Recycling chrome liquors

1. Abstract

This study presents our experiences in the setting up and operation of a recycling system for the final liquors from chrome tanning. The design of the system was based on the studies and proposals made by Davis and Scroggie from CSIRO Australia18,19,20,21,22,23,29 and complementary experiences in various works by other authors.10,14,16,17,24,25,26,27,28,30,31

The system recycles final tanning liquors for use as the main float source in preparing the pickling baths. The chrome present in the residual liquors is precipitated as hydroxide and then diluted with sulfuric acid2,3,4,5,6,7,8,9 before being integrated into the pickling bath.

The system generates important savings of chrome sulfate, sodium chloride water and sodium formate (the masking salt).

The plant began operation in June 2005 and currently 500 hides/day are processed with this system, generating important savings and reducing the impact of the effluent generated.

2. Introduction

The main objective of the chrome tanning process is to convert a raw hide or skin into a product which can be used for the manufacture of usable goods.

Research1,12,15 has shown that with conventional chrome tanning systems, only two thirds of the chrome offered in the tanning bath fixes to the protein, while the remaining third is disposed of in the waste baths at the end of the process and during the sammying operation before shaving.

Our company was prompted by this fact to look for a solution which would reduce the impact generated by the wastewater discharge and allow recovery of the wasted tanning agent.

On evaluation of the available information, it was found that the reduction of chrome in the effluent can be achieved by different strategies such as:

• simple modifications in the parameters of the tannage such as time, pH, volume of the bath13
• the re-use of the residual chrome by precipitation and subsequent solubilisation2,3,4,5,6,7,8,9
• the use of high exhaustion tanning systems based on dicarboxylates11 added during the tannage or incorporated in the tanning agent or other systems which operate with the same logic
• the use of direct recirculation techniques of the final tanning baths,14,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31 a system which has been widely used in Australia, New Zealand and some Asian countries with great success and has been studied in Europe as part of the Life Project 00 ENV/E/000498 in 2002.26

After studying the various options available, we decided to apply the direct recycling system proposed by CSIRO Australia.

It was clear to us that the recycling of final tanning baths is based on two important points:

• reducing process costs in order to save consumables.
• reducing the levels of pollution generated by the discharge of the final baths to the company's water treatment system.

In practice, the project has generated greater environmental advantages than expected and we have recovered our investment in a very short time frame.

The biggest doubt we had about the system was related to the quality of the wet-blue produced but after three years of operation, we assured that this technology has complied with the requirements of the tanning process, achieving better results in quality and homogeneity of the production, maintaining and improving the characteristics sought: a light, even colour, absence of stains, homogenous distribution of chrome through the cut, and containing less than 0.5% fatty matter in the wet-blue.

The controls demanded for the recycling system have given the operators a greater understanding of the parameters to control and a greater awareness of the means of carrying out all the operations. 

In addition to the results obtained in terms of production, the system generated the following environmental benefits:

• reduction in the discharge and in the consumption of the chrome tanning salt, sodium chloride, sodium formate and water
• reduction of the chrome content in the sludge generated in the primary phase of the treatment plant
• notable reduction of odours

in the homogenization tank caused by the discharge of low pH corresponding to the final tanning baths

• substantial reduction in the precipitation of solids at the bottom of the homogenization tank, which complicates the operation of the shakers and often generates problems with odours because of the deposition of sludge in the bottom of the tank
• feasibility of using the sludge generated by the primary treatment in the preparation of compost for use in agriculture.

3. Investment

Introducing a recycling system requires significant capital investments, which can vary according to the sophistication of the system desired. I visited the automated recycling plant in Geelong, Melbourne, Australia, in the VHSP tannery which was operated from the same control centre as the tanning drums. The degree of sophistication for the design and operation of the system is practically infinite.

In our case, the system is manually operated following the strict operation protocols. The total investment in the recycling plant was US$116,083; of which US$55,733 was for the physical recycling plant (four decanters of 12 m3, two decanters of 18 m3 with mechanical shaking system, Volume measurement tank for the pickling bath, preparation tank for magnesium oxide, Volume measurement tank for sulfuric acid, dilution tank for the precipitated chrome, support structure, underground tanks for the storage of the final tanning/ washing baths, assembly and electrical/ pneumatic pumps) and US$60,350 for a modern tanning drum of 4m x 4m which was included in the project to comply with the suggestions in the literature of a good filtration and efficient evacuation of the baths to be recycled.

4. Operational results of the plant

The savings in chemical products generated in the 36 months of operation are US$157,041 (see Table 2) and the consumption of magnesium oxide and sulfuric acid is US$9,152 (see Table 3). The savings generated by the recycling plant in the 36 months of operation, during which time 351,686 hides have been tanned, amount to US$147,889.

From the figures included in tables 2 and 3, it can be easily concluded that the total investment has been returned in 28.3 months and the rate of return on the investment in the recycling plant equipment, excluding the tanning drum, was of 13.6 months, which clearly indicates the ease and speed of the recovery of the investment.

