Acid Quality

Properties - Sulphuric Acid - Acid Quality
March 12, 2003

Introduction
Federal Specification O-S-801

Associated Links

The quality of acid is often defined in terms such as commercial grade, technical grade, electrolytic grade, battery grade, food grade, etc. These terms are general in nature and may differ from supplier to supplier and from country to country. The way to get around this ambiguity to have national standards that define the physical properties and maximum levels of contaminants permissible in various grades of acid. In the United States grades of acid are defined in the Federal Specification and by the FDA in the Food Chemicals CODEX.

Ultimately it is the purchaser that defines the quality of acid that is produced and sold while complying with the applicable regulations and standards (i.e. food industry).

Component		Typical Ranges	Federal Spec (Note 1)	Comments
Ammonium	NH₄	< 10 ppm	10 ppm	-
Antimony	Sb	< 1 ppm	1 ppm	-
Arsenic	As	< 0.1 to 1 ppm	1 ppm	Smelting of ores containing arsenic will result in off-gases containing arsenic.
Beryllium	Be	-	-	-
Cadmium	Cd	< 0.5 ppm	-	-
Chloride	Cl	< 1 to 50 ppm	10 ppm	Smelting of ores containing chlorine compounds will result in off-gases containing HCl. Removal of HCl in the gas cleaning system depends on equilibrium conditions in the scrubbing circuit. Chlorides may also be present in makeup water used in the gas cleaning system and as dilution water.
Chromium	Cr	< 1 ppm	-	Chromium can be present in smelter off-gases or result from the corrosion of stainless steel piping and equipment.
Copper	Cu	1 to 50 ppm	50 ppm	Copper is present in the smelter gases as a dust. Proper design and operation of the gas cleaning system will control the amount of copper reporting to the product acid.
Fluoride	F	-	-	Smelting of ores containing fluorine compounds will result in off-gases containing HF. Removal of HF in the gas cleaning system depends on equilibrium conditions in the scrubbing circuit. Addition of sodium silicate will enhance removal of HF.
Iron	Fe	< 30 to 50 ppm	50 ppm	Generally the result of corrosion of ductile iron and carbon steel piping and equipment. The amount of iron can be reduced by the use of stainless steels as material of construction and anodic protection of carbon steel storage tanks.
Lead	Pb	< 1 to 5 ppm	-	Lead is present in the smelter gases as a fume which is condensed in the cleaning system. Proper design and operation of the gas cleaning system will control the amount of lead reporting to the product acid
Mercury	Hg	< 1 ppm	-	Smelting of ores containing mercury will result in off-gases containing mercury. Removal of mercury from the gas before entering the contact section of the plant can be done by a number of different technologies. Due to the toxicity of mercury, reduction to levels less than 0.1 ppm may be legislated.
Magnesium	Mg	-	-	-
Manganese	Mn	< 0.5 ppm	0.2 ppm	-
Nickel	Ni	< 1 ppm	1.0 ppm	Nickel can be present in smelter off-gases or result from the corrosion of stainless steel piping and equipment.
Nitrate	-	< 5 to 10 ppm	5 ppm	NOx is formed in the smelter from the burning of nitrogen containing fuels and the fixation of atmospheric nitrogen. Control of nitrate in the acid is achieved by the segregation of candle drainings and oxidation with chemical such as hydrazine.
Platinum	Pt	None	None	-
Selenium	Se	< 20 ppm	20 ppm	Smelting of ores containing selenium will result in off-gases containing selenium. Proper design and operation of the gas cleaning system will control the amount of selenium reporting to the product acid.
Sulphur Dioixde	SO₂	< 50 ppm	40 ppm	SO₂ is dissolved in the acid when as it comes in contact with SO₂ containing process gas in the towers. Air stripping will remove SO₂ to the desired levels. Treatment with hydrogen peroxide will oxidize SO₂ to H₂SO₄.
Zinc	Zn	< 1 to 40 ppm	40 ppm	Zinc is present in the smelter gases as a fume and dust. Proper design and operation of the gas cleaning system will control the amount of zinc reporting to the product acid.
Organic Matter			None	-
Fixed Residue		< 100 to 300 ppm	300 ppm	-
Colour	-	-	-	A ‘water white’ acid is desireable. Coloured acid is the result of the presence of colloidal carbon which can impart a light brown to black colour to the acid.
Transmittance/Clarity	-	-	-	The presence of suspended solids such as Fe(III) will cause the acid to appear cloudy.

Note 1: Federal Specification O-S-801F, Class 1, Sulfuric Acid, Electrolytic

Federal Specification O-S-801

Federal Specification O-S-801 Revision F, Sulfuric Acid, Electrolyte (For Storage Batteries) defines four grades of sulphuric acid:

Class	Acid Content (% weight)		Specific Gravity at 15.6ºC/15.6ºC (60ºF/60ºF)
Class	Minimum	Maximum	Minimum	-Maximum
1	93.2	-	1.8354	-
2	49.5	50.5	1.3945	1.4042
3	36.5	37.5	1.2767	1.2853
4	28.3	29.5	1.2085	1.2185


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