Inspiration for a greener tanning industry from the Arzignano tanning district.

Valuing the oxidative depilation byproducts

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by Filippo Meneguzzi, Gruppo Dani

A description of the research made by Dani Group over the last year of experimentation.

The first goal was to remove the hair from the drum during the liming process, with the following objectives:

  1. Separate keratins rich in protein, still little affected by the action of the depilatory oxidative mixture ;
  2. To allow a more targeted action of depilatory mixture to hair still attached to the bulb, to those already detached;
  3. Avoid the clogging of the heat exchanger.

To do this separation, we have identified a filter-cochlea as the best device: the bath to be grilled, extracted from the drum side pan, passes through an external circuit and is conveyed on filter screen leaving the entire material that is larger than the diameter of the holes (mainly fur); at this point you operate the rotary movement of the central cochlea and the brushes clean the screen depositing the screened material on the cochlea which lift and drain it; simultaneously, with a pump, this liquid is fed back into the drum.

During the tests, we have checked several variables such as rotation speed of the cochlea, rates of activation of the pump that enters the bath inside the drum, opening times of the inlet valve in the bathroom of the filter-cochlea; nevertheless, although varying these parameters, we were able to extract a lower percentage of hair than normally we obtain by conventional liming and this is a confirmation of the fact that the hair separation mechanism is not the same as a sulfide liming and that we must intervene on the hair immunization to protect stem and make it attacked only in the bulb.

An other research was related to the baths treatment at the end of the liming process in order to extract as much material as possible (material that may find added value in other industrial fields) and then re-use the clarified bathroom in the soaking phase of the subsequent liming.

To achieve this we’ve covered the bathroom at the end of liming with a vertical centrifuge; in the first step the end of liming bath has been treated in a centrifugal obtaining a densified (1 ° mud) and a clarified (1 ° clarified); subsequently the 1 ° clarified was brought to pH = 4 with 85% phosphoric acid, to induce the precipitation of further protein substances, and retracted with the centrifuge to obtain a second thickened (2 ° mud) and a second clarified (2 ° clarified ).

Acidification has been pushed up to a pH value equal to 4 as in laboratory tests we indentified it to be a good compromise between reagent consumption (phosphoric acid) and amount of precipitate obtained. The analyzes carried out showed that the 2nd mud contains interesting values of carbon and nitrogen as shown in the following table:

 

 

N   TOTAL C   TOTAL SO3   TOTAL
(%) (%) (%)
1A TEST 1° MUD 2,9 27,7 1,3
2° MUD 9,4 37,0 2,7
2A TEST 1° MUD 2,2 23,9 0,7
2° MUD 8,3 38,9 2,0
3A TEST 1° MUD 1,9 24,1 1,0
2° MUD 9,1 38,3 2,4

At last, a heat exchanger was installed to the plates downstream of the filter-cochlea (their use must necessarily be combined to prevent residues of the hide or hair which can block the heat exchanger, reducing the yield) with the aim to cool down the bath and avoid excessive increase of temperature inside the drum, especially in the exothermic phase of the liming.

The exchanger was tested especially after the insertion of the depilatory mixture (exothermic phase of the reaction) with excellent results; working with the exchanger as confer a less thermal stress to the hides and avoiding the risks of damage to the grain that will usually occur at temperatures close to 35 ° C (below the final carry-plant scheme currently used).

articolo Dani 2016

In collaboration with Acque del Chiampo we made an analysis of the main parameters of comparison between the exhaust of a conventional liming baths and those of a liming oxidative. Some parameters have improved (sulfides, sulfates, sulfur, COD, TOC, calcium, sodium) while others have deteriorated (chlorides, total nitrogen and suspended solids).

In the  coming period, in collaboration with Acque del Chiampo, we are planning to repeat the withdrawals to see if the clarified bath from the previous liming, once soaked in the next liming, beyond the obvious water saving, makes any changes to these parameters.

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