Leather and Hides

Decalcination and/or Pickling 

The application of hydrogen peroxide at the beginning of the decalcination and/or pickling results in a cleaner hides, without interfibrillar dirts and bleaching of the natural pigmentation of the skin.

The use in decalcination offers the advantage of additional sulphide oxidation, that are present in the bath and hides, avoiding that those are released to the atmosphere of the tannery, providing wastewater free of such compounds.

Note: Due to the oxidant characteristics of the hydrogen peroxide, it should be avoided to use it together with reducing substances, such as bisulfites, sulfites and others.

 

Production of Soles

The use of hydrogen peroxide at decalcination will provide a brighter and more uniform final product, with a better standardization of the cut of the soles. 
 
Note: Due to the oxidant characteristics of the hydrogen peroxide, it should be avoided to use it together with reducing substances, such as bisulfites, sulfites and others.

Sulfides

The presence of sulfides in wastewaters and working environment of tanneries is especially due to: 

• Processing baths of depilation or washing of the leathers before and after the deliming and bating 
• Action of sulphate-reducing bacteria over organic and inorganic compounds of the wastewaters in anaerobic conditions 

Problems caused by sulfides:

Odor 
The odor of rotten eggs of sulfides is well known and is perceptible even in concentrations below 0,3 ppm. In elevated concentrations, the sulfidric gas inhibits the olfactory system, eliminating a factor that would serve as alarm in a danger situation. 

Toxicity 
Sulfides constitute an environmental threat for being poisonous to aquatic life in general. The sulfidric gas is equally toxic and, in concentrations over 1000 ppm in the air, causes death in a few minutes. Eye and respiratory tract irritation, headaches and fatigue sensation are symptoms of an exposition to concentrations over 5 ppm. Due to its low solubility and elevated volatility, the danger represented by the sulfidric gas is of the same order of the cyanide gas. The presence of sulfidric gas in the working environment in tanneries depends mainly of the processing step that the hides go through and air circulation inside the factory. 

Corrosion 
It also corrodes piping, pumps and installations, even concrete parts. 

Unfavorable action over biological treatment  
In elevated concentrations, sulfides are toxic to the biological treatment, reducing the efficiency of the process and inhibiting microbial activity. They also favor the growing of filamentous bacteria in the processes of treatment by activated sludge. To avoid disturbances of the active biomass, sulfide concentration must not be superior to 25 mg/l, and should be kept constant to avoid shocks that hamper the biological activity in the processes. 


Oxidation of Sulfides by hydrogen peroxide: 
 

Our hydrogen peroxide products are easily handled and their application doesn’t involve large increases in cost. They provide partial oxidizing of sulfides to intermediate compounds that do not exhale bad smell, and that can be treated by aeration, efficiently and at low cost. 

Sulfides are formed as follows: 

H⁺ + HS^- ↔ H₂S (Meio neutro ou ácido) 
HS^- ↔ H⁺ + S^2- (Meio básico)

In basic medium, there are species of low corrosive power in equilibrium: S2- and HS-, with a low concentration of H2S (less than 1% of dissolved sulfur). However, in this pH range, nor domestic sewage nor industrial effluents could be dumped over rivers. They are dumped with neutral pH, which is when S2- and HS- ions are converted into the volatile, toxic and corrosive H2S. 

-In an acid or neutral solution prevails the following reaction:

H₂S + H₂O₂ → S(s) + 2 H₂O

Where the biggest part of sulfide is transformed into elemental sulfur. The rest is constituted of different soluble compounds of sulfur, and according to their structure might be oxidized later. 

The reaction is relatively slow in acid medium, but can be catalyzed through ions of transition metals. After addition of dissolved iron (such as iron (III) sulfate), the reaction completes in few minutes, even at room temperature.

-In alkaline reactions, the oxidation occurs according to the following equation:

 

S^2- + 4 H₂O₂ → SO₄^2- + 4 H₂O

In that case, the reaction is considered faster than in acid medium. At ambient temperature it concludes itself in few minutes, even without catalyst addition. To avoid byproducts, the proportion of hydrogen peroxide that must be used should be above the stoichiometric.

Application

There are mainly four practical and economically viable ways to oxidize the sulfides with our products:

• Complete Oxidation
• Auxiliary Oxidation
• Polishing
• Preventive Oxidation

 

Complete Oxidation 

The complete oxidation of sulfides to sulfates by hydrogen peroxide is used preferably by companies with a great wastewater with sulfides flow e small area for treatment, or by those who are still constructing their wastewater treatment.

It can also be used in case of accidental spill, momentary inoperability of the treatment station or emergency cases.

The speed of implanting the dosage system, the operational easiness and reaction effectiveness in a very short time are inherent characteristics of the hydrogen peroxide.

Auxiliary Oxidation 

To the companies that already oxidize its wastewaters by aeration or oxygen injection, the hydrogen peroxide is a valuable partner, since the use in large enough amounts for a partial oxidation of the wastewater eliminates the bad smell immediately and accelerates the operation of the subsequent aeration, guaranteeing an increase in efficiency of the process with energy economy.

Polishing

The polishing is an adequation of the wastewater to the conditions required by the environmental control organs. In case of the wastewater still having sulfide content above the allowed emission levels, it can be adequated by adding a small dosage of hydrogen peroxide.

Preventive Oxidation

When used as a preventive measure to the development of sulfides, hydrogen peroxide is primarily used as source of oxygen

H₂O₂ → H₂O + ½ O₂

The decomposition level to oxygen will depend of the presence of contaminants in the wastewater, particularly transition metals. The concentration of hydrogen peroxide to keep the aerobic conditions is lower than those necessary of molecular oxygen.

Elimination of sulfides in the working environments

Many times the content of up to 40ppm of sulfidric gas can be found in the working atmosphere of a tannery, depending of air circulation and of the work in progress. As seen before, contents this high can cause problems to the respiratory tract, eyes, headaches and fatigue, that affect the productivity of the employees. On the other hand, openings pots for felt preparation by the end of the deliming and/or purge exposes the operators to even higher concentrations of this gas. 

The sulfidric gas can be eliminated from the atmosphere by the addition of hydrogen peroxide at the beginning of the deliming.

Disinfection 
In some countries, for the disposal of sewage in rivers and the sea, where is foreseen use for recreation of primary contact, it is mandatory by law to disinfect it to eliminate the thermo tolerant coliforms. For this application, the oxidizing action of the hydrogen peroxide works as a first step of disinfection, reducing the peracetic acid necessary for complete elimination of pathogenic microorganisms.