Salt Dedusting

Q.    How could salt content be reduced in the wastewater of soaking?

A.  The salt content in the wastewater could be reduced through salt dedusting. After this operation, most salt on the surface would be removed. Although there will still be salts within the hide, but they are very difficult to remove physically. Therefore, the salt content in the wastewater of soaking will be reduced to a good extent but not completely.

Q.    What is the best method for salt dedusting?

A.   The best suitable method for salt dedusting is selected on the basis of the least potential of mechanical damage to the hides or skins. For bovine material, caged drum is most suitable. But it should be ensured that the drum is run at a reasonably slow rpm so as not to cause excessive mechanical action which might damage the hides.

     Use of brushes is more common for skins. The bristles of the brush must not be very stiff, but rather flexible. Apart from the bristles, the distance between the rollers is also important. If the rollers are too close, they might cause abrasion.

Q.    What will be the impact of salt dedusting on water consumption in soaking?

A.   Typical soaking ratios are 200% water on weight basis for dirt soak and the same amount for main soak. The volume of water will reduce automatically after dedusting as the weight of the raw lot will go down. Typical reduction in the weight is around 7% which causes a corresponding reduction in the water volume.

Q.  What will be the impact on grain quality if salt de-dusting is applied on sheepskin?

A.   Most tanneries using salt de-dusting technique are bovine tanneries. It is also suitable for sheep tanneries processing short hair sheepskins, as less mechanical action is needed and there is little impact on grain quality. For normal sheepskins, salt de-dusting operation might have adverse impacts on the grain quality because higher level of mechanical action is required to de-dust salt from normal or long woolskins.

Q.  What are the techniques employed in the European tanneries for reuse of dusted salt?

A.   Reuse of dusted salt in any down stream process is not very common in European tanneries. Most of these tanneries are not processing salted hides, rather they are processing chilled hides. Chilling is the most favored method of raw hide preservation across Europe. In Pakistani conditions, this method is not practical and use of salt for preservation is unavoidable. Therefore, the option of reuse of dusted salt is more relevant to Pakistani industry.

Lime Recycling

Q.   Some tanneries in Pakistan are using only Sodium Sulfide for de-hairing and liming. They do not use any quantity of lime even for the liming process. Is recycling of such liquor possible?

A.    There are no examples of such practices anywhere else in the world, therefore no practical example of Sodium Sulfide recycling could be cited. Normally combined liquors are recycled and only solid impurities are removed from them. However, theoretically there is no reason against it, as the same principles and same system is applicable here as in case of lime liquor recycling. Screening is the critical factor as removal of all insoluble solid impurities is absolutely essential. If there are any insoluble impurities left in the recycled liquor, it will hamper the process of liming. For this reason, membrane technology is the best possible option, but this technology is expensive and cannot be justified financially. Therefore, conventional filtration systems with sophisticated arrangements should be used; till such time that the cost of membrane technology comes down to a viable level.

Q.   Membrane system will not be commercially viable for Sodium Sulfide recycling, as the cost of Sodium Sulfide is only Rs. 10/kg.

A.   If only coarse material can be filtered from the waste stream by using ordinary filtration system, the stream will become recyclable.

Q.   Is the system described above only for pure lime?

A.   In practice, the system is more suitable for pure waste streams (waste streams containing only one type of chemical). However, the same system is applicable to streams containing both lime and sulfide in it. Lime has a very low solubility in water and most of it is separated during filtration. The filtrate might be recycled after making appropriate adjustments in the concentration of both or any one constituent, as per the process scheme employed by the individual tannery.

Q.   Are there any practical examples of any tanneries practicing CO₂ deliming and lime recycling?

A.   The techniques are being practiced in various parts of the world and have been proved to be very successful. It must be noted that cleaner production technologies do not have any negative impacts on the quality of leather. For example, tanneries in Italy are practicing lime recycling and some of them have also employed CO₂ deliming. Italian tanneries are known to manufacture premium quality fashion leather and using cleaner technologies has not affected their quality. Similarly tanneries in Australia are also using CO₂ deliming.

Q.    How many times can liquor be recycled?

A.   The number of times any liquor may be recycled depends mainly on the type of recycling process and the chemicals present in it. Typically recycling liquors can be recycled around ten times. Improvement in filtration system may make it possible to recycle the liquors more the ten times.

