Recovery and reuse of chromium from chrome tanning waste water aiming towards zero discharge of pollution

The leather industry faces very challenging task because of the regulations enforced by the various pollution control bodies to maintain the discharged norms of the chemical in the treated water. The pollution due to the use of chromium and other chemicals in the leather industry is critically analyzed for the harmful effects. Many studies have been undertaken to optimize the amount of chemicals used in leather processing. The pollution due to chromium is one of the major constraints in the leather processing operation. Chrome recovery and reuse methods have become the compulsory recovery methods in the leather industry to recover and reuse the chromium present in the effluent. Several precipitation methods are emerging but still they are not feasible. In this paper, an attempt has been made to recover the chromium present in spent tan liquor using neutralized wattle extract. The wattle extract at different pH conditions (pH 7, 8, and 9) was used for recovery of chromium. The neutralized wattle extract was made to react with chromium in the spent liquor and allowed to settle for a period of 6 h. Major amount of chromium settles at the bottom leaving minimal amount in the supernatant. The chrome was recovered and reused for tanning the pelt. The wattle extract that was left in the tanning bath was reused for post-tanning process as a retanning agent. Therefore, complete utilization of the chromium and wattle extract has been achieved in this work. It has also been noticed that increased level of chrome exhaustion of 85% and 87% (precipitate and supernatant) in experimental leather could be obtained in comparison with the exhaustion of 74% in control sample. The physical strength and color properties of the experimental leather were comparable to the control leather. The overall results show that the recovered chromium can be well used for tanning purpose to obtain the leather of comparable properties.     

Reducing the environmental impact of the unhairing–liming process in the leather tanning industry

The environmental impact of the tanning industry is generally significant with outputs of wastes, i.e. high concentrations of organics, salts and heavy metals (chromium compounds), both in solid and liquid form, as a result. In order to bring the tanning industry more in line with present environmental thinking, various methods have been devised to reduce impacts.The underlying study proposes a modification of the method for unhairing–liming of hides where the unhairing–liming liquids are reused several times after being recharged by reduced quantities of chemicals. The study, therefore, aims at reducing both the economic and environmental costs of the unhairing–liming process.Experiments were carried out at lab scale with a simulation apparatus designed for the purpose. Life cycle assessment was used to evaluate the net environmental benefits of the modified method. The present value approach was used to evaluate the economic feasibility of the modified method. The quality of the produced leather was assessed by experts from the tanning sector (tanners).On the environmental level, the modified method reduced the environmental impact of the process by 24%, COD was reduced by 50% as well as sulfide which was reduced by 73% when the process water was recycled four times.The modified method requires some investment in new equipment and is a little more labor intensive as compared with the conventional method but does permit for savings in water up to 58% and chemicals up to 28% as well as wastewater treatment cost which was reduced by 58%. The modified method allowed for four times reuse of the unhairing–liming liquor without visibly affecting the quality of the final product of leather.It was concluded that both the economic and the environmental costs of the unhairing–liming process were reduced relative to the same of the conventional method.     

An improved product-process for cleaner chrome tanning in leather processing

Severe restrictions imposed by the pollution control authorities on the disposal of chromium, total dissolved solids and chlorides in tannery effluents have forced the tanners to look for low-waste, high exhaust chrome tanning salts. An improved chrome syntan with more than 90% uptake of chrome has been developed. The new product serves both as tanning and retanning agent and can be applied directly to delimed pelts thus eliminating the conventional pickling stage in the leather processing. This modified process helps to reduce the chemical oxygen demand (COD), total dissolved solids (TDS) and chlorides in the spent tan liquor by 51, 81 and 99%, respectively. The product offers full, soft leathers having shrinkage temperature comparable to conventional chrome tanned skins. Since the developed product is highly reactive, it saves time and reduces the water requirement when compared to the conventional chrome tanning method. Thus the novel product/process developed not only has advantages in reducing pollution loads but also seems to be techno-economically viable.     

