Recovery of Cr (Ⅵ) from tannery sludge and chrome-tanned leather shavings by Na2CO3 segmented calcination
Graphical Abstract
Recovery of Cr (VI) from tannery sludge (Ts) and chrome-tanned leather shavings (CTLs) by first calcination and segmenting calcination.
Introduction
In 1797, French scientist Louis Vauquelin discovered an element with different colors when it was present in different chemical forms. The term “chroma” means “color”, and because one of the compounds that contained this element was green, the element was termed chromium (Cr) [1]. Cr exists mainly as Cr (III) and Cr (VI) in compounds in nature [2]. Cr (III) and Cr (VI) are both ionic forms of Cr, but they have different chemical properties. Cr (III) can form insoluble and relatively stable complexes with oxygen and donor ligands, which makes Cr (III) flow with difficulty; Cr (VI) exists mainly in the form of chromate and dichromate, which are highly toxic, fluid; and soluble [1]. Cr (Ⅵ) is second in toxicity after the arsenic oxygen-containing anion according to the CERCLA 2011 Priority List of Hazardous Substances [3]. Cr (Ⅵ) can cause acute toxicity, irritate nasal septum ulcers and cause respiratory sensitization (asthma) after inhalation [4], [5]. For this reason, the United States Environmental Protection Agency recommend a total Cr limit of 100 µg/L in drinking water [6]. If a large amount of Cr (VI) is present in environmental media for prolonged periods, it would be harmful to the ecosystem. Regarding technology to resolve this problem was reported in extensive literature, including the use of adsorption materials (for example, biological materials and activated carbon materials) to fix Cr [7], [8], [9], [10], [11]. Furthermore, toxic Cr (VI) can be reduced to the less toxic Cr (III) by chemical methods [12], [13], [14], [15]. However, these methods solidify and stabilize Cr and its compounds, and they cannot be recycled fully. An approach to the effective disposal of such " hazardous materials" has been a topic of great interest in the scientific research field.
Tannery sludge (Ts) and chrome-tanned leather shavings (CTLs) originate from the fur-tanning and product-processing industries. They are classified as HW21 Cr-containing wastes according to the national hazardous waste list [16]. Ts is a wastewater treatment sludge that is produced by chrome tanning and retanning using a chrome tanning agent. Approximately 2.5 × 106 tons of sludge require disposal in China annually [17]. CTLs are chrome-containing leather scraps that are produced by leather cutting technology. They are composed mainly of protein, collagen and Cr [18]. A ton of raw hides can generate about 600 kg of solid waste, including about 250 kg (CTLs) [19], [20]. Two main disposal methods exist for the hazardous Ts and CTLs wastes. In the first, Ts and CTLs are solidified and stabilized by cement and sent to landfill for disposal [21], [22]. To prevent the leaching of harmful materials, the pretreatment process requires a large amount of cement materials [23], [24], which increases the disposal cost. Because of the reduction of the number of available landfills, this disposal method may be abandoned in the future. The second approach involves sending the Ts and CTLs to an incineration plant [25]. The incineration approach can reduce the waste volume to less than 10%. A high-temperature environment can eliminate bacteria, pyrolyze toxic substances and recover the energy that is released during disposal [26]. However, a high temperature can promote toxic heavy metal volatilization. Although flue gas treatment systems can capture the gases [27], [28], a residual amount could harm the atmosphere.
Both Ts and CTLs contain significant amount of Cr, which was 3.26% and 4.22% respectively in this study. They can be oxidized into Cr (VI) in large quantities, and the recovered Cr (VI) can be transferred to the electrochemical industry to prepare chromic acid or can be used as a passivator for the passivation treatment of galvanized sheet. This treatment method can avoid problematic waste disposal and provide economic benefits.
In recent years, it has been reported that Cr (III) can be converted into Cr (VI) in an oxidizing atmosphere, but incineration under anoxic conditions substantially reduced the extent of oxidation of Cr (III) to Cr (VI) [29]. O2 is a necessary requirement for Cr (III) oxidation to Cr (VI). Besides, the thermodynamic calculations in Fig. 2 show that Cr (III) (Cr2O3) can be oxidize to Cr (VI) when the temperature is above 1200 ℃. Mao indicated that Cr (III) can be converted to CaCrO4 by CaO at an appropriate temperature [26]. Verbinnen et al. found that Cr (III) can be converted to Cr (VI) by alkali metal hydroxides (NaOH, KOH) and alkaline earth oxides (Cao, MgO) in a certain temperature range [3]. The reaction equations are:Cr2O3+1.5O2(g)+4NaOH→2Na2CrO4+2H2OCr2O3+1.5O2(g)+4KOH→2K2CrO4+2H2OCr2O3+1.5O2(g)+2CaO→2CaCrO4Cr2O3+1.5O2(g)+2MgO→2MgCrO4
Therefore, Cr (III) favors recovery of Cr (VI) from environments with an oxidizing atmosphere and alkaline substances. Most raw materials that were used in experiments have been Cr-containing compounds [3], [26], [30]. However, because of the complex composition of Ts and CTLs, the real recover of Cr cannot be measured this way.
This work aimed to explore the feasibility of using Na2CO3 as a "catalyst" to convert Cr (III) from Ts and CTLs into Cr (VI) in an oxygen-enriched environment. The successful development of this method has a good potential to be a low cost and simple process with a high the recovery of Cr (Ⅵ), which would be beneficial to the disposal of Ts and CTLs in industrial production.
Section snippets
Materials
Air dried Ts (10 kg) was recovered from a wastewater treatment site of the cluster in the Wuji county industrial zone of North China. This type of solid waste that contain Cr is composed of materials with a weak adhesion. Ts was homogenized by a crusher, and crude Cr-containing solid waste was obtained. The same amount of waste chrome leather scraps (CTLs) was collected from local leather factories.
The crude products were placed in a 105 ℃ vacuum oven (XY-GZL-500) and dried at constant
Thermodynamic calculation
The feasibility of the experiment was discussed by thermodynamic calculation. Cr (III) in Ts exists as Cr (OH)3 [30], and as Cr2O3 in CTLs [33]. The reaction equation between these compounds and O2 is:2Cr(OH)3+1.5O2(g)→2CrO3+3H2OCr2O3+1.5O2(g)→2CrO3
Fig. 2 shows that Cr2O3 and O2 cannot react directly to form CrO3 (Eq. (5)) (Δ Gr > 0) under normal conditions. Paoletti proposed that the oxidation of Cr (III) to Cr (VI) would not occur spontaneously at the temperature was below 1500 ℃ [34]. With Na
Conclusions
Thermochemical calculations indicated that a high temperature, sufficient oxygen and rich alkali (Na2CO3) are conducive to the oxidation of Cr2O3 and Cr (OH)3 to Cr (VI).
Raw material analysis showed that Ts and CTLs contain organic matter that can affect the recovery of Cr (VI).
Na2CO3 does not affect the decomposition of organic matter in Ts and CTLs, but Na+ can accelerate the recovery of Cr (VI).
The effect of organic matter and glassy chromate phase was eliminated by staged calcination
CRediT authorship contribution statement
Hua Long: Methodology, Writing - original draft, Investigation, Resources, Data curation. Xiaolu Huang: Formal analysis, Supervision, Investigation. Yang Liao: Writing - review & editing, Funding acquisition. Jidong Ding: Validation, Conceptualization.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China under Grant (41641010 and 21406147), the Foundation of Sichuan Science & Technology Committee under Grant (2020YFH0162).
Author contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
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