      Apart from build-up of undisclosed impurities, the main cause to render the liquors no more recyclable is the sulfate build-up. In case of lime and chrome liquor recycling, the composition and pH of the liquor is adjusted using sulfuric acid, which results in gradual build up of sulfates in the bath. In lime liquor, sodium sulfide also oxidizes to give sulfates. After around ten recycles, the sulfate content of the bath grows to such high concentrations that it could not be recycled further.

CO2 Deliming

Q.   What are the disadvantages of Ammonia salts?

A.   The disadvantages of Ammonia salts are mainly related to legislative requirements in many parts of the world, and occupational health and safety in all parts. The legislation regarding emission of gaseous ammonia to the air and nitrogenous compounds in the effluent is getting more and more strict in many parts of the world, forcing the industries to alter their practices.

    Another major disadvantage of using ammonium salts is discharge of ammonia into the occupational air. This causes health related problems as well as bad odor.

      At the same time, some tanneries have also reported improved leather quality by the use of CO₂, compared to that by the traditional ammonium based deliming systems.

Q.   What is the impact of CO₂ deliming on human health?

A.   CO₂ deliming system is quite safe in terms of human health and environment. The only major issue with CO₂ deliming is the formation of high amounts of Hydrogen Sulfide. The concentration of gaseous H2S is higher at lower pH (pH of CO₂ deliming is 7 compared to that of ammonia systems of 8-8.2). Exposure to high H₂S concentrations is very dangerous for human health. However, this problem is easily solved by adding a small amount of any oxidizing agent to the bath e.g. Hydrogen peroxide or Sodium meta bisulfite before starting the CO₂ deliming. The oxidation converts sulfides into soluble sulfates, which are safe.

Q.   How many tanneries are practicing Carbon dioxide deliming.

A.   Carbon dioxide deliming system is more appropriate for thinner substrates such as sheep or goatskin. There are four to six tanneries, which have employed CO₂ deliming system. Some tanneries in the UK are also practicing this method, including the world-renowned parties in Leeds. Some tanners in the Scandinavian countries e.g. Sweden are also doing deliming with Carbon dioxide.

Q.   How is CO₂ stored and used in the CO₂ deliming system?

A.   The tanneries using CO₂ deliming system usually buy the gas from its commercial suppliers. It is stored in a tank and pumped from there to the process vessels i.e. drums, or to the recirculation system. The conveyance arrangement may be a fixed piping system, facilitating many liming batches at one time, just like the plumbing system for providing water to all process vessels. Otherwise, it is also possible to install one flexible hose to provide CO₂ to the process drums one by one. For measuring and controlling the quantity of the gas, flow meters would need to be installed. In some cases, the storage tank and the piping system is provided by the gas suppliers, depending upon the size of the supplies needed.

     CO₂ cylinders may also be used. The disadvantage of using CO₂ cylinders is that their handling is cumbersome.

     If a tannery is using a combination of CO₂ and Ammonia based deliming systems, or in some other cases as well, the amount of CO₂ needed is small. In such cases, CO₂ generated as a flue gas from the boilers or generator of the unit, may be collected and utilized. If small quantity of CO₂ is to be collected and stored, cylinders could be used instead of tanks.

Q.   How is the quantity of carbon dioxide measured in CO₂ deliming?

A.   The normal practice is to measure the volume of the gas through flow meters. This volume relates with a specific mass at a given pressure. For operational ease, recipe for carbon dioxide deliming is developed in the laboratory is a manner so as to take into account the volume of the gas at a fixed pressure.

Q.   If deliming is done using mixture of lactic and acetic acids on pH 7, and then new bath is taken for addition of bate after adjusting the pH to 8-9 with ammonia, would there be any CP concerns? How would this system compare with the proposed CO₂ deliming?

A.   The system described above does not pose any particular environmental concern, apart from some minor chances of Ammonia discharge when it is used to adjust the pH. However, since only small amounts of Ammonia are used, the fugitive emissions are less likely.

      The cleaner deliming technique, i.e. carbon dioxide deliming has been developed as an improved alternative to ammonia based deliming systems. The advantage of carbon dioxide deliming over a system based on organic acids is that it requires a lesser degree of process control. Moreover, carbon dioxide based system is relatively cheaper compared to the cost of organic acid deliming.

