Investigation on pyrolysis and incineration of chrome-tanned solid waste from tanneries for effective treatment and disposal: an experimental study

Depollution of Polymeric Leather Waste by Applying the Most Current Methods of Chromium Extraction

The leather industry is one of the most polluting industries in the world due to the large amounts of waste following raw hide processing but also due to the high content of chemical substances present in leather waste. The main problem with chromium-tanned leather solid waste is related to the storage, due to the ability of chromium to leach into soil or water, and also owing to the high ability of trivalent chromium to oxidize to its toxic form, hexavalent chromium. The purpose of this article is to present the most current methods of chromium extraction from solid tanned leather waste in order to obtain non-polluting leather, which can constitute secondary raw material in new industrial processes. The extraction methods identified in the present study are based on acid/basic/enzymatic hydrolysis and substitution with the help of organic chelators (organic acids and organic acid salts). In addition, this study includes a comparative analysis of the advantages and disadvantages of each identified extraction method. At the same time, this study also presents alternative chromium extraction methods based on the combination of conventional extraction methods and ultrasound-assisted extraction.

Fate of sulfur and chlorine during co-incineration of municipal solid waste and industrial organic solid waste

In China, the co-incineration of municipal solid waste (MSW) with industrial organic solid waste (IOSW) is increasingly adopted. Compared with MSW, IOSW contains higher levels of sulfur (S) and chlorine (Cl), presenting significant challenges for controlling S/Cl emissions in MSW incineration plants. In this study, the impact of co-incinerating IOSW was investigated in a 500 t/d incinerator grate, focusing on the emissions and transformation behaviors of S/Cl. IOSW, with a consistent sulfur content of about 0.22 wt% and a more variable chlorine content averaging 0.53 wt%, contains over 40 % organic sulfur and >90 % organic chlorine, higher than in MSW. The results of co-incineration experiments showed that the median SO2 concentration in the flue gas was stable at 50 mg/m3, while HCl concentration decreased initially and then increased as the co-incineration ratio of IOSW rose from 20 % to 40 %. Furthermore, the concentrations of SO2 and HCl were not significantly influenced by wind flow but were positively affected by the rising furnace temperatures. Besides, the co-incineration ratio had minimal impact on sulfur in fly ash before deacidification, primarily derived from the gas stream. However, the (Na + K)/Cl ratio in fly ash progressively increased from 1.5 to 1.9, and the Ca content decreased from 0.35 % to 0.15 % as the co-incineration ratio rose to 40 %, indicating more chlorine migration into the fly ash at higher co-incineration rates. This research offers essential guidance for effectively controlling pollutant emissions during the co-incineration of IOSW, specifically the S/Cl pollutants.

Cleaner Leather Tanning and Post-Tanning Processes Using Oxidized Alginate as Biodegradable Tanning Agent and Nano-Hydroxyapatite as Potential Flame Retardant

In this study, sodium alginate (SA) was oxidized with potassium periodate to produce an alginate-based tanning agent. Using OSA as a biodegradable tanning agent and a nano-hydroxyapatite (nano-HAp) low concentration suspension to give flame retardancy to leather, eco-design concepts were applied to establish a chrome-, aldehyde-, and phenol-free tanning process. Micro-DSC, 1H unilateral nuclear magnetic resonance (NMR), attenuated total reflection mode Fourier transform infrared spectroscopy (FTIR-ATR), and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) were used to investigate the complex matrix collagen-OSA-nano-HAp. Micro-differential scanning calorimetry (micro-DSC) was used to assess OSA's ability to interact with collagen and stabilize the collagen-OSA matrix, while 1H unilateral (NMR) was used to investigate the aqueous environment and its limitations around collagen molecules caused by their association with OSA and nano-HAp. Industrial standard tests were used to assess the mechanical properties and fire resistance of the new leather prototype. The findings reported here indicate that both OSA and nano-HAp are suitable alternatives for cleaner tanning technologies and more sustainable leather.

