Waste not29 April 2019
A team from Biosk, including Richard Daniels, Jiasheng Su, Falei Zhang and Zhuangdou Zhang, examine ways to eliminate effluent from liming, acid and salt pickling, and chromium tanning processes through high-volume wet-blue manufacturing.
It is acknowledged that within the chemical processing of hides into leather that liming, acid pickle and chromium tanning are the major contributors of pollutants. This is in terms of biological load, suspended solids, sulphide, nitrogen, chromium, salinity and the resultant sludges for disposal. This loading includes unused chemicals that are discharged from these processes due to their poor efficiencies.
Many attempts have been made to lessen this waste at source, but technologies in common use have basically remained unchanged for decades. Solutions to these problems are far-reaching, and the details here describe a technology that addresses and resolves these matters. This information was compiled through independent on-site studies within three major tanneries, where approximately 60,000 wet-salted US, European and Australian hides per week have been processed since 2013. Accordingly, the authors describe in detail this radical new approach to leather manufacture.
The initial investigations by Biosk required the building of a full-scale wet-blue manufacturing plant, and five years of developments before the technology was introduced to industry. The practicality and value of this technology has since been verified by continuous full-scale manufacture by tanners, which commenced in 2011. The technology involves the complete retention and reuse of used floats from liming, pickle and tanning in self-contained loops, and eliminates associated washing cycles. This ensures the complete uptake of processing chemicals, a significant reduction in water use, and zero effluent discharged from these critical manufacturing stages.
Conventional liming, acid and salt pickling, and chrome tanning processes are inefficient in the use of chemicals, and create considerable pollution that has to be addressed within waste water treatment. This effluent is high in biological load, nitrogen, sulphide and suspended solids from the dehairing and liming processes, together with neutral salts and chromium as residual chemicals from tanning. High-volume sludges are also created for disposal.
A five-year investigation took place by Biosk to seek the most effective way to make full use of the in-process chemicals, avoid waste water pollution, and optimise the management of unwanted solids. These objectives were achieved by creating an advanced recycling system – known as BIOcycle – which resulted in no waste waters being discharged from unhairing and liming, acid and salt pickling, and chromium processes. This technology was introduced to industry in 2011 for full-scale manufacture.
From initial small-scale trials it became clear that the way towards high-efficiency liming, pickling and chromium tanning processing would be based on recovering and reusing used floats from processes that remained undiluted by washing cycles. The technology was then developed in a custom-built experimental plant fitted with three 1.8×2.2m diameter drums for loadings of 40 hides per load, and a series of collection sumps for holding used floats from process.
These findings were refined one year later in a purpose-built, full-scale pilot plant. This included three stainless steel processing drums 5.8×6.3m in diameter working with loadings of up to 600 hides per load, two whole hide fleshing machines, and 15 sumps, each of size 16×3.3×2.5m. These sumps were built into the fabric of the plant during its construction for holding various used floats and wash waters from wet-blue manufacture, to optimise their collection, delivery and reuse.
The technology was considered suitable for use on commercial scale one year later, and it was then offered to industry. This included the three tanneries where information for this paper was sourced in June 2016, which were manufacturing 60,000 hides per week combined at the time of the study (see Table 1, page 35).
The outcome is high-quality, high-volume manufacture of chromium-tanned hides for sales as wet-blue, contract tanning, and in-house processing for finished shoe upper leathers. Central to the system are two self-contained loops:
¦ A liming recycling process that includes hair recovery.
¦ A pickling and tanning recycling process that includes a combined acid pickle and chrome pre-tanning stage.
Furthermore, there are no washing cycles following either of these procedures. Both loops rely on the continuous reuse of concentrated used floats that are collected from processes without any dilution with wash waters. Accordingly, due to the absence of washing cycles, there are no residual waste waters for treatment. The technology has been amended by each tannery to suit individual needs, but the details that follow present an accurate summary of the working process.
The processing vessels are conventional wooden drums, loaded with between 450 and 500 wet-salted hides. These hides receive a dirt soak, with the used float being discharged. After green fleshing, the hides receive a main soak, with the used float either discharged, or recovered and reused for the dirt soak.
The technology for the application of BIOcycle technology in the liming process follows these basic steps:
¦ Undiluted used lime floats that have been filtered, collected and mixed (in a lime-holding tank that serves all of the liming drums) replace the fresh water normally used as float for conventional liming and unhairing.
¦ Chemicals are added for a hair recovery process.
¦ After two hours the hair is filtered from the float, and the filtered float returned to the liming drum.
¦ The float is increased using recovered lime liquors from the lime-holding tank, followed by water for controlled osmotic swelling. This can be performed in several additions.
¦ When the liming process is complete, the float is refiltered to remove residual hair.
¦ The load is discharged without any washing and delivered to a dedicated area for loading onto a line conveyor for feed to the lime-fleshing operations. This is followed by lime splitting.
¦ All of the spillage and drainings from these procedures are collected using dedicated drainage and delivered to the lime-holding tank for reuse in subsequent operations.
There are no washing cycles at the end of the liming process, so the recovered floats remain undiluted. Moreover, they are free of odour and hair residues. The chemicals offered to the process are conventional – lime, sodium sulphide and sodium hydrosulphide – although auxiliary products can be included. These additions are at a reduced level when compared with conventional processing due to the presence of unused chemicals that remain in the recovered used liming floats.