Pickle & De-pickle Recycling

Q.   How could the quantity of de-pickle liquor be reduced?

A.   The scheme to reduce de-pickling effluent is based on recycling into pickling stage. Chemicals used in de-pickling are salt and some weak base. The de-pickling liquor could be clean4ed through screening and fat trap. Once cleaned, it should be acidified by adding calculated amount of sulfuric acid. As the salt content is already there, the float could easily be used in the pickling process.

Q.   Can the de-pickle and pickle floats be stored together? How could the problem of volume build up be overcome in such case?

A.   The de-pickle and pickle float can easily be stored together. In fact it might be a little advantageous, as the amount of acid required for the acidification de-pickle float would go down to an extent.

      However, the problem of volume build up will be encountered in that case. This problem might be overcome by using the de-pickle water in dirt soak rather than pickling. Only pickle spent liquor will then be reused in pickling process, thereby eliminating the problem of volume build up. If de-pickle liquor is used in the dirt soak, then a little more fresh water will be required in the main soak. The overall equation will require less water than in the traditional scheme of things.

Chrome Management (Recycling and Recovery)

Q.   Experience has shown that the color of the wet blue is different if recycled chrome is used. What should be done to overcome this problem?

A.   Generally leather produced with recycled chrome is darker in color, due to impurities like proteins and fats in the recycled stream. Therefore, filtration step is critical for the process. These impurities must be removed to the maximum practically possible extent. Possibility of passing the recycling liquor through membranes before finally feeding it to the drum may be considered.

      It should also be noted that the difference in wet blue color is important only when fresh chrome crust and recycled chrome crust is mixed in the same lot for dyeing. A better alternative is to use a standard mix of recycled and fresh chrome for all the lots of wet blue. A uniform, though slightly darker, color will thus be obtained. Then the dye recipe could be set accordingly.

Q.   Recycled or recovered chrome gives a relatively darker wet-blue. What would be the impact on the final product?

A.   Both the recycling and recovery options of chrome have been tested throughout the world, and international tanners are utilizing both options. These options do not cause any loss in the final leather quality.

      As far as the darker shade of wet-blue is concerned, it is more relevant to the suede or aniline leather. Darker wet-blue does not pose any problems for the darker shade end leathers, e.g. black or dark brown. Similarly, if pigment coating is applied during finishing operations, it will overcome the problem of darker wet-blue. 

Q.   Would the darker wet-blue be a problem for those tanneries producing only wet-blue as their end product?

A.   There are a number of points to consider. First and foremost, the producer of the wet-blue would have to convince his buyer that there is no difference in quality. For this, the producer should have in-depth knowledge about the issue. From a business perspective, all the facts should be made known to the buyer straightaway. It is easy to convince a client after telling him total facts, rather than by concealing things.

     Secondly, it should be noted that the color of wet-blue almost always differs from tannery to tannery, even if fresh chrome is used. Color adjustment is easily possible by adjusting the recipe through trial runs. There is no quality difference though, just because the shade is slightly different.

      Furthermore, it is possible to lighten the shade of wet-blue made by recycled or recovered chrome. If membrane technology or other fine screening system is used, the wet-blue color through recycled or recovered chrome will be very close to the color of fresh chrome wet-blue.

Q.  Would the color change vary from lot to lot?

     The wet-blue made by using recycled chrome would be of darker shade, but if the system is properly controlled, it will be a consistent shade without variations. Initially there will be a sulfate build up in the recycling system, causing some shade variations. But once the equilibrium is achieved, that is after around 5-6 lots, there will be no more variations.

Q.   Darkening of wet-blue due to use of recycled or recovered chrome might cause some problems for lighter color leathers.

A.   It depends on how dark the wet-blue is, compared with the wet-blue of fresh chrome. Experience have shown that usually there is no great difference between colors of wet-blue produced by recycled or recovered chrome and fresh chrome. There are some important points to be noted.

     Firstly, the combination of recycled and fresh chrome could easily be adjusted to control the shade of wet blue. As discussed earlier, if same combination is applied to all the lots, there will be no difficulty in post tanning processes.

