Recovery and Reuse of Chromium Ions from Tannery Wastewater by Precipitation Reaction Method

The global concern about leather industries are increasing as leather industries grows bigger each year. These industries face very challenging task with an increase in stringent pollution control regulation enforced by various bodies due to environmental concern and human risks. The chromium salts are most widely used chemical for tanning process in leather industries, about 35% of chromium used for tanning process remain as metal and discharge to wastewater stream. The removal and recovery of this quantity of wasted chromium are necessary for environmental pollution control and economic reason. This paper shed light to chromium recovery and reuse system of Chromium salts in tanning wastewater by using NaOH as effective chemical precipitation method to regenerate chromium solution, adapted chrome recovery plant and evaluated the system technically and economically.

Hydroxide etc. This paper shed light to present NaOH as superior chemical precipitation method used to recover and reuse chromium from wastewater (7). Sodium Hydroxide is attempted to be a promising alternative for overcoming a Precipitation reaction method for recovery and reuse of Chromium.
The Chromium recovered by added of Sodium Hydroxide as Chromium Hydroxide, the Chromium Hydroxide is dissolved by using Sulphuric acid the recovered Chromium uses as tanning liquor (4) . This method is more effective and cleaner solution which can not affect the quality of leather.
Daily pollution discharge details.

Technology Adopted for Chrome Recovery and Reuse System
The list of equipment installed into the chrome recovery plant are stainless steel Screens fixed before the collection tank pump, effluent collection equipped with reactor and stirrer, sodium hydroxide tank, with stirrer, chromium regeneration tank with mixer and dosing tank with dosing pump filled with Sulphuric acid and chromium liquor storage tank with Pump for regenerated chrome liquor.

Chrome Recovery Operation Procedures:
The chrome tanning drums discharge solution into chromium drain channel ,tanning solution will be filtered with stainless steel screen fixed at inlet point of chrome liquor's collection tank to remove large impurities, from this point the sample was collected and used for all the tests in this study like Cr +3 , Cr +6 , biological oxygen demand(BOD) chemical oxygen demand (COD) pH, total solids, suspended solids (SS), etc. the spent tanning solution pumped by the submersible pump from collection tank into separation tank the solution then stirred with the mechanical mixer in separation tank the solution then basified to pH vale of 11 by addition of caustic soda and a known quantity of calcium carbonated added to the solution with continuous stirring for a period of one hour and stopped to allowed the liquor to settle for 5 hrs. The two layers of supernatant and precipitate layers are formed. The two layers will be separated. The precipitate layer which contained Chromium trivalent separated from the salt and transferred to Chromium regeneration tank and stirring by mechanical stirrer connected to Chromium regeneration tank during stirring the Sulphuric acid is added from Sulphuric acid dosing tank to acidified solution and adjust the pH range to 2-3 in regeneration tank, theoretically, Cr 3+ removal depends in solution pH and temperature, the treated chromium is transferred to the chromium liquor storage tank.

FUNDAMENTAL METHOD OF PRECIPITATION: Chemical Precipitation:
The chemistry of chromium is usually try to described clearly particularly on reaction of hexaaquachromium (III) ion {Cr (H2O)6} 3 complex ions with ligands of water.
To achieve stabilization, the free hexaaquachromium (III) ion {Cr (H2O)6} 3 Coordinated with anion containing free pairs electrons the stability depends on concentration of chromium metal and ligand, the number and orientation of and ligand of water as donor groups, the formed number and size of chelate ring and PH of solution (3) . The reaction of free hexaaquachromium(III) ion {Cr (H2O)6} 3 with sodium hydroxide NaOH removed hydrogen ions from water ligand in the hexaaquachromium(III) ion, the hydrogen removed from the three water molecules attached to hexaaquachromium(III) ion {Cr (H2O)6} 3 the chromium complex become without charge (neutral complex) it will become insoluble in water and it precipitated in the solution.

Reactions of hexaaquachromium ions with sodium hydroxide
Step-1 Adding hydroxide ions to 2+ hexaaquachromium ions The formed precipitate of chromium hydroxide is written without including the water ligands as Cr(OH)2.
Step -2 Adding acid to hexahydroxochromate(III) ions The characteristic of tanning wastewater The tanning liquor was acidifying and treated wastewater contained concentrated Cr 3+ , BOD, see table (1) Measurements for the inlet streams of the treatment pilot plant and chromium recovery plant.

Cost benefit analysis:
The cost-benefit analysis is limited by the investment, performance, maintenance costs, and supply of chemicals. The recovery of chromium is profitable. Each year, the price of the new chromium salt increases, which brings additional benefits. According to Table 4, the total capital investment in the chromium recovery system is $ 52,000 and the annual consumption of BCS is 240 tons per year. The waste is around 80 tons. The efficiency of the Chrome recovery system is greater than 99% We can collect 90% of the fluid from the Chrome exhaust host from Chrome Drum. Therefore, 70% of the chromium salts can be recycled and reused from a cost-benefit analysis, and you can see that the cost recovered from chromium, including all costs, is around 400US / T, while that the new chromium salt can cost more than $ 800 / t. The recovery period for the maintenance of the vineyard is less than three years. In addition to the direct economic benefits, the cost to operators, and the maintenance of waste management facilities is low.

CONCLUSIONS:
Recovery of chromium(III) salts discharge through spent tanning solution from tannery effluent by using chemical precipitation reaction to precipitate chromium as chromic hydroxide(Cr(HO)3.nH2O or Cr(OH)3 or CrH6O3) is performed this design show the Effect of sodium hydroxide as best precipitant coagulant which give a voluminous chromium sludge and it is therefore separate the chromium sludge by decanted the supernatant liquor and chrome slurry is dissolved in Sulphuric acid and chromium Sulphate {Cr2(SO4)3} formed again as reusable liquor. The system is more than 99% efficient in recovery chrome from chrome tanning drum the level of chromium in wastewater reduce from 100 -150 mg/L to 10 -20 mg/L and it re-use water and this result in increasing financial benefit addition to prevent the disbursement of chromium pollution in environment.