Cleaner tanning practices for tannery pollution abatement: Role of enzymes in eco-friendly vegetable tanning

Concern about pollution related problems in the global scenario are persuading all the processing industries to adopt cleaner manufacturing practices. Thus, the leather industry is also under pressure to look for effective alternative tanning materials for chromium. Natural products like vegetable tannins are regaining importance. However, there are limitations in the use of vegetable tanning materials because of its high organic load in the effluent, which are difficult to degrade leading to high chemical oxygen demand (COD). Moreover, conventional vegetable tanning process requires partial pickling that involves the use of sodium chloride, to suppress osmotic swelling. This results in very high amount of total dissolved solid (TDS) content in wastewaters. In this investigation, an attempt has been made to design an eco-friendly vegetable tanning process combining pickle-free tanning and application of proteolytic enzymes to improve the exhaustion of vegetable tannins. Such an approach has resulted in more than 95% tannin exhaustion in the case of the experimental process, an increase of 10% compared with the conventional vegetable tanning process. The tanned leathers showed slight improvement in hydrothermal stability. Physical and tactile evaluation of experimental leathers has been better than conventionally tanned leathers. Surface colour values illustrated negligible variation in colour and shade between control and experimental leathers. The resultant leather showed opened up, split compact fibre structure that has been well coated, indicating that the enzyme assisted tanning process did not bring about any major change or destruction on the fibre structure of the leathers. The optimized system has been field tested in a commercial tannery. The results showed that the enzyme assisted tanning process is efficient in terms of improved quality of leather and also led to reduction in total solids (TS), chlorides and COD loads. The enzyme assisted tanning system presented appears to be a viable option for combating pollution arising from the conventional vegetable tanning system.     

基于鞣制工艺参数的铬污泥产污系数核算方法

针对无明确化学计量关系、均相或非均相封闭系统的一次性化工过程中产生的具有固定组成的污染物,其单位产品污染物产生量(即产污系数)的实测会因产品加工周期与污染物产出量相脱节而得不到准确值.制革行业铬鞣工段的诸多工艺参数对皮中铬的吸收率具有显著影响,通过实验室试验及历年工艺手册数据整理,在建立单一工艺参数与铬吸收率之间的关系基础上,采用逐步优化回归分析法,建立了鞣制工艺参数与铬污泥产生量之间的定量关系,并与企业实测铬污泥数据进行了对比.结果表明,通过工艺参数计算的铬污泥产污系数完全可以反映实际情况.      

A chemo-enzymatic pathway leads towards zero discharge tanning

Leather processing has been an important industrial activity, which has gained significant economic relevance in India. The recent practices of leather manufacture cause difficulties with regard to environmental challenges. The conventional method for making the skins ready for tanning and the tanning by itself employs a wide variety of chemicals which result in an increase in chemical oxygen demand (COD), total dissolved solids (TDS), chlorides, sulfates and chromium in the tannery effluent. In this study, an integrated chemo-enzymatic methodology has been explored which would minimize or to some extent eradicate the unsafe chemicals involved in the process to provide a clean environment. The sequence involves an enzymatic dehairing, NaOH based fibre opening and a pickle-less chrome tanning. The modified process results in decrease in COD and TS (total solids) loads by 67 and 78%, respectively, as compared with control process. The process explored appears to be economically viable.     

Reclamation treatment of the chrome leather scrap

ＩｎｔｒｏｄｕｃｔｉｏｎＣｈｒｏｍｅｌｅａｔｈｅｒｓｃｒａｐｉｓｔｈｅｓｏｌｉｄ ｗａｓｔｅｏｆｌｅａｔｈｅｒｉｎｄｕｓｔｒｙ .Ｉｔｓｍａｉｎｃｏｍｐｏｓｉｔｉｏｎｉｓｐｒｏｔｅｉｎ(ｍａｉｎｌｙｃｏｌｌａｇｅｎｐｒｏｔｅｉｎ ,ｇｌｏｂｕｌｉｎａｎｄａｌｂｕｍｉｎ) ,ｓｏｍｅＣｒｃｏｍｐｏｕｎｄｂｏｎｄｉｎｇｗｉｔｈｔｈｅｐｒｏｔｅｉｎ ,ａｌｉｔｔｌｅｃａｒｂｏｈｙｄｒａｔｅ ,ｆａｔａｎｄｓｏｍｅｉｎｏｒｇａｎｉｃｓ.Ｔｈｅｃｈｒｏｍｅｌｅａｔｈｅｒｓｃｒａｐｓｔａｃｋｅｄｗｉｌｌｐｕｔｒｉｄｇｒａ…     

