Ferric Chloride Applications & Advantages
Ferric Chloride Applications & Advantages
Various iron salts are used in water treatment, although ferric chloride and ferric sulphate are the most commonly used . One of the reasons that ferric salts find wide application in water treatment, is that ferric hydroxide is insoluble over a wide pH range .
In recent years the demand for ferric chloride has increased and the chemical is purpose made .Ferric chloride is highly acidic and the solution contains free hydrochloric acid. The chemical is normally supplied as a solution of about 40% strength as FeCl3 with a SG of about 1.4 and a pH of less than 1 .
General description :
Ferric chloride is sold commercially in solution form and the solution is a characteristic red brown colour. The solution is acidic and corrosive to most metals. It is regarded as hazardous in terms of handling and transport.
The Characteristics of the Product :
Ferric chloride is sold commercially, for use in water treatment, as a dark red-brown solution containing about 14% iron or 40% ferric chloride. A solution containing 14% as iron (Fe) has the same concentration as a solution containing 40% as ferric chloride (FeCl3) .The decision to manufacture the product at 40% is based on factors such as the freezing point of the solution and the tendency in more concentrated solutions for ferric chloride to crystallize out of solution. This concentration has been found to be the most practical and is the accepted standard for commercially produced solutions for water treatment applications.
Application of Ferric Chloride in Wastewater Treatment :
The primary reason for removing phosphate from wastewater is because phosphate is a
nutrient and if released into surface waters, it can result in nutrient enrichment of the water body (especially when released into a river flowing into a dam (reservoir). This in turn can lead to excessive growth of algae and aquatic plants, eventually causing oxygen depletion in the water, fish kills and poor water quality. This condition is known as ‘eutrophication’. Excessive growth of algae is of course a major aesthetic and environmental problem .
Africa and in some sensitive catchment are as specific regulations exist regarding the discharge of phosphates with the intention of controlling algae growth. Of course if we could eliminate phosphates from our water courses it would be effective in controlling the growth of algae. Unfortunately however, phosphorus has a wide range of unique uses such as in laundry detergents and fertilisers. There have been regulations in some countries to limit or prevent the use of phosphates in laundry detergents but unfortunately the alternative products are expensive and not as effective. Furthermore, phosphates are essential to fertilisers and therefore agricultural run-off would still account for phosphate loadings. Natural sources of phosphate also occur in domestic wastewaters, so limiting phosphates in detergents and fertilisers would not necessarily solve the problem. Fortunately for water treatment chemists, the phosphate salts of ferric (Fe3+) iron are quite insoluble and this is the key to the most common chemical means of phosphate removal. The reaction is one of precipitation of insoluble ferric phosphate according to the reaction (Metca and Eddy, 1999) :
Fe3+ + HnPO4
3-N ↔ FePO4 + nH+
We can ignore the parts of the reaction we are not interested in and note that 56 g (or kg or tons) of iron will react exactly with (31 + 16 + 16 + 16 + 16) = 75 g (or kg or tons) of phosphate. So 56 g of iron will, in theory remove 75 g of phosphate. It can now be seen that if you know how many g (or kg) of phosphate are flowing into a plant over a period of time, the quantity of iron required to remove the phosphate can be calculated, and therefore the quantity of ferric chloride required can be determined.
However it is not as simple as that.
Although in theory the above is correct, these reactions do not account for all the competing reactions that also occur as well as the effects of alkalinity, pH, trace elements, etc. that are found in wastewater. The result is that in practice the theoretical calculated amount provides nothing more than a rough estimation, but hopefully this provides some understanding of how chemical reactions occur and how calculations can be used to at least provide an approximation of dosage rates, etc. Theory would suggest that we need 75 ÷ 56 = 1.34 times as much iron as there is phosphate.
This is the stoichiometric amount. In practice though, because of the competing reactions, it is usually found that 1.52.5- times the stoichiometric amount of Fe:P is required. 28 There are many different designs of wastewater plant and as a result the best place to add the ferric chloride also varies. In some plants it is added near the beginning of the works, right before the primary settling tanks, while in others, it is added to the activated sludge plant. A third option is add it near the end of the process into the secondary setting tanks.
Finally, before leaving the subject of ferric chloride we should mention its use as part of a process called ‘Chemically Enhanced Primary Treatment’ or ‘CEPT’. As its name implies this process involves adding ferric chloride (or other metal salts) to the first, or primary settling tanks in a wastewater works. Primary settling tanks rely on gravity and natural flocculation to remove some of the suspended solids from the raw sewage before the settled sewage overflows into the secondary stage. By adding an iron salt to the primary settling tanks, the flocculation process becomes much more efficient and a significant proportion of the phosphate can be removed in the sludge.
This reduces the solids that normally flow into the second stage allowing the plant to cope with a higher flow rate. Of course the result is that the amount of solids produced in the primary stage is much greater and this is only acceptable on plants that have enough solids handling capacity, (e.g. digester capacity) to cope with this extra load. Under these circumstances however, CEPT can provide a cost effective means of increasing the effective capacity of a treatment works without having to spend capital expenditure on plant extensions.