30 WORLD LEATHER APRIL/MAY 2018

Special technical theme: Salt

Due to the limited options availableand the higher cost involved intreating wastewater, Total

Dissolved Solids (TDS) in effluent is anincreasing concern to the tanner. WhileTDS levels are affected by severaldifferent processes and chemicals, oneof the biggest contributors is commonsalt (NaCl). Effluents with high levels ofNaCl concentration can increase thesalinity of soil and therefore render theeffluent unsuitable for use in irrigation.While salt from curing is responsible forthe major portion of TDS, salt used inpickling is the next biggest contributor toTDS levels in effluent. It is estimated thataround 75% of TDS in the effluent due toadded salt (not including the salt fromcuring) is from pickling. As tanners donot have any control over the incomingsalt from curing (unless they processfresh substrates), their best option forreducing TDS in effluent is to reducepickling salt. Any major reduction in saltapplication in pickling, or indeedcomplete elimination of the picklingstage, could significantly reduce TDSlevels in the tanning liquor. This papergives an overview of an application thatcompletely eliminates the picklingstage, reduces the use of basicchromium sulfate and also eliminatesthe basification stage in chrome-tanningskins and limed split hides. This processhas demonstrated significant reductionsin TDS in the tanning liquor, in the use ofwater, in the levels of chrome powderused and in the length of process time,whilst leading to improved wet blue andcrust properties.

The pickling processIn conventional beamhouse

processing, following the deliming andbating stages, the pelt is still in a highlyanionic and alkaline state. Since the pHof the pelt is much higher than its pK,the carboxylic endings of collagen areionised (-COO-). While most of theamino endings of the collagen are alsoionised in the cationic form NH3

+,some amino side chains from lysineand arginine remain in their anionic

NH2 form. Providing the deliming hasbeen done appropriately, the calciumions will no longer be bound and mostof the ionised carboxylic groups willtherefore be “free”, as shown in Figure 1. This makes collagen a perfectreactive site for cationic compounds,such as basic chromium sulfate.Conventional chrome-tanning employs

a pickling process in which organic andinorganic acids are used to bring down

the pH from an alkaline to acidic state.When pelts are pickled to a pH slightlybelow 3.0, the remaining un-ionisedNH2 groups from lysine and arginineare ionised to NH3

+ and the carboxylicgroups of collagen convert to theirmolecular and “blocked” form -COOH.When the carboxylic groups areblocked, and the amino groups arepositively ionised, no bonding site forbasic chromium exists and the pelts

Salt and acid-less tanning systemfrom Buckman

H+

H+

H+ H+ H+ H+ H+ H+ H+ H+

H+H+

H+ H+ H+ H+H+

H+H+

H+

H+

+H3N

+H3N +H3N +H3N

+H3N +H3N

COOH COOH COOH

COOHCOOHCOOH

Sivasankaran GanapathyLeather Technical Manager, Buckman

Figure 1

+H3N +H3N +H3N

H2NH2N H2N

OH- OH- OH-

COO- COO- COO-

COO-COO-COO-

Figure 2

Process % Products °C Run Stop # pH Be Remarks

Pickle 100.00 Water 20

8.00 NaCl 5' 7

0.60 NCOOH 15'x 2

1.00 H2SO4 20'x 5

120' 3.8

Chrome tan 7.00 Chrome powder

0.15 Busan 30L

0.50 HCOONa 120' Penetration OK

100.00 Water 15'

0.25 MgO 60'

0.20 MgO 420' 3.8

Table 1: Conventional chrome tanning process

become less reactive (see Figure 2)and therefore the penetration of basicchromium sulfate is improved.Since pickling is generally done

down to a pH of around 2.8 – 3.0,common salt (NaCl) is used to controlacid-swelling of the pelts. As NaCl ischemically neutral and has very highsolubility in water, it is difficult toremove using common effluenttreatment methods.

Trials with the complexing systemBusperse 7865Busperse 7865 is a novel complexing

agent which simplifies chrome-tanningby eliminating the pickling andbasification processes. This producthas been designed to form weakcomplexes with basic chrome powder,thus reducing the affinity of basicchrome to pelt. Even though the pH ofboth the pelt at the surface and thecross-section are higher than withconventional chrome-tanning, theweakly complexed chrome hasreduced reactivity and can penetrateinto the pelt. Since the affinity exertedby the carboxylic groups to chrome isstronger, this weak complex will breakdown to allow the released basicchrome to covalently react with thecarboxylic group to complete thetanning. Tables 1 and 2 outline theprocesses for conventional chrome-tanning and for chrome-tanning usingBusperse 7865 respectively.The simplicity of the process using

Busperse 7865 is quite clear whencomparing the systems shown in Table1 and Table 2 where the processingtime can be reduced by one-third,process water can be reduced by half,and the addition of basic chromiumsulfate can be reduced by one-quarteras shown in Graph 1.In addition to the savings in time,

chemicals and water, results show that akey benefit of this complexing system isan approximately 65% reduction in TotalDissolved Solids (TDS) from the chrometanning float. However, when the floatlevels are normalised to theconventional system, which uses morewater (100% float versus 60% for thecomplexing system), this equates toaround 80% reduction in TDS levels.Similar reductions in salinity andconductivity are also observed asshown in Graph 2. Laboratory trials have indicated that,