     Secondly, no appreciable color difference should practically be encountered in recovered and fresh chrome wet-blues. If there is a considerable difference in the shades of the two leathers, there is a strong possibility that the filtration arrangement in the recycling or recovery system is inadequate. In this case, efficiency of this arrangements needs to be checked and improved.

      Finally, the minor difference in wet-blue colors of recovered / recycled chrome and fresh chrome must not pose any serious problems in dyeing. There are tanneries in Pakistan and abroad that are using recovered chrome. They overcome the shade difference of wet-blue by adjusting their dyeing recipes to a minor extent.

Q.    How long can the regenerated chrome be stored?

A.    The normal practice throughout the world is to use the regenerated chrome within a day, except for those factories, which have a regular practice of annual shut down for around 2 weeks per annum. However, apart from the large storage arrangement, there is no apparent limitation of storing regenerated or recovered chrome for larger time intervals.

Q.  Will there be any effects on quality if the chrome is stored for longer periods?

A.   There will not be any adverse impacts on the quality if fresh or recycled chrome is stored for long. The reason behind this is that the storage capacity and stability of basic chromium sulfate is quite indefinite without any degeneration or decomposition.

Q.   Is recycled chrome less reactive than the fresh chrome?

A.   No. It is not likely for chrome to become inert or less reactive at suitable pH range. If this were the case, there would have been a build up of chrome in the waste stream. But there is no evidence to suggest that chrome builds up in the waste or recycled stream. Therefore, it can be safely said that the reactivity of chrome is unaffected in recycling system.

Q.   While using recycled or recovered chrome, the basicity of this and the fresh chrome should be the same. How is it ensured?

A.   The method to ensure that the regenerated chrome has the required basicity is to utilize calculated amount of sulfuric acid for regeneration. The control measure is pH. Normally, it is not a cumbersome process if proper pH control is practiced.

Q.   Chrome recovery and reuse is based on MgO. Sometimes when chrome is settled and the pH is attained through Sulfuric acid, the lab basicity is different than the required basicity. What are the possible reasons?

A.   There are many possible reasons behind this.

      Firstly, there is a possibility that the reaction is not complete, due to too short time span given or inadequate mechanical action in the reaction tanks.

      Secondly, the screening operation performed on the influent to the recovery plant may be inadequate. In that case, there will be insoluble impurities in the bath, which could both hinder the chemical reaction as well as consume some of the sulfuric acid themselves.

     Finally, there is a possible presence of electrolytic fat liquors in the bath if fat liquors have been applied at the wet-blue stage. The electrolytic fat liquors are too stable to exhaust easily during the process and might come in the wastewater. They react readily with sulfuric acid. Efforts should be focused to exhaust maximum amount of fat liquors during the process. 

Q.    Can spent chrome liquor from tanning be recycled in re-chroming?   

A.   This is not practiced anywhere, because spent chrome liquor does not have a consistent chrome concentration in it, and might be difficult to adjust for re-chroming bath. Furthermore, re-chroming is mainly done to ensure uniform application of chrome on leather surface, and chrome penetration beneath the surface is not the objective; therefore, chrome used in re-chroming usually have higher basicity than the chrome used in tanning.

     However, if these problems are not very severe in any particular tannery, then the recycling of spent chrome liquor into the re-chroming process might be a feasible option, after necessary adjustments of chemical concentrations and pH of the bath.

Q.   During trials in a particular tannery, it was found that dye shining was better on the leather re-chromed with fresh chrome rather than that re-chromed with recycled chrome?

A.   As has been described earlier, practice of recycling spent tanning liquor to re-chroming bath is not common. There is practically no data available in this regard, therefore no comment can be made on this.

Water Conservation

Q.   What is the most suitable method for water conservation?

A.   The first and foremost step is to install flow meters on process vessels and incoming supply. Monitoring of the amount of incoming water would give a measure of how much water is being wasted. Unless this is known for a unit, specific and efficient water conservation steps are difficult to suggest.

For Pakistani tanneries in general, application of batch washing to replace continuous washing and reducing the amount of non-process water are two easy and practical steps. Some tanneries have adopted these practices and reported typical reduction of 15-20% in the water consumption.

