After reading this article you will learn about:- 1. Search for Better Uptake of Chromium in Tanning 2. Preparation of Low Waste Basic Chromium Tanning Salts 3. Recycling and Reuse of Spent Chrome Liquors 4. Sludge Disposal 5. Alternative Tanning Materials for Chromium.
According to B. Chandrasekaran, leather industry is one of the major industries that discharges many toxic pollutants like sulphide, phenolic compounds, chromium and other mineral salts, dyes, solvents, etc. Chromium contributes a major share to the potentially hazardous nature of tannery effluents.
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The toxicity of Cr (VI) has already been reported. Although Cr (III) has been found useful as a trace element in the control of diabetes, at higher concentrations, it seems to exhibit toxicity in general.
Despite the pollutional threats posed by chromium, chrome tanning has incomparable benefits for the manufacture of softer, brighter and lighter leathers over other mineral, synthetic and vegetable tanned leathers.
In addition, quicker processing time and higher hydrothermal stability of the leathers produced by chrome tanning have made the tanners to resort to the practice of chrome tanning. The efficacy of chrome tanning salts is well established by the fact that more than 80% of the leathers produced are by chrome tanning.
The consumption of basic chromium salts by the Indian leather industry is about 24000 tonnes per annum. The conventional chrome tanning practices lead only to an uptake of 65-70% leaving behind about 7000-8000 tonnes of Cr (III) salts in the effluent annually.
This trend, if not attended to, will certainly lead to a huge material loss as well as creating an ecological imbalance. The permissible levels set by several environmental agencies (ISI, 1985: IULTCS Report, 1987) at < 2 ppm have necessitated the tanners to look for long term and safe solution to the problems arising from chromium loss and pollution.
Several physicochemical and biological methods to contain the amount of chromium in the tannery effluent have been reported. Unfortunately, such methods lead to substantial build-up of chromium containing sludge.
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The problem of sludge disposal has been widely stressed and different governments have enforced guidelines for the disposal of chromium sludge in agricultural fields or as a landfill. In the light of the difficulties arising from viable economical exploitation of these methods, in-plant control measures have become necessary.
A solution for the better management of chromium in tanneries lies in adopting in-plant control measures with improved tanning techniques such as:
(i) Use of alternatives to chromium in tanning
(ii) High exhaust tanning salts/methods
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(iii) Recycle and reuse methods
Search for Better Uptake of Chromium in Tanning:
The poor uptake of chrome tanning species by collagen may arise from:
(a) kinetic inertness of Cr (III) ions;
(b) Lower binding constants for complexation with functional sites in collagen;
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(c) Diffusion problem in case of aggregates; and
(d) Lack of availability of Cr (HI) binding sites in protein.
Therefore, any attempt in improving chrome fixation needs to:
(a) Identify poor affinity Cr (III) species; and
(b) Seek methods to modify their chemical structures; or
(c) Avoid the formation of such species during the preparation of chrome tanning salt
Ion exchange chromatographic techniques were employed for the separation of various Cr (III) complexes using Dowex 50W X4 cation exchange resin at a flow rate of 1 mL/min.
After the absorption of ‘he species has been complete, about 20 mL of water was passed through the col inn to collect the non-cationic species. Then, 1+ species were cluted using 1M HC1, whereas 2+ charged species needed the clution with 2M HC1. An apparently 3+ charged complex was obtained using 3M HC1 as an eluant.
Some polymeric species with charges greater than 3+ were also found on the column after elution with 3M HCI. All the fractions eluted were collected quantitatively and the percentage composition analysed.
The typical composition of basic chromium sulphate before and after tanning is given in Table 1. It is evident from the table that the cationic charged species contributed most (>80%) to the effluent. Typically, the species of 3+ and > 3+ accounted for 50-60% of the unfixed chrome.
A new polymeric species of chromium which behaved as a tri positive ion in 3M HC1 and tetra positive ion in 4M HCIO4 has now been isolated and identified as a tetra positive tetramer: The species wise uptake characteristics of complexes present in the basic chrome sulphate reveal that the tetramer is one of the chief contributors to the effluent.
The poor affinity of tetramer is due to poor thermodynamic affinity and high kinetic liability. An effort was therefore made to reduce the amounts of such a species in basic chromium sulphate by altering the method of preparation.
Preparation of Low Waste Basic Chromium Tanning Salts:
Generally the familiar process variables such as temperature; concentration of reagents; time of reaction; order of addition of reductant/acid; the nature of additives; etc., can be expected to influence the nature of Cr (IV) and Cr (V) intermediates formed and therefore the chrome tanning salts produced.
The preparation of glucose and sulphur dioxide reduced chrome tanning salts under modified conditions in the laboratory can also be done.