After lime splitting, the hides are delimed and bated, then drained and washed as conventional practice. The hides may be unloaded from the deliming vessels, then loaded in to the tanning drums. If, however, they are delimed in the tanning drums they must be thoroughly drained before the pickle stage. The technology then follows these basic steps:
¦ A minimum volume of fresh water is added as float for the distribution of salt and acid in the initial pickle stage.
¦ A small amount of common salt is added, then an acidification using 0.5% formic acid diluted with water.
¦ Recovered float from previous chromium tanning processes is pumped into the drum at a constant rate together with diluted sulphuric acid. This acid has been previously diluted with recovered chrome float.
¦ The hides are then run for 90–120 minutes (according the weight range) in this simultaneous acid pickle and chrome pre-tannage.
¦ The main tannage commences with an addition of standard chromium tanning agent, and runs for another four hours.
¦ More recovered chromium float is added, then run for a further four hours.
¦ The hides are dumped without any washing, then delivered to a dedicated area for stacking before blue samming.
All of the chromium containing spillage and drainings is collected using dedicated drainage, screened or filtered to remove suspended solids and grease, then delivered to two holding tanks. One tank is for reuse in subsequent acid dilution and for pre-chroming. The second tank is for the offer of chromium that follows the main tannage operations.
There are practical aspects that are required to ensure that this technology works correctly. The plant must be clean and well managed. All of the pipework and drainage arrangements need to be separate for each stage to avoid any contamination.
The collection tanks are of sufficient size to hold all of the used floats from the processing drums. These tanks are fitted internally with coils for heat exchange, and the temperature adjusted precisely by heat pumps. Typically, the recovered lime floats are adjusted to 220°C, the recovered chromium float used for pickle and pre-chroming to 220°C, and the recovered chromium float for tanning to between 55°C and 650°C. These temperatures vary according to the ambient temperature and to accommodate summer/winter variations.
This is a hair recovery system, and the hair recovered is very clean, intact and compact. Being fibrous and clean, it is easy to manage and dispose of compared with contaminated sludges following conventional effluent treatment. This hair can be composted and has potential to be used in higher-value products.
Water is added in the liming process. This makes up the volume balance as it replaces water taken up by the hides in the swelling phase in previous liming cycles. This continuous removal of water together with dissolved neutral salts causes a levelling or a plateau of the salts content in the otherwise closed loop system.
The pickle and tanning system works in a similar way. The hides swell with water from the delimed state to the final chromium-tanned condition in previous cycles, and this uptake is replaced by the addition of fresh water. This continuous water removal enables a levelling of the salts content at a concentration suited to chromium tanning.
Proven benefits of BIOcycle
The technology offers 100% use and uptake of active chemicals. Moreover, the conditions required within the chemical processing do not need to become extreme to achieve a high uptake.
The pH at the end of tanning does not need to be unusually high, the floats very low, or specialised products and control employed to achieve a chromium uptake that might be good, but always less than 100%. For example, in this situation the final float at the end of tannage is higher than found in many processes and may be as great as 150%. More moderate conditions can prevail through the process.
The leathers produced are clean, have no remaining hair or residues, show no signs of chrome staining, and are relaxed and well extended. They have high commercial value, as indicated by the volume of production and the acceptance of this technology in the three tanneries that form the base for this paper.
Each tannery has adjusted the technology to meet their own individual needs, and so there are no absolute values to judge advantage. However, savings in chemical use and water have been given, and can be expressed as a spread of values across the three tanneries (see Table 2, page 39).
A radically different approach to the manufacture of chromium-based tannages has been developed. This technology centres on self-contained systems for unhairing and liming, and pickling and tanning that ensure the uptake of processing chemicals in these manufacturing stages is complete.
This approach reduces the offers of active chemicals that are required for processing, eliminates the chemical waste from these process stages, reduces water use, and avoids the treatment of sulphides and chromium from wet-blue manufacture. This has not been previously achieved.
The technology requires that only the most basic chemicals and equipment, although auxiliary products can be included. In manufacture, the focus can remain on making a high-quality product and sidestep the many technical problems associated with high-uptake systems that may be sensitive to control and never 100% efficient.
The technology provides very significant environmental advantages, although these cannot be accurately quantified. This is because each of the tanneries not only manufactures wet-blue hides, but continues to process to the finished state. Some of the wet-blue are also sold to other tanners or are part of contract tanning. Moreover, process changes have been made since the introduction of this technology to retanning and fatliquoring processes to meet changing customer demands. There were also changes in water consumed to take into account, and although the waste waters from each tannery came from different departments, these waste waters were mixed together for effluent treatment.
However, sulphides are no longer an issue for treatment, resulting in an absence of odour across the effluent treatment plants. There are no chromium-based compounds in the waste water from wet-blue manufacture, thus avoiding the route of separation, collection, precipitation and possible regeneration.
The biological load is reduced, there is less nitrogen from hair breakdown, and the sodium chloride and TDS content is reduced. The volume of settled solids is significantly reduced, and the management associated with these sludges and their disposal as waste. The hair that is recovered is very intact, low in moisture and volume, and is also uncontaminated by products from subsequent processing.
The energy use is also reduced as aeration is no longer needed for the removal of sulphide. There are also energy savings for aeration in the biological stage and sludge dewatering. Energy is also saved due to the reduced water requirement and associated temperature controls in manufacture.
Not only does this technology provide advantages when it comes to the environment, its commercial viability and value is demonstrated by long-term, high-volume leather manufacture of high quality.