Reclamation treatment of the chrome leather scrap

A novel method to extract protein and Cr(III) from the chrome leather scrap discarded by leather industry is described.Chrome leather scarp was hydrolyzed under basic condition to remove chromium compound and extract protein. The extracted protein was mixed with substrate and used as feed protein after being dried and ground. The basic residue was treated with sulfuric acid to obtain chrome(III) sulfate which can be used as tanning agent again after adjusting pH with base. The acidic residue was processed and used as flower fertilizer. The safety of the protein powder produced has been by toxicology and nutriology test. This reclamation method has been industrialized.     

Screening of leather auxiliaries for their role in toxic hexavalent chromium formation in leather—posing potential health hazards to the users

With increasing concern about the potential health hazards of toxic chemicals in consumer articles such as leather products, many countries importing these articles have introduced stringent stipulation for the permissible levels of such chemicals in these articles. Hexavalent chromium is one such toxic metal ion included in the list of regulated substances. The problem is unique to leather industry as it uses only trivalent chromium based salts in the tanning process but faces the problem of presence of Cr(VI) in the end product. Efforts are being made to understand the reasons for the presence of hexavalent chromium in leather and to devise strategies for eliminating the same in processing. This paper describes our attempt to investigate the role of potential post-tanning (wet finishing) and finishing auxiliaries used in leather processing in the conversion of Cr(III) to Cr(VI) in the chrome-tanned leather. The auxiliaries were also screened for the presence of oxidizing functionality and the amount estimated as persulfate. The study reveals that the auxiliaries suspected of having a role in Cr(VI) formation do have considerable quantities of residual oxidizing functionality capable of converting Cr(III) to Cr(VI). The study with persulfate as the model oxidizing agent confirms the role of oxidizing agents in the formation of quantities of Cr(VI) well above the permissible limit of 3 mg/kg in leather matrix, which basically provides a reducing environment.     

A salt-free pickling regime for hides and skins using oxazolidine

During the last few decades new materials and technologies have being studied and applied in order to reduce the ecological impact of leather production. Severe restrictions imposed by the pollution control authorities on the disposal of chromium, total dissolved solids and chlorides in tannery effluents have forced the tanners and researchers to look for eco-benign pickling and tanning processes. In this study, we would focus on the novel process development of pickling with the presence of oxazolidine based product. Investigations indicated that oxazolidine is a potential material to replace neutral salts in pickling and, realize high chrome exhaustion in the subsequential chrome tanning.     

Reducing sulfates concentration in the tannery effluent by applying pollution prevention techniques and nanofiltration

The use of large quantities of sulfuric acid and other sulfur-containing chemicals causes high sulfate concentrations in the wastewater of a tannery. The aim of this work was reducing the sulfate concentration in the final wastewater from a tannery. For that, firstly a study about the main sulfate sources in a tannery was carried out and the total sulfates load in the tannery wastewater was evaluated. Two measures for sulfates reduction were studied: the recycling of unhairing wastewater to the soaking drums and the reuse of the chromium sulfate from the tanning washing wastewater after its separation by nanofiltration (NF). The first measure proposed was studied experimentally in laboratory drums of 5 L of volume. Two series of experiments with different volumes of unhairing wastewater in the soaking bath were carried out. The quality of the final leather was evaluated by means of mechanical tests. NF experiments were carried out in a laboratory pilot plant with a spiral wounded membrane element. Concerning the results, the combination of 50% unhairing wastewater and 50% of fresh water was appropriate in order to obtain leather with an acceptable quality. Besides, it drove to a diminution of approximately 10% in the addition of sulfide in the unhairing. Related to the NF experiments, 97% of the sulfates were rejected by the membrane. The separated ions could be recycled to the tanning drums. The application of the two measures (firstly the recycling of the unhairing wastewater and secondly the NF of the tanning washing wastewater) drove to a reduction of 14.82 kg SO₄⁻² t⁻¹ of raw hide.     

