The manufacture of alum entails just one step additional to the aluminum sulfate production process. The uses of aluminum sulfate and alums are similar and the compounds are largely interchangeable. The term "alum" is loosely applied.
A true alum is a double sulfate of aluminum or chromium and a monovalent metal (or a radical, such as ammonium). Aluminum sulfate is very important industrially and although it is not a double sulfate, it is often called alum or papermakers' alum. Alum has been know since ancient times. The writings of the Egyptians mention its use as a mordant for madder and in certain medical preparations. The Romans employed it to fireproof their siege machines and prepare it from alunite which is plentiful in Italy.
True Alum: Ammonium aluminum sulfate, or ammonia alum, NH4Al(SO4)2·12H2O, is used in tanning, in dyeing and fireproofing textiles, in vegetable glues and porcelain cements, and in water purification. Chromium potassium sulfate, or chrome alum, KCr(SO4)2·12H2O, is used as a mordant in dyeing, in tanning, and in photographic fixing baths to harden gelatin films and plates.
Alum has been replaced to a large extent by aluminum sulfate, which has a greater aluminum equivalent per unit weight. Aluminum sulfate is Al2(SO4)3·18H2O. The most important single application of it is in clarifying water, more than half of its total amount being manufactured being so consumed. In water treatment aluminum sulfate is used to produce an aluminum hydroxide floc.
Aluminum sulfate, also called alum, became an industrial product in the 19th century. It was made by treating either bauxite or china clay with sulfuric acid. Unlike true alum, aluminum sulfate can not be conveniently purified through recrystallization because of its greater solubility in water. This is one of the reasons why it often contains varying proportions of silica, iron and free sulfuric acid.
Because of its greater concentration of alumina (Al2O3) and cheaper production procedures, aluminum sulfate saved costs in papermaking and therefore replaced aluminum potassium sulfate for most purposes in papermaking and particularly in rosin sizing towards the mid-19th century. This earned it the name "papermaker's alum."
Al2O3 is the active ingredient and that is what you are paying for when you buy aluminum sulfate. 8.5% Al2O3 is sold because that concentration has the highest freezing temperature which is practical consideration for shipping and handling.
Percent Al2O3 is a common method of quoting the strength of aluminium-based coagulants on a w/w basis. Another method of referring to concentration is to state the aluminium content, which is roughly half the Al2O3 content. For example, liquid alum of 7.5% w/w Al2O3, which is the same as 4.0% w/w aluminium (Al).
The term percent weight/weight or % w/w is similar to the previous term and is the number of kilograms of active chemical per 100 kilograms of liquid chemical. For example, liquid alum at 7.5% w/w Al2O3 has 7.5 kg of Al2O3 in solution for every 100 kg of liquid chemical delivered.
When calculating coagulant doses, it is important to first state on what basis the dose is to be expressed. The most commonly used unit in water treatment is mg/L, which is a weight per volume unit. This is also the same as parts per million (ppm), but only when quoted as ppm on a weight/weight basis.
If you state a chemical dose as ppm using the volume of chemical dosed to the volume of raw water, then what you have calculated is ppm v/v and this is not the same as
mg/L.
To calculate the dose of a coagulant or other chemical in mg/L, you will need to know its % w/w strength and specific gravity.
The formula is:
mg/L = 10,000 X % w/w X SG
For example, consider alum at 49 % w/w strength aluminium sulphate (as Al2(SO4)3.18H2O) and with a SG of 1.29. Using the formula above, the concentration of the alum in mg/L, will be: 10,000 X 49 X 1.29 = 632,000 mg/L or 632 g/L. If you are dosing alum at a rate of 150 mL/min as found from a drop test, and the raw water flow is 50 L/s, then the alum dose will be: (150 X 632,000/(1000 X 60))/50 = 31.6 mg/L or ppm (w/w) or: 1,000,000 X (150/(60 X 1000))/50 = 50 ppm (v/v).
In customary US units:
Chemical Feed Setting (mL/min) = [(Flow, MGD)(Alum Dose, mg/L)(3.785L/gal)(1,000,000 gal/MG)]/[(Liquid Alum, mg/mL)(24 hr/day)(60 min/hr)]
The only element which requires explanation is the liquid concentration. The liquid concentration, given in mg/mL, is the amount of dry chemical (in mg) mixed with the amount of water (in mL):
So, if you added 230 mg of alum to 100 mL of water, you would have a liquid alum solution with a concentration of 2.3 mg/mL.
Consider a situation in which the flow of the plant is 5 MGD and the alum dosage is 10 mg/L. The liquid product is shipped to the plant as 40.6% alum (6.9% Al2O3).The liquid alum has a concentration of 516 mg/mL. The chemical feeder setting would be determined as follows:
Chemical Feed Setting (mL/min) = [(5 MGD)(10 mg/L)(3.785L/gal)(1,000,000 gal/MG)]/[(516 mg/mL)(24 hr/day)(60 min/hr)]
The setting on the liquid alum feeder should be 254 mL/min.
As an another example, the liquid product is shipped to the plant as 48.5% alum (8.3% Al2O3). The Alum solution that you use has approximately 642 mg dry Alum per mL of liquid.
If the plant is treating 830 gpm water with 394 mL/min alum, the calculation is as follows:
394 mL/min X 1 L / 1,000 mL = 0.394 L per min
0.394 L/min X 1440 min/Day X 642 mg/mL Alum X 1 lb/453.6 grams = 803 lb/Day.