Potential of biofuel production from leather solid wastes: Indian scenario

India is one among the major leather-producing countries in the world which shares close to one-fourth of the world's leather solid wastes and most of these wastes are not effectively utilized. These wastes are rich in protein and lipids that could be a potential feedstock for biofuel production, i.e., biogas, biodiesel, etc. Among the 150,000 tons of daily leather solid wastes in India, approximately 87,150 tons are shared by pre-tanning operations (i.e., raw trimmings, fleshing, and hair wastes) while the rest of the 62,850 tons are shared by tanning, post-tanning, and finishing operations (i.e., wet blue trimmings, chrome splits, shavings, buffing dust, crust trimming wastes). This review article shows that there is considerable bioenergy potential for the use of leather solid wastes as a green fuel. The biogas potential of leather solid wastes is estimated to be 40,532.9 m3/day whereas the biodiesel potential is estimated as 15,452.6 L/day. The bio-oil and bio-char potential of leather solid wastes is estimated to be 80,513.0 L/day and 45.8 tons/day, respectively. Several factors influence the biofuel process efficacy, which needs to be taken into consideration while setting up a biofuel recovery plant. The overall biofuel potential of leather solid wastes shows that this feedstock is an untapped resource for energy recovery to add commercial benefits to India's energy supply. Furthermore, in addition to the economic benefits for investors, the use of leather solid wastes for biofuel production will yield a positive environmental impact.

R, Choudhary S, Singh R, Verma AK. Optimization of biogas potential using kinetic models, response surface methodology, and instrumental evidence for biodegradation of tannery fleshings during anaerobic digestion

The optimization of the batch size experiment was run for a hydraulic retention time of 45 days using proteolytic enzyme pretreatment. The highest amounts of biogas were produced in comparison to conventional BDS (25:75), which is not processed with enzymes, and there was an increase in the biogas generation of 13.9 and 18.57%. The kinetic models show the goodness of fit between 0.993 and 0.998 and the correlation coefficient's value domain was [-1, 1] from a statistical perspective. The Box-Behnken design was carried out using the response surface methodology at different levels of independent parameters to optimize the process. Different instruments were evaluated to determine the chemical structure change and the contamination of the different treatments and the raw sample of tannery fleshings was determined. Thermogravimetric analysis was conducted to determine the loss of weight on thermal degradation. The Fourier transform infrared spectrometry was carried out to determine the different functional groups, such as -OH, -CH, -NH, and C-O, present in the samples of tannery fleshings. Scanning electron microscopy and energy dispersive X-ray analysis were carried out to determine the morphological alterations in the substrate, digestate, enzyme-pretreated fleshings, and the chemical composition of samples.

Carbonized Leather Waste: A Review and Conductivity Outlook

The carbonization of collagen-based leather waste to nitrogen-containing carbon is reviewed with respect to the preparation, characterization of carbonized products, and applications proposed in the literature. The resulting nitrogen-containing carbons with fibrous morphology have been used as adsorbents in water pollution treatment, in electrocatalysis, and especially in electrodes of energy-storage devices, such as supercapacitors and batteries. Although electrical conductivity has been implicitly exploited in many cases, the quantitative determination of this parameter has been addressed in the literature only marginally. In this report, attention has been newly paid to the determination of conductivity and its dependence on carbonization temperature. The resulting powders cannot be compressed into pellets for routine conductivity determination. A new method has been used to follow the resistivity of powders as a function of pressure up to 10 MPa. The conductivity at this pressure increased from 9.4 × 10^-8 S cm^-1 for carbonization at 500 °C to 5.3 S cm^-1 at 1000 °C. The conductivity of the last sample was comparable with conducting polymers such as polypyrrole. The carbonized leather thus has the potential to be used in applications requiring electrical conduction.

Ultrasound-assisted extraction of chromium from tanned leather shavings: A promising continuous flow technology for the treatment of solid waste

In this work, a continuous flow extraction system assisted by ultrasound (US) was developed for the extraction of Cr(III) from residual tanned leather shavings. US energy was delivered into the system by a tubular applicator (clamp-on tube US applicator). The effect of the US energy was investigated at 20 kHz of frequency and electrical input power of 75, 150, 300 and 600 W. Residence time and temperature profile were also evaluated. It was observed that the internal temperature profile was affected by the presence of US and inverted in comparison with the conditions without US. In this way, the temperature profile generated by the US was reproduced by using electrical resistances in order to compare the obtained results. The US intensity was measured using a hydrophone connected to a sound pressure meter. The use of the US did not alter the dynamic behavior of the system but increased the extraction efficiency when compared to the silent condition. US power above 75 W did not lead to increased extraction efficiency, when the residence time was 30 min. However, when 60 min of residence time were employed, the optimized US power was 150 W, resulting in an extraction efficiency of 71.7 ± 0.7 %, about 28 % higher when compared to the silent condition in the same temperature and other conditions. The US energy allowed a reduction in processing time and operational temperature when compared to the silent condition with the same temperature profile. The overall energy consumption with US was similar or lower than that observed without US, showing the feasibility of the proposed extraction system.