Recouping the wastewater: a way forward for cleaner leather processing

Leather processing employs copious amounts of water. This leads to the generation of enormous amounts of liquid effluent. The high effluent volume requires huge investments for effluent treatment plants in order to meet the required specification for the discharge of liquid effluents to various water bodies. Increasingly therefore, water use minimization in leather processing assumes greater significance due to increased treatment costs. End-of-pipe treatment methods alone do not meet the requirements and hence, in-plant control measures are gaining importance. The new era of cleaner technology has begun in leather processing. Pre-tanning and tanning operations contribute about 57% of the water consumption in leather processing and the washings about 35%. The proper adoption of integrated cleaner technologies provides a viable solution to the conservation of water in leather processing. This paper presents an integrated approach for water use minimization through recycling and optimization in leather processing. The integrated approach provides considerable reduction in the use of process water.     

Studies on the use of power ultrasound in solid–liquid myrobalan extraction process

Focus on eco-friendly processing techniques makes vegetable tanning a viable option in leather processing and establishes the subsequent need for the more efficient methods of extraction in tannin manufacture. Application of ultrasound has been tried in the extraction of tannins from myrobalan nuts in order to improve the extraction efficiency, to perform the extraction under milder process conditions and to reduce the process time. The influence of process parameters such as ultrasonic output power, time and temperature has been studied. Scale-up trials and the use of ultrasound in pulse mode have also been attempted. The results show that a three- to fivefold improvement is possible with ultrasonic output from 20 to 100 W. Extraction efficiency has been calculated from the maximum extractable materials from myrobalan nuts. Extraction efficiency is found to be 90% for ultrasound, 100 W without external heating as compared to 77% for control process at 70 °C for 4 h. Therefore, ultrasound could be employed even dispensing with provision for temperature controls. The use of ultrasound in pulse mode offered 70% extraction efficiency of continuous mode. Scale-up trials indicate that there exists an optimum ultrasonic output power depending on the amount of nuts used, to achieve better extraction efficiency. The effectiveness of ultrasonically extracted tannin solution has also been tested in the tanning process for its applicability. The degree of tanning efficacy has been assessed by shrinkage temperature measurement. The results indicate that ultrasonically extracted tannin solution is suitable for tanning process. Therefore, application of ultrasound in tannin extract manufacture is a viable option with added advantages.     

Material flows in the life cycle of leather

This paper presents a study on the resource and environmental profile of leather for communicating to the consumers about the environmental burdens of leather products. The results indicate that significant environmental impacts were caused during the tanning and finishing of leather as well as the electricity production and transportation required in the life cycle. The use of fossil fuels in the production of energy has greater impact with increased emissions leading to about 15190 kg CO₂ equivalent of global warming and about 73 kg SO₂ equivalent of acidification while producing 100 m² of leather for shoe uppers. Further resource use of 174 kg of coal, 6.5 kg of fuel oil, 17.4 m³ of water and 348 kg of chemicals of which about 204 kg are hazardous are consumed, and wastewater of about 17 m³, BOD of 55 kg, COD of about 146 kg, TDS of 732 kg and solid waste of about 1445 kg are generated during the life cycle for the production of 100 m² of leather. The total solid waste generated is 1317 kg, out of which about 80% is biodegradable contributed by slaughtering, tanning and finishing stage, 14% is non-biodegradable contributed by tanning, finishing and electricity production stages and 6% is hazardous mainly from tanning and finishing stage of leather.     

