of the ceramic filter could not be achieved; on the contrary, the zeolite liquid phase may block the pores.

When simultaneously using fly ash and zeolite as fluxing agents, although the properties were improved compared with that of adding zeolite alone, there was still a mismatch in melting points between zeolite and the other raw materials, and the overall properties were not very good. Thus, the properties of the ceramic filter produced by adding 15% fly ash as the fluxing agent were better and we selected this condition for the following experiments.

Effects of roasting temperature

High-temperature roasting was a crucial process to allow materials melting, particle bonding, and strength forming in the preparation of the ceramic filter. The feed composition was as follows: 55% tailings, 20% silica, 15% fly ash, together with 10% charcoal; the holding time was fixed at 30 min.

The effects of roasting temperature on the performance of ceramic filter are shown in Fig. 9. At the elevated roasting temperature, the porosity and water absorption of the ceramic filter first increased and then remained stable at 1100 °C. There was little reduction in these properties at 1125 °C; however, when the temperature overtook 1125 °C, the porosity and water absorbency declined significantly. The main reasons for the above results were as follows: although liquid was generated when the temperature was below 1100 °C, its fluidity was poor, resulting in poor properties of the ceramic filter; when the temperature was between 1100 and 1125 °C, crystal growth inside the ceramic filter was completed and the liquid phase started to diffuse uniformly, which increased the anti-bending strength of the ceramic by bonding on the surface of the agglomerates; when the temperature exceeded 1125 °C, the liquid phase continuously flowed into the pores under the effect of the capillary force. Based on the aggregative properties of the ceramic filter, we chose 1100 °C for subsequent experiments.

Fig. 9
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Effects of roasting temperature on ceramic filter properties

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Effects of holding time

There was a complementary relationship between holding time and roasting temperature. The holding time had a significant effect on the formation and growth of pores. When the holding time was too long, the pores in a sample would be continuously penetrated to form large holes of random shapes at a given temperature; when the holding time was too short, the degree of inner corrosion of the sample was inadequate and there were only some unformed pores. Regular and homogeneous pores can only be formed under proper holding conditions. The composition of the feed was as follows: 55% tailings, 20% silica, 15% fly ash, and 10% charcoal; the roasting temperature was 1100 °C, and the holding times were 30 min and 60 min, respectively.

The porosity of the ceramic filter was 41.06% at a holding time of 30 min, while that of the sample after 60 min holding time was 35.6%, showing a significant difference. SEM images of the ceramic filter formed at different holding times are shown in Fig. 10. When the holding time was 30 min, the holes of the specimen were evenly distributed, with a pore diameter of less than 100 μm. The pores were interconnected and presented a three-dimensional structure, so the porosity was higher. When the holding time was 60 min, the liquefaction effect of the specimen was very strong, the glass phase increased, small pores expanded into irregular large holes with a diameter greater than 100 μm, and the porosity was lower. Hence, the properties of the ceramic filter were better at 30 min holding time.

Fig. 10
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Scanning electron micrographs of ceramic filter formed at holding times of a 30 min and b 60 min

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Filtering ceramsite prepared under optimal conditions

On the basis of the over test results, the optimum preparation parameters for the ceramic filter were: 55% magnetic separation tailings, 15% fly ash, 20% silica, and 10% charcoal, 1100 °C roasting temperature, 30 min holding time. The morphology and SEM images of the ceramic filter media prepared under these optimum conditions are shown in Figs. 11 and 12, respectively; Table 7 illustrates the properties of this ceramic filter compared with JB/T 11098–2011 Chinese National Standard. The pores were even and regular, and the properties of the ceramic filter met the requirements of the requisite standard.

Fig. 11
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Appearance of ceramic filter specimens prepared under optimal conditions

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Fig. 12
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Scanning electron micrographs of ceramic filter prepared under optimal conditions: a 100 × magnification; b 200 × magnification

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Conclusion

In this work, we explored a zero-waste recycling technology for textile dyeing sludge by a magnetizing roasting–magnetic separation process and ceramic filter preparation. We provided a potential method to solve an environmental problem associated with textile dyeing sludge resources. The main verdicts were as follows:

  1. (1)

    The best parameters of magnetization baking procedure included blending the textile dyeing sludge with 5.0% content of coal and roasting at 800 °C for 45 min. The main iron-bearing mineral (Fe2O3) was converted to Fe3O4. The roasted ore was beneficiated by low-intensity magnetic separation (0.15 T magnetic field intensity). A concentrate grading 57.85% iron was obtained with 68.11% iron recovery. Magnetic roasting magnetic separation procedure was an efficient way to recover iron resources from textile printing and dyeing sludge.

  2. (2)

    Magnetic separation tailings were suitable raw materials for the preparation of ceramic filters for water treatment. The scale of tailings would be raised up to 55%, and they were blended with 15% fly ash, 20% silica, and 10% charcoal and roasted at 1100 °C for 30 min. We produced ceramic filter specimens that met the national standards for qualified water treatment materials.