The use of modifying agents such as benzophenone tetra carboxylic acid; sulphophthalic acid; adipic acid; and succinic acid, during the reduction of Cr (VI) is expected to form reactive intermediates with higher affinity to collagen.
Tanning studies were carried out with the laboratory prepared as well as with commercially available basic chromium sulphate. The results are given in Tables 2 and 3. Among the various additives, benzophenone tetra carboxylic acid shows good promise.
When used at 0-05 mole/mole of Cr, the exhaustion was as high as 85% as compared to the uptake of 65-70% chromium in the case of commercial tanning materials. While efforts are being made to prepare high exhaust chrome tanning salts, methods for more efficient recycling of chromium need to be explored.
Recycling and Reuse of Spent Chrome Liquors:
There has been considerable effort in developing methodologies for the recycling and reuse of spent chrome liquors. It has become necessary to devise techniques for efficient recycle and reuse of spent chrome liquors with a view to save chromium as well as water and contain chromium pollution.
The traditional practice for recycling suggests the reuse of spent chrome in pickling and in chrome tanning. The chemical composition of spent chrome liquor is given in Table 4. In view of the presence of 6-7% neutral salts, the reuse and recycling of spent chrome liquors in pickling appears more attractive.
(i) Recycling in Pickle Bath:
This procedure would enable containing salt pollution in tanneries and poor affinity chromium species may undergo hydrolysis under relatively alkaline conditions and bind to protein. The percentage uptake of chrome when spent chrome liquor was used in pickle bath has been reported in Table 5. It is seen from the Table that 80 – 85% of chrome present in spent liquor was taken up by the pelt.
However, it seems unlikely that such chromium will not get displaced during subsequent chrome tanning. These investigations indicate that this technique has limited value in the reuse of chromium contained in spent liquors although neutral salts and water can be advantageously recycled.
Recycling in Tanning Bath:
The reuse of spent chrome liquor in the tanning bath in two successive cycles was examined. When effluent tanning solutions were replenished with fresh chrome and reused, the percentage uptake of chrome (Table 6) showed no net increase in the uptake of chrome.
An improved uptake of chrome in this method cannot be expected unless the chemical structure of the poor affinity species was altered during the recycling conditions.
Excessive build-up of neutral salts in spent chrome liquor with every recycle is also one of the reasons why reuse methods have found limited commercial applications. A true scientific solution to the problems in the reuse of spent chrome liquors awaits a technological innovation for the selective separation of neutral salts.
(ii) Electro Dialysis for the Removal of Neutral Salts:
Electro dialysis of spent chrome tanning solutions using perm-selective membranes (Selemion AMV and CMV, Asahi Glass Co. Lt., Japan) was carried out on samples obtained from commercial tanneries with a flow rate of 30 mL/min at controlled currents of 2-4 A for 7 hrs. The characteristics of the spent liquor before and after the separation are listed in Table 7.
Thus, it has now become possible to selectively remove 90% of NaCl and 60% of Na2S04 from spent chrome liquor without any loss of chromium. There is now a high promise in the reuse of stream containing neutral salt in the pickle bath and the chromium concentrate in tanning.
The removal of neutral salts seems to bring about marginal (10-15%) increase in the uptake of chrome when recycled in the tanning bath. However, optimization of the energy requirements, overall cost effectiveness is the important factors to be considered in the use of electro dialysis.
Recovery of Chrome after Precipitation:
The effect of neutral salts in the recycling of spent liquor can be overcome by the precipitation of chromium as Cr (III) hydroxide and re-dissolution prior to use. Precipitation was carried out using lime, Na2CO3 and MgO. The Chrome Exhaustion and uptake data obtained when chrome recovered from spent chrome liquor was used in tanning are given in Table 8.
The percentage uptake of chrome was almost identical regardless of the alkali used for the precipitation of the metal salts. The addition of polyelectrolyte can be used to enhance the settling time of the precipitate.
Sludge Disposal:
A mixture of lime yard wastes with liquid wastes of chrome yard would lead to the precipitation of chromic hydroxide in the formation of chrome sludge. The disposal of chrome sludge poses major problem of chrome leaching.
The possible ways by which the chrome sludge could be effectively utilized are:
(i) Composite for energy source and related use.
(ii) The manufacture of refractories;
(iii) The manufacture of industrial bricks;
(iv) Filler in road construction; and
Alternative Tanning Materials for Chromium:
The ecological acceptance and relative case of after-treatment methods have necessitated the search for alternative to chrome tanning salts.
The use of other mineral tanning agents based on aluminium, titanium, zirconium, iron has been extensively carried out over the years. Recently, aluminium based syntan (Alutan) [along with Chromium-aluminium based syntan (Alcrotan)] have been developed at CLRI and the technology has been commercially transferred to the industry.