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DKL Engineering, Inc.
Handbook of Sulphuric Acid Manufacturing
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Sulphuric Acid
Decolourization
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Process Engineering Data Sheets - PEDS
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Introduction
Bibliography of Sulphuric Acid Technology
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Preface
Contents
Properties -
Sulphuric Acid - Colour
December 20, 2001
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Pure sulphuric acid is colourless or "water white". However, the presence of small amounts of organic material, will discolour the acid. These may be present in the incoming gas and are not totally removed in the gas cleaning section of the plant.
The degree of discolourization varies from a light brown to black depending on
the quantity of organic matter.
This colour would make an otherwise good quality acid sell at a discounted price
or not at all.
An arbitrary standard scale is used as a means for matching of colour intensity. A colour of 5 units means that the intensity of the colour of this sample is equal to the intensity of the colour of a sample of sulphuric acid 50% in water, containing 5 milligrams of platinum (as potassium chloroplatinate) per liter.
Blending of acid is of limited effectiveness in improving the colour of the acid
due to the large amount of colourless acid required to achieve an acceptable
product quality.
Hydrogen peroxide (H2O2) is effective in removing colour
from the acid and the rate at which hydrogen peroxide clears up the acid varies
with the acid temperature, the quantity of SO2
dissolved in the acid and the strength of H2O2.
Bench scale test should be performed to determine the amount of hydrogen
peroxide to be added, the temperature and residence required to produce a
product acid of acceptable quality.
Based on the results of the testing, bleaching system can be designed based on
conservative design parameters.
One acid producer found that to reduce the product acid colour to acceptable
levels required between 0.5 to 1.4 kg of 50% H2O2
per metric tonne of 98% H2SO4 at an operating temperature
of 80ºC. The residence
time required for the reaction to take place was 2 hours.
A conservative design of a bleaching system based on these test results would be
for double the dosing rate and a residence time of 4 to 5 hours.