5. Recommendations for the operation of the system

The recycling system should reproduce the parameters of the normal tanning process. To adjust the system it is necessary to know all the conditions in which the current tanning process is operated. To achieve this it is necessary to measure and know the real operating conditions of the process in detail, registering the exact values of the different parameters, such as volumes of the bath entering the process and the pH and density values.

In addition, we must bear in mind that the system was developed based on the direct recirculation of the final tanning baths to be used for the pickling baths.

For this reason there is a tendency to generate excesses of baths which contain chrome, which must be incorporated into the system precipitating the chrome salts present by conventional means, with increased pH, then separating this sludge and subsequently solubilizing them with sulfuric acid.

For that reason it is important to establish protocols which avoid incorporating unnecessary volumes of baths before beginning the recycling process.

To achieve this it is necessary to make the following adjustments to the normal working system:

• eliminate the final wash which is carried out at the end of tanning to prevent formation of chrome stains during the storage before samming. In this case it is necessary to adjust the basification system guaranteeing the absolute exhaustion of the basifying agent.
• one must control the water used for the washing of the drum at the end of the process. These baths are incorporated in the system to recover the chrome present; obviously the greater the dilution we generate, the more complicated the recovery.
• it is necessary to use where possible products which are in concentrated powder form to avoid causing dilutions. For example: chrome powder, basifying agents in powder form.
• Draining the drum very well at the end of each process, in order to avoid increasing the volumes and altering the values of density required by the system and generating dilution of the baths.
• the recycling system stabilizes between the fourth and fifth cycle. For this reason, it is necessary to make adjustments in the quantity of salt, which is added to the system in the early cycles. It is very important to have the density measurements from the normal procedure in order to be able to reproduce the identical conditions in the new process. In our case the system recycles 66.7% of the salt from the process, generating important savings of the product and reducing the salinity of the baths discharged to the treatment plant.
• the recycled liquors must be controlled for use in the pickling bath, analysing the pH, density and chrome content. The chrome added to the system at this point must be discounted from the normal chrome offer, generating savings of the product shown in Table 2.
• the pickling bath must be pumped quickly into the drum. It is clear that the pumping stage is the most critical moment in the process and, therefore, it is advisable to use a high volume pump system which allows the transfer of the whole bath to be recycled in the shortest possible time.  

6. Adjusting the formulation

The adjustments in the formulation for the recycling, once the pH, density and volume of baths which enter the process have been identified clearly, are really very simple:

• one must identify the volumes of the bath which enter the current tanning system. For example, in our case the volume of water used to begin the pickling process was 30% and the volume of water used in the predilution of sulfuric acid which entered the normal tanning system was 14.6%
• the total volume of the bath which entered the system (which in our case was 44.6%), is replaced by the volume of the recycled bath, to which is added sulfuric acid, corresponding to the pickling process.
• all parameters related to density, pH during pickling and basification should remain unchanged in the new process.
• the quantity of salt to be used is reduced, since the recycled baths at the moment of stabilizing the system (between the third and fourth cycles) contain a quantity equivalent to 4%. In our case we use 2% salt at the start of pickling, but at the beginning of the system we used 4% salt in the first process and have gradually reduced the salt offer until we could equalize/level out the density.
• when using masking products such as sodium formate or formic acid, the research recommends reducing the offer to 1/3 of the quantity used, to avoid accumulation and generating an excessive masking of the system which would result in low chrome absorption in the leather. In our case the offer of sodium formate was reduced from 0.5% to 0.39%. The following will show an original formulation compared with the modifications for the recycling process.

7. Process flow

The process is based, as we have explained, on the recovery of all the chrome-containing effluent. In the process we really generate two types of liquors, some with high and some with low chrome concentrations. The high concentrations are in baths generated at the end of the tanning process and the low concentrations are the baths from washing in the drum, the baths generated during the resting of the leather before samming and the baths generated during sammying.

In our case we recover all the liquors mentioned, with the exception of the samming wastes, which are to be incorporated into the process over the course of this year.

In general the discharge should be classified and managed/operated bearing in mind its concentration:

• baths with a high concentration of chrome should be directed to a group of two decanters for purification (removal of decantable material and fats). These baths can later be used for the preparation of the pickling bath which will be recycled
• the diluted baths should be directed to another set of two decanters in which after purification (removal of decantable material and fats) they will be treated with magnesium oxide to precipitate the chrome present therein.

The ‘cake' of chrome precipitate obtained is then redissolved and this product is added to the pickling bath which will be recycled.