Q.   What is batch washing?

A.   Batch washing is a means of washing in which a fixed amount is water is taken into the drum and then it run for a short period to facilitate the completion of the process, before finally draining it. This mode of washing ensures optimum results with low water use, as compared to continuous washing which is water intensive.

Q.   Are there any practical examples of water conservation in sheep tanneries?

A.   There are examples available both at the national and international level. Within the Korangi cluster, Shafi & Company has already taken some steps in this direction. They have reduced non-process water in floor washing and domestic consumption of workers. They have also converted some of their continuous washes into batch washes. Through these steps, their water consumption have considerably reduced but the quantification of this would be done by CTP team at a later stage.

     The use of batch washing rather than the continuous washing is becoming common internationally, as it is easily adaptable and results directly into saving of sufficient amounts of water without any cost.

      Another possible way of reducing water consumption is optimizing the process by changing the vessel type or design. Most tanneries at the international level are replacing their old process drums with newly designed segmented drums. These segmented drums allow use of very low floats without any adverse effect on the product quality. However, this is a costlier solution and not economically justifiable for water conservation purposes only. This option may be adopted if a tannery is looking to replace its existing wooden drums due to some other reasons as well.

Q.   Is use of short float disadvantageous for the leather quality?

A.   It is true that use of excessively short float might negatively affect the leather quality due to increased mechanical action. This is particularly relevant to sheep skins. There is a limited extent to which a float could be reduced. The emphasis of CP technique is on optimum float, and to avoid excessive use of water.

      Shorter float technique is more applicable to processing of bovine hides, which can withstand slightly more mechanical action. Moreover, the traditional equipment might need some modifications, according to the raw material used and the local climatic and other conditions, before use of shorter float could be applied. New equipment is designed in a manner so as to support use of low float, e.g. Y compartment drums. Example of local and international tanneries are available which are already employing low float techniques, after taking into account the above mentioned considerations.

Q.   If the float is shortened, patches and such patterns occur in dyeing and crusting. What are the reasons and remedies of this phenomenon?

A.   The first reason might be that the process is already being at the optimum float, i.e. the minimum float necessary to carry out the process in a satisfactory manner without negatively affecting the hides or skins. In such cases, no further efforts should be done to further reduce the float, without a design change in the process vessels.

      However, in the present situation, a more realistic reason might be that the chemical distribution in the vessel is not adequate. This problem may be overcome by slowing down the reactivity either by adding some slowing agents or by changing the conditions e.g. pH. Slowing the fixation process through slight modifications in the recipe e.g. uses of a combination of formic and acetic acids in place of formic acid alone might also counter it. Another way of slowing the reaction is by modifying the vessel design.

Q.   If short float is used in dyeing for shoe leather, looseness in leather is encountered. How to overcome this problem?

A.   It might be that the less than the optimum float is being applied. It should be noted that shorter floats could be applicable to some cases, and not applicable to others. The main purpose of introducing cleaner technologies is to make quality leather at low or no environmental cost, and CT is not introduced at the cost of the leather quality.

CT for Post Tanning

Q.    Is it possible to recycle the dyeing liquor?

A.    Recycling of dye liquor is difficult as it contains various chemical substances of different nature. The cleaner technology approach for dyeing liquors is to improve the dye uptake through process or recipe modification, so that there is less quantity of dyestuff in the effluent. But that does not qualify for an increase in the amount of dye in the bath, a practice that many tanners adopt. The right approach is to increase the efficiency rather than increasing the amount of dye.

Q.   What is the CT approach for waste streams of dyeing and fat liquoring etc.?

A.   Wntil now, CTs are more common and practical for process stages upto tanning, mainly because of the simplicity of process chemicals. In post tanning processes, the CT approach is to increase the uptake of chemicals rather than recovery and recycling. Therefore, post tanning processes should be restructured to avoid wastage in the first place. Adjusting the process parameters and the recipe accomplishes this target.

General CT  Options

Q.   Why should one adopt CT measures when there is no benefit in terms of cost?

A.   The perception that CTs do not present possibilities of financial benefits is wrong. In fact, many CTs provide a pay back period of less than a year. Typically, pay back periods have been reported to be between 8 to 189 months for different CT options.

Q.   CT options are more relevant for higher production.