Choosing the Right Cold Lime-Soda Softener for Your Water Treatment Needs

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Introduction

Water is an important resource that is crucial for a variety of industrial and household uses. Hard water, which contains high levels of minerals, can lead to problems like scale buildup and decreased efficiency of appliances. Cold lime-soda softeners are a dependable option for dealing with hard water issues.This blog will explore the various kinds of cold lime-soda softeners and their distinct features.

Types of cold lime-soda softeners

There are four basic types of cold lime soda softeners
(1) The intermittent type (batch process)
(2) The conventional type
(3) The catalyst or spiractor type
(4) The sludge blanket type

(1) Intermittent or batch process

  • The intermittent type of cold lime-soda softener consists of a set of two tanks which are used in turn for softening of water. Each tank is provided with inlets for raw water and chemicals, outlets for softened water and sludge, and a mechanical stirrer (Fig. 1).
  • Raw water and calculated quantities of the chemicals are slowly sent into the tank simultaneously under agitation with the help of the stirrer. Some sludge from a previous operation is also added which forms nucleus for fresh precipitation and thus accelerates the process.
  • Thus by the time the tank is full, the reaction is more or less complete.Stirring is stopped and the sludge formed is allowed to settle. The clear softened water is collected through a float pipe and sent to the filtering unit. The sludge formed in the tank is removed through the sludge outlet.By employing a set of tanks planned for alternate cycles of reaction and settling, continuous supply of softened water may be ensured.
Figure 1

(2) Conventional type

  • In this process, the raw water and the chemicals in calculated quantities are continuously fed from the top into an inner chamber of vertical circular tank provided with a paddle stirrer (Fig. 2).
  • The raw water and the chemicals flowing down the chamber come into close contact and the softening reactions take place. The sludge formed settles down to the bottom of the outer chamber from where it is periodically removed through the sludge outlet. The softened water rising up passes through the fibre filter where traces of sludge are removed and filtered soft water passes through the outlet provided.
  • Water treated by the cold lime-soda process generally produces softened water containing about 50 – 60 ppm of residual hardness.
Figure 2

(3) Catalyst or spiractor type

  • The spiractor consists of a conical tank which is about two-thirds filled with finely divided granular catalyst (Fig. 3). The tank used may be either open (for gravity operation) or closed (for operation under pressure).
  • In both the cases the raw water and the calculated quantities of chemicals enter the tank tangentially near the bottom of the cone and spiral upwards through the suspended catalyst bed. The catalyst employed is a finely granuled (0.3 to 0.6 mm diameter) insoluble mineral substance such as graded calcite or sand or green sand.
  • The retention time is about 8 to 12 minutes. The sludge formed during the softening reactions deposits on the catalyst grains in an adherent form and hence the granules grow in size. The softened water rises to the top from where it is drawn off.
  • The catalyst or spiractor type of continuous water softener is of interest as it gives a granular sludge which drains and dries rapidly and can be handled easily.
Figure 3

(4) The sludge blanket type

  • The sludge blanket type of water treatment equipment is extensively used for coagulation and settling as well as water softening by cold lime soda process. These softeners differ from the conventional type in that the treated water is filtered upwardly through a suspended sludge blanket composed of previously formed precipitates. Thus in a single unit, all the three processes namely mixing, softening and clarification take place.
  • In the conventional type of equipment, some of the added lime suspension is carried down in the sludge formed by the precipitates, before it has time to dissolve and react with the hardness causing impurities of the raw water and thus some of the lime is wasted. In the sludge blanket type,this does not happen because the upward filtration through the suspended sludge blanket ensures complete utilization of the added lime.
  • With the conventional type of equipment, it is generally observed that after precipitates or after deposits form on the granules or filter media employed, and in pipe lines or distribution systems carrying the filtered effluents. This usually necessitates recarbonation with CO2 to obviate the formation of such deposits.
  • However, in the sludge blanket type of equipment, the intimate contact of the treated water with a large mass of solid phase mostly prevents super-saturation or the formation of after deposits. This results in the production of the effluent which is clear enough (turbidity usually less than 10 mg/l) for many industrial applications, so that subsequent filtration is often unnecessary.
  • The retention period required with sludge blanket type equipment is one hour as against four hours with the conventional units. Further, silica is removed better in sludge blanket units.The sludge blanket type of water softening equipment, owing to its higher efficiency,shorter detention period and smaller space requirements, is rapidly displacing the conventional type.

Hot lime-soda process

  • The reactions during water softening take place in very dilute solutions (about 0.001 M) and hence proceed very slowly. The rate of these precipitation reactions can be greatly accelerated by increasing the temperature, because, this not only increases the rate of the ionic reactions themselves, but also the rate at which particles of measurable size are formed.
  • The effect of temperature on the velocity and completeness of precipitation reactions involving the removal of scale forming constituents is shown in Fig. 1.4. It can be seen that at 96°C the precipitation is more complete in 10 minutes than after several hours at 10°C. Thus effect is more pronounced with the precipitation of magnesium compounds.
  • Hot lime-soda plants carry out softening at 94° – 100°C which has several advantages.For efficient softening, cold lime-soda softening plants must be of considerable area and water-storage capacity, whereas hot lime-soda softeners are much more rapid in operation and therefore for a given through-put, much more compact. Elevated temperatures not only accelerate the actual chemical reactions but also reduce the viscosity of the water and increase the rate of aggregation of the particles.
  • Thus, both the settling rates and filtration rates are increased. Thus the softening capacity of the hot lime-soda process will be several times higher than the cold process. Since the sludge formed settles down rapidly, there is no need of adding any coagulants. A smaller excess of chemicals is needed than with the cold process. Further, dissolved gases are driven out of the solution to some extent at the high temperature.The hot lime-soda process yields softened water having relatively lower residual hardness (about 17 to 34 ppm) as against the cold process (about 50 – 60 ppm).
  • A typical hot lime soda water softening unit is shown in Fig. 4, which includes a reaction cum settling tank and a filte . If the water is alkaline, filtration through sand and gravel beds might contaminate the water with dissolved silica, particularly if the quartz used is of inferior quality.
Figure 4
  • Other filtering media used are anthracite coal, calcite and magnetite.If the precipitation is incomplete in the softening tank,“after-precipitation” occurs in pipes, storage tanks and even in boiler itself. If slight excess of chemicals are used over that theoretically required, more rapid and more complete removal of hardness will result.But if larger excess of chemicals are used, naturally they will appear in the softened water. Lime soda plants do not produce water of zero hardness.
  • Tips for the solvening problems on water treatment by Lime-soda process.On the basis of the various reactions taking place in lime-soda process given earlier, the following deductions can be made:

(i) One equivalent of calcium temporary hardness requires one equivalent of lime

`Ca{left(Hco_3right)}_2+Ca{left(OHright)}_2rightarrow2CaCo_3+2H_2o`

(ii) One equivalent of magnesium temporary hardness requires two equivalents of lime

`Mg{left(Hco_3right)}_2+2Ca{left(OHright)}_2rightarrow2CaCo_3+2H_2o+Mg{left(OHright)}_2`

(iii) One equivalent of calcium permanent hardness requires one equivalent of soda,

`Caso_4+Na_2Co_3rightarrow Caco_3+Na_2so_4`
`Cacl_2+Na_2Co_3rightarrow Caco_3+2Nacl`

(iv) One equivalent of magnesium permanent hardness requires one