Figure 1 shows the general flow of the process

8. Process scheme

8.1 Operation with high chrome concentration:

a The final tanning bath is discharged into an underground storage tank to then be pumped to one of the two decanters which store baths with high chrome content.

b Then, these baths from the end of the process are retained for two to three days to allow sedimentation of the proteins and solids present in the waste baths and the flotation of fats. The pH conditions at the end of the process guarantee the elimination of protein and fats from the system in this first stage. Excess liquor, which can be generated in this stage, is pumped before resting in the precipitation decanters.

c Then the baths are degreased and the decanted solids and proteins are transported from the first group of decanters to a new group, for the preparation of the pickling bath, where sulfuric acid and the redissolved chrome obtained from the precipitation of the baths with low chrome content generated by drum washes, floor washing, samming leathers etc are added.

d Once the sulfuric acid has been added and, if appropriate, the redissolved chrome obtained from the baths with diluted chrome, the contents of the decanter are shaken and then this bath is retained for two to three days to allow the sedimentation of proteins and solids and the flotation of the fats which will have passed from the first group of decanters. The pH values in this point vary between 0.6 and 0.75 after addition of the acid, which is why the precipitation of proteins and the breaking of fatty emulsions present in the system is guaranteed.

e the prepared pickling bath, decanted and degreased, is ready to be passed into a measuring tank and from there be pumped to the process drum when required.

The plant has two decanters of each type to allow the retention times of two to three days which are necessary for the purification of the baths. The decanters work alternately.

In order to calculate the volume of the groups of decanters, it is necessary to measure/cali- brate the volumes of the final tanning baths and calculate the necessary volume of the baths to be used in the pickling. In this way, we must calculate the capacity of the group of decanters for precipitation, measuring the volumes of the used baths in the drum wash and the water discharged from the tanned hides during the rest before samming. Normally, the volumes of this type of decanter are greater.

8.2 Handling/management with low chrome concentration

The final bath from washing the drum is discharged to an underground storage tank for baths with low chrome, to then be pumped to one of the two storage and precipitation tanks, which feature a mixing system designed for the process. 

The area where the leather is unloaded and temporarily stored was adapted with slopes which allow the collection of waters from washing the floor. Draining liquor is also collected this way. These waters are channeled and deposited in the underground storage tank of baths of low chrome concentration. Waters generated by the samming process will be pumped to this same tank later.

a the excess liquors with high chrome content from the end of the tanning process which have been purified are pumped to the precipitation decanters

b The baths are retained in the precipitation decanter without shaking for two or three days to allow the sedimentation of proteins and solids present in the waste baths as well as the flotation of the fats.

The pH conditions of these diluted liquors guarantee the elimination of proteins and fats from the system.

c Once the decanted proteins/ solids have been eliminated, the bath is analysed and the quantity of magnesium oxide required for the precipitation and the quantity of sulfuric acid required for solubilising the chrome hydroxide obtained in the precipitate are calculated.

The mechanical agitation of the decanter is begun and the magnesium oxide solution is pumped for a period of time which guarantees the complete reaction of the basification agent. Then agitation is suspended, leaving it to rest for one to two days.

Equation 1 enables us to calculate the quantity of product to be used to precipitate the chrome solution, obviously making the calculations in accordance with the concentration of magnesium oxide and the concentration of chrome present in the bath.

d After analysis, the clarified content is discharged to the effluent plant and the precipitated chrome hydroxide is transferred to the tank for solubilisation.

e Mixing is begun in the solubilisation tank and the sulfuric acid is added according to the prior calculations. The dissolved chrome is pumped to the decanters for the preparation of the pickling bath.

As in the previous case, there are two precipitation decanters to allow the necessary retention times of two to three days for purification of the baths and the decantation of the chrome hydroxide precipitate. The decanters work alternately.

The final pH of the process should be around 2.5.

From equation 2, the quantity of products to use to dissolve the chrome hydroxide can be calculated, obviously making the calculations according to the concentration of sulfuric acid and the concentration of chrome present in the bath.

9. Special precautions

It is clear that as this is a closed system, all variation generated in the system has an impact on the processes. In our experience, the system can become imbalanced for any of the following reasons:

• it is important to keep control of the pH, density and controls which verify the efficiency in the savings of the system. For example a final pH of basification in the tanning which is less than that sought, indicates the presence of too much acid. This excess of acid indicates a reduction of the pH of the pickling bath, which will in turn affect the basification process.
• an accidental mixing of the final deliming liquors in any of the underground storage tanks will lead to precipitation of the present chrome, causing its loss at the point of elimination of the decanted sludge after the rest during the purification process of the baths to be recycled or precipitated.
• the inappropriate/inadequate elimination of the waste generated by decantation and flotation of fats can contaminate the system and generate leather which is of inadequate colour and contains greater quantities of fatty matter than is required.

For references, please see the website www.leathermag.com

Reaction 1: Precipitation of waste bath with chrome

Cr2(SO4)3 + 3MgO + 3H2O -> 2Cr(OH)3 + 3MgSO4

ph 8.5-8.9 shake 3 h rest 24-48 h

1.5 moles of MgO = 1 mole of Cr(OH)3

1.16g MgO = 1g Cr3+

Reaction 2: dissolution of chrome hydroxide

Cr(OH)3+ 3H2SO4 -> Cr2(SO4)3 + 6H2O

3 moles of H2SO4 = 1 mole of Cr(OH)3

294.0 g/mol H2SO4= 103 g/mol Cr(OH)3

2.9g H2SO4 = 1g Cr(OH)3