KASUR TANNERIES POLLUTION CONTROL PROJECT (KTPC)
  - A DOCUMENTARY

The purpose of the chromium recovery pilot plant is to recover the chromium from the wet blue liquor discharged during the tanning process. It is a pilot project to check the feasibility for the future use. It is intended that every tanner will install his own in - house chromium recovery plant. It is decided because plant is cheap to install and run, furthermore it is in the benefit of tanners.

Basic chromium sulphate, a chemical compound having chromium as the active ingredient is used for tanning of hides and skins universally. After pre treatment of raw hides and skins these are charged in a wooden drum where water and basic chromium sulphate along with the other chemicals is added. The drum is sealed and rotated for certain duration. The hides absorb nearly 70% of chromium present in the drum. After some time, the drum is opened and chrome tanned hides are recovered from the drum. The water solution contains 30% of the unabsorbed chromium sulphate, along with other chemicals originally present in the drum. A reasonable amount of organic matter is in the form of fleshing is also released by the hides and is present in the left over chromium solution in the drum. The idea behind the project is to:

· Recover the valuable chromium salt from the waste tannery effluent.

· Determine the quality of recovered chromium salt for tanning purposes.

· Practically demonstrate the benefits of chromium recovery plant to the tanners.

· Reduce the chromium pollution from tannery waste solution.

CHROME RECOVERY PROCESS

The collection of the chrome liquor is done, by the open mouth drums, of 120 liters capacity each. The transfer of the tanning drum to the plastic drum is manual. The transport from the tannery to the pilot plant is by donkey cart. The liquor is then transferred from the drums into a reception tank. It is done through a sump pump of capacity 100 liters/min.

The reception tank is a steel tank of length 1200 mm, width 650 mm and height 600mm. The tank incorporates a sieve of the 2 mm opening. The purpose of the sieve is to block the fleshing present, in the effluent, as fine suspension. Another screen is installed at the bottom of the reception tank to check the fleshing from entering the transfer pump.

Storage Tanks

The purpose of the storage tanks is to store the waste liquor for the time being and provide to the reaction tanks when required.

The tanks have the capacity of 15,000 liters. The centrifugal pump is installed to pump the solution from the reception tank to storage tank. A bayonet valve is used to block the entry of any one of the two tanks. Each storage tank has 2,130-mm diameter and 4,300 mm length, fabricated from the stainless steel SS 316 and mounted horizontally on the carbon steel saddles. Each tank is fitted with the manhole, level indicator, outlet valves and drainage valves. A scale indicating the volume is fitted in the center of the two tanks.

Reaction Agents

Chromium is one of the heavy metals and its hydroxide is insoluble in water. This chemical property is the basis of methods used for removal of waste from the tannery effluent. Thus the waste liquor is mixed with an alkali to precipitate chromium. Any alkali can be used.

§ Calcium hydroxide (burnt lime) is the cheapest material, however it forms the calcium sulfate, which is insoluble in the water and is likely to pose problems when recovered solution is recycled back to the tanneries.

§ Magnesium hydroxide is used for this purpose in some countries. It's major advantage is the quality of the precipitate formed with chromium. It is reported that this precipitate settles to the 25% of it's original volume in eight hours. This may eliminate the need of a filter press. However according to use in Pakistan, it is not feasible because:
1. The price of the pure magnesium oxide is Rs. 120/kg.

2. Since the settled precipitate retains the 25% of the original volume of water, the purity of the chromium obtained is low. The recovered chromium retains upto 25% impurities present in the solution.

3. The concentration of the recovered chromium solution is
very small. Almost 75% of the chromium is wasted, present in the original solution.

§ Sodium hydroxide is manufactured in Pakistan and thus is readily available in forms of solid, granules, flakes and liquid. The rate of the sodium hydroxide flakes is Rs.25/kg. Also it's use has enabled the 99% recovery of the chromium from the original solution. The plant uses the sodium hydroxide solution and is designed to handle the 50% solution. But the use of magnesium hydroxide is also possible.

Reactors

The rectors are the tanks in which the chromium is precipitated from the spent liquor. These are two conical tanks of capacity 5,000 liters. The dia of each tank is 1,700 mm and shell height is 2,200mm. The cone angle is 45 degree. The volume of the cone is 850 liters and that of shell is 5,560 liters. Each tank is fitted with the mechanical agitator rotating at the 100 rpm. The caustic soda is added to bring the pH of the waste chrome liquor in the reactor to around. At which point the chromium will start to precipitate from the solution. The amount of the alkali required is to be determined in the laboratory first.

There are two tanks provided for the dissolution and storage of the alkali. Each tank is conical with the 550mm dia and 1200mm shell height and fitted with the agitator rotating at 100 rpm. the working volume is 250 litres.