回收电镀废液制取鞣革剂方法的研究

在高浓度的含铬、铜电镀废液中加入亚硫酸钠还原Cr6+为Cr3 +;再加入硫化钠去除铜 ;然后加入氢氧化钠调节pH值使Cr3 +生成氢氧化铬沉淀 ;最后加入一定量硫酸溶解氢氧化铬制成鞣革剂———碱式硫酸铬 ,且出水水质达到国家工业废水排放标准     

硫酸亚铁酸洗废液在制革废水处理中的应用

用混凝沉淀－微滤工艺对制革废水进行试验研究。结果表明,用含硫酸亚铁酸洗废液作混凝剂是行之有效的,经此工艺处理后,ＣＯＤ、ＢＯＤ_５、ＳＳ和色度的去除率分别为８０．４％、８７．２％、８８．６％和９１．７％,处理后废水水质达到排放标准,而且其中５０％回用于生产。此工艺具有很好的应用前景,尤其适合北方寒冷地区。     

A cleaner production method for the synthesis of bronopol – A bactericide that is useful in leather making

In leather industry environmental concern has been growing since 1980s. Increasing levels of pollutant emissions have reached saturation point in spite of the conventional technology that highly industrial countries have adopted to keep them under control. This has started a technology revolution in several industries. These critical levels of pollution may have been caused by high industrial density, by high population density or by the use of old polluting technology. This situation has boosted emerging green technology which has begun to take up a larger portion of research work in the supply and raw materials in industries. Together with consumers growing awareness of the environment, this new technology has ended up affecting the consumer good industry. In leather industry due to the sector's bad public image regarding pollution more emphasis has been given to clean technologies and waste reduction. At this time nearly every part of the tanning process has several cleaner or environmentally friendlier alternatives. In leather industry to prevent damage to the hide and skin, bactericide is added during soaking process to kill the growth of microorganism. Bronopol is used as a bactericide. The active ingredient of bronopol is 2-bromo-2-nitropropane-1,3-diol. Here we have reported simple and cleaner technology of the bronopol by using nontoxic and non-hazardous raw materials and belongs to the modern concept of environmentally friendly low waste or nonwaste technology. Bronopol was characterized by IR and Mass spectral data.     

污水回用中的循环浓缩水质模型与污水回用率

污水回用中因循环浓缩,存在着水质变差问题．探索其变化规律,定量确定运行参数,做到既节水又保证回用水质合格、稳定,具有重要理论和实用意义．通过数学归纳法推导出一个循环浓缩水质数学模型和计算杂用水质和污水回用率的公式．应用该模型,以厕所污水回用为例,就污水水质、污染物去除率、处理水质和杂用水质标准对杂用水质、污水回用率的影响进行讨论．结果表明,在符合城市杂用水水质标准的前提条件下,分别按生化需氧量、浊度、氨氮及色度等水质指标计算得出的污水回用率是65．3％,82．9％,57．5％和37％．在厕所污水回用中,色度为水回用率的限制指标．由于色度指标的限制,经计算水循环厕所的污水回用率一般不宜超过40％。     

混凝微滤法处理制革废水

对混凝微滤法处理制革废水进行了试验研究,通过大量试验,确定了最佳工艺条件。在最优控制的基础上,制革废水经混凝沉淀－微滤处理后,COD,BOD5,SS和色度的去除率分别为80%,87%,89%和92%左右,处理后废水可达到排放标准或回用于制革工艺。结果表明,在较低温度下,用硫酸亚铁作混凝剂仍能较好地完成混凝沉淀,验证了硫酸亚铁的低温混凝效果。因此,该工艺值得在北方寒冷地区大力推广。      

某制革企业污染控制与环境管理中若干问题研究

通过对数家制革企业开展调查分析,探讨了制革企业在含铬、含硫废水分流处理和各生产环节臭气防治方面存在的若干技术与管理问题.首先,含铬废水和含硫废水收集与管理不规范统一致使后续废水预处理及生化处理受到影响;部分企业忽略了原皮存放及转运、皮革水场加工车间、污水处理设施等臭气产生环节的细节控制.最后提出针对性的对策与措施建议,包括统一规范含铬废水和含硫废水的收集与预处理范围,加强原皮仓库、水场和污水处理系统的恶臭全过程控制.