when compared to conventional tanningsystem, absorption and fixation of

31WORLD LEATHER APRIL/MAY 2018

Salt and acid-less tanning system from Buckman

Process % Products °C Run Stop # pH Be Remarks

Chrome tan 60.00 Water 20

2.50 Busperse 7865 30’ 5.5 Cut section will be more than 7.0

5.00 Chrome power

0.15 Busan 30L

0.50 NCOONa

0.50 Chrome stable 120’ Penetration OK fatliquor

40.00 Water 420’ 3.7 Drain, wash, pile

Table 2: Busperse 7865 process

Savings in process time Savings in chrome Savings in water0

10%

20%

30%

40%

50%

25%

50%

34%

Graph 1: Percentage savings in chrome tanning

0

10

20

30

40

50

60

70

80

90

TDS Conductivity Salinity

Busperse 7865 system - As received

Busperse 7865 system - Float adjusted to conventional tanning

Conventional tanning

% Reduction for Busperse 7865 system - As received

% Reduction for Busperse 7865 system - Float adjusted

82.00 82.0080.00

66.0060.00 62.00

Graph 2: Reduction in TDS, salinity and conductivity in tanning liquor

chrome was significantly better with thenew complexing system. Thesebenefits not only allow tanners toreduce the dosage of basic chromiumsulfate, but also lead to a reduction inchrome content in the effluent.Numerous laboratory trials showedaround an 18% reduction in chrome onan ‘as-received’ basis was noticed inthe effluent. This percentage takes onmore significance as the total effluentload from Busperse 7865 was onlyaround 25% of the regular effluent load.As such, when the chrome content iscalculated by equating float-loads,theoretical values showed a huge 82%reduction. This is another importantbenefit to the tanners as, in addition tocommon salt, chrome in the effluent isa major concern. Graph 3 highlights thereduction of Cr2O3 in effluent.It was observed from the numerous

trials that even after reducing thedosage of basic chrome powder by25%, the Cr2O3 content in hidesprocessed to wet blue with thecomplexing system was significantlyhigher than in conventionally tannedwet blue hides. Results from thesetrials indicate an increase of around12% in Cr2O3 content in wet blue asindicated in Graph 4.In addition to higher Cr2O3 levels in

wet blue, crust leathers developed fromthe complexing system in the laboratory trials showed comparatively betterbursting strength, tensile strength and elongation-at-break properties.Percentage increases of theseproperties have been noticed to bearound 6% to 8% as shown in Graph 5.Wet blue tanned by the complexing

system can be further processed likethe wet blue from conventional tanningto various articles and various colourswith slight adjustments in wet-endprocessing, as shown in Figure 3. Inaddition to the improvement in thestrength properties mentioned, finishedleathers from the complexing systemwet blue generally exhibit better fullness,roundness and body.In addition to all the above benefits,

the new complexing system alsominimises the risk associated withissues due to improper basificationsuch as formation of chrome patches,an increase in pH with ageing, chromeprecipitation and so on, as this systemdoes not employ a basification step inprocessing. Risks associated withhandling highly concentrated inorganicacids such as sulfuric acid are

32

Salt and acid-less tanning system from Buckman

WORLD LEATHER APRIL/MAY 2018

0

100010%

0%

20%

30%

40%

50%

60%

70%

80%

90%

Normal LiquorCr203 ppm

Busperse 7865 liquorCr203 ppm

Busperse 7865float adjusted liquor

Cr203 ppm

82%

2000

3000

4000

5000

6000

18%

5100

4200

900

Graph 3: Chrome reduction in chrome tanning effluent float

4.7

4.8

0

0.02

Normal Busperse 7865

12%

5.03

5.64

4.9

5

5.1

5.2

5.3

5.4

5.5

5.6

5.7

0.04

0.06

0.08

0.1

0.12

0.14

Leather - Cr2O3% % Increase

Graph 4: Cr203 % in wetblues

0

5.00

0%

1%

2%

3%

4%

5%

6%

7%

8%

9%

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

50.00

Bursting strengthN/mm2

Tensile strengthN/mm2

Elongation at break%

Busperse 7865Regular % Increase

Graph 5: Improvements in strength

33WORLD LEATHER APRIL/MAY 2018

Salt and acid-less tanning system from Buckman

eliminated completely as the new complexing system doesnot employ any inorganic acids in processing.Commercial applications of this system have been

successfully implemented in several tanneries in number ofcountries on substrates such as goat, sheep, cow calf, buffcalf and lime-split cow hides. Figure 4 shows a bulk lot ofsheep wet blue produced from sheepskins.

ConclusionIt has been proven by numerous trials that wet blue

produced by the new complexing Busperse 7865 system canbe successfully used to produce various types of articles,including conventional shoe upper leathers, shoe softyleathers, shoe nappa leathers, garment leathers, leathers forvarious types of leathergoods and upholstery leathers. Thissystem presents a viable and simple solution to the tanner’ssearch for ways to minimise salinity in effluent, water load,chrome load and time. In addition to those benefits thissystem also produces leathers with better fullness,roundness, body, tightness and strength properties.

Figure 3

Figure 4: Sheep wet blues from Busperse 7865 system

THIS ARTICLE WASPUBLISHED IN WORLD LEATHER

APRIL/MAY 2018

THE WORLD’S NO.1 MAGAZINE FOR THE LEATHER INDUSTRY

April/May 2018 | [Published 27th April] Volume 31, Number 2

IN THIS ISSUE:Tannery of the Year: the 2018 results

Time to call a halt to salt

How to make waterproof leather from wet blue

Calculation of leather carbon footprint agreed