Current trends in solid tannery waste management

The tannery is one of the leading revenue-generating sectors in developing countries. The ever-increasing demand for leather products in the global market requires converting large amounts of rawhide/skins into resilient non-putrescible finished leather. Only 20% of the raw material is converted into a finished product; the rest 80% is discarded as solid and liquid wastes during leather processing. A heavy discharge of improperly treated solid tannery waste (STW) causes a severe impact on the surrounding environment by polluting soil, surface water, and groundwater resources, posing severe hazards to human and animal health. STW comprises proteinaceous untanned and tanned waste, which requires proper treatment for eco-friendly disposal. Several strategies have been developed over the years for the reduction and recycling of STW for producing renewable energy (biogas and biohydrogen), biofuels (biodiesel and briquettes), construction material, fertilizers, commercial products (adsorbents, animal feeds, proteins, fats, and enzymes), and biodegradable packaging and non-packaging materials. In this review, we discuss various strategies adopted for recycling, reutilization, and reduction of STW in an environment-friendly manner. Furthermore, an overview of the current perspectives toward achieving a zero-waste policy is also presented to reduce the environmental burden using green-clean technology to aid the survival of present-day tanneries.

Slaughterhouse and poultry wastes: management practices, feedstocks for renewable energy production, and recovery of value added products

The slaughterhouse and poultry industry is possibly one of the fastest-growing sectors driven by the increasing demand in food availability. Subsequently, the wastes produced from the slaughterhouse and poultry industry are in huge quantities, which could be a promising resource for the recovery of value added products, and bioenergy production to minimize the dependence on fossil fuels. Furthermore, the wastes from slaughterhouses and poultry are a hub of pathogens that is capable of infecting humans and animals. This demands the emerging need for an effective and safe disposal method to reduce the spread of diseases following animal slaughtering. In light of that, the state of the production of slaughterhouse and poultry wastes was presented at first. Following this, the impact of solid waste exposure in terms of air, water, and soil pollution and the associated health challenges due to improper solid waste management practices were presented to highlight the importance of the topic. Secondly, the potency of these solid wastes and the various waste-to-energy technologies that have been employed for effective management and resource utilization of wastes generated from slaughterhouses and poultry were reviewed in detail. Finally, this review also highlights the opportunities and challenges associated with effective solid waste management, future requirements for the development of effective technologies for the recovery of value added products (like keratin, fibreboards), and biofuel production.

Research on the Influence of Combustion Methods on NO x Emissions from Co-combustion of Various Tannery Wastes

To further increase combustion efficiency and reduce nitrogen oxide pollution caused by tannery wastes, three raw materials, including tannery sludge, chrome-tanned buffing dust, and chrome shavings, were burned together in a dual-bed model reactor under various conditions. In addition, a thermogravimetric analysis of co-combustion of three tannery wastes was studied in this study, which was conducive to understanding the combustion characteristics and positive effects. The comprehensive combustibility index S, the flammability index K _r, and the stable combustion characteristic index G _b all increased when the tannery sludge was blended with chrome-tanned buffing dust and chrome shavings, indicating that the combustion behavior was improved by co-combustion. For normal combustion, decreasing the gas volume flow and temperature resulted in a decrease in the oxidation of nitrogen compounds, consequently lowering the NO _x emission. During air staged combustion, at an appropriate secondary gas ratio of about 10-40%, the NO _x reduction would be increased from 10.9 to 19.3%. By increasing the tertiary gas volume flow from 0.2 to 1.1 L/min in decoupling combustion, an average relative NO _x reduction efficiency of 47% was attained compared with normal combustion. The results offered a viable technology that resulted in a lower NO _x emission and realized the application of decoupling combustion.

Valorization of rubberwood sawdust and sewage sludge by pyrolysis and co-pyrolysis using agitated bed reactor for producing biofuel or value-added products

This study investigated experimentally pyrolysis of rubberwood sawdust (RWS), sewage sludge (SS), and their blends (25:75, 50:50, and 75:25 by weight) in an agitated bed pyrolysis reactor. The yields and characteristics of liquid product and biochar were determined for pyrolysis at 450, 500, and 550 °C and were affected both by temperature and feedstock type. The liquid and biochar yields were in the ranges 27.30-52.42 and 21.43-49.66 (wt%). Pyrolysis of RWS at 550 °C provided the highest liquid yield, while SS gave a high biochar yield. Co-pyrolysis of SS with RWS improved yield and quality of liquid and biochar products. The liquid product had 57.54-70.70 wt% of water and a low hydrocarbon content. The higher heating value (HHV) of water-free liquid product was 14.73-22.45 MJ/kg. The major compounds of liquid product included acetic acid, 2-propanone, 1-hydroxy, and phenols according to GC-MS. The biochar from RWS had a high carbon content (83.37 wt%) and a high HHV (33.57 MJ/kg), while SS biochar was mainly ash (67.62 wt%) with low carbon content. The SS biochar also had high contents of Si, Ca, Fe, K, and Mg as determined by XRF. Co-pyrolysis of SS with RWS improved the biochar by increasing its carbon content and reducing ash and inorganic elements. The surface of RWS biochar was more porous, while SS biochar had the larger specific surface according to SEM and BET. Based on these results, co-pyrolysis of 75:25 feedstock mix is recommended for further studies on applications of liquid product and biochar.

Regeneration of native collagen from hazardous waste: chrome-tanned leather shavings by acid method

The collagens (COL2, COL4, and COL5) were extracted from chrome-tanned leather shavings via three distinctive routes of acid method. The dechroming degree of COL2 extracted with the easiest operation was the highest (95.6% ± 1.2%) and the yield exceeded 90%; however, the total amount of acid was the most and the cost was the highest. In the second route, although the three-step dechroming process brought cumbersome operation, the dechroming degree and yield of COL4 were 90.5% ± 0.8% and 92.2% ± 0.6%, respectively, and the acid amount was less than that in the first route. For COL5, the dechroming degree and yield was the lowest; nevertheless, this route had the advantages of lowest cost and simpler operation. Electrophoretic patterns showed that all the collagens contained α1, α2, and β chains without low molecular weight components and were close to those of type I collagen. Compared with native collagen extracted from fresh calf skin, the regenerated collagens also maintained unique triple helix conformation determined via ultraviolet, infrared spectra and X-ray diffraction, confirmed by the similar values of AIII/A1455 and Δν. Additionally, the collagens existed in the form of fibrils with D-period pattern of ~ 67 nm. Furthermore, the denaturation temperatures of COL2, COL4, and COL5 were 71.2, 79.1, and 85.4 °C, respectively, which were relevant to the tighter arrangement of fibrils with the increased chromium content.

Selection of Tanned-Leather Waste in Recovering Novel Raw Material for Manufacturing Rubber Artifacts: Towards a Zero-Waste Condition

Zero-waste is an ambitious goal to encourage sustainable production, consumption optimum recycling and resource recovery. The recycling and resource recovery of chrome-tanned leather waste is a difficult proposition because of the three-dimensional chemical network of collagen, which renders leather waste incompatible to polymer matrices. Many attempts have been undertaken to reuse leather waste, most of them require technological processes and challenging chemical pretreatments, which make the reuse an economically disadvantageous industrial operation. The paper aims to show how a very fine separation of leather solid waste leads to raw materials suitable for mixtures with natural rubber, without any chemical pretreatment. In other words, the paper intends to be a stimulus to manage leather solid waste carefully to start a cleaner and more profitable production. In particular, various industrial compounds containing different concentrations of post-tanning dyeing, or chrome shaving or oil-tanning and natural rubber were vulcanized through a standard preparation. Some compounds were also reinforced with carbon black and the vulcanization kinetics were monitored through rheometer. Experimental results indicate that there exists an optimal concentration of waste such that post-tanning dyeing post-tanning and chrome shaving are suitable for manufacture floor mats, soles and heels while oil-tanning waste can be used for making hose or conveyor belts. The mechanical performances of the articles prepared by direct mixing with leather waste are the same as those of commercially distributed rubber products.

Chromium Concentrate Recovery From Solid Tannery Waste in a Thermal Process

Leather processing requires substantial inputs of energy, water and chemicals. Additionally, it generates significant amounts of liquid and solid waste, severely impacting the environment. Processing 1 Mg of raw hides yields up to 600-700 kg of waste, considerable amounts of which are solid tannery waste. Such waste contains chromium (Cr) compounds, which are commonly used as tanning agents. This paper reviews solid tannery waste treatment technologies, with emphasis on waste incineration in a specially designed experimental tunnel incinerator. Three different types of tannery waste were subjected to tests: trimmings, shavings and buffing dust. As the research revealed, the process can be applied to all types of solid tannery waste. Moreover, it enables the reuse of the heat of the process and results in a Cr concentrate in the process residues. The conducted analyses (carbon, hydrogen and nitrogen elemental analysis; inductively coupled plasma optical emission spectroscopy; powder X‑ray diffraction) proved that there is no or little organic content in the obtained residual ash, which contains up to 53.1%(w/w) Cr in the form of Cr (III) oxide. Such material may be used as a Cr ore substitute in the chemical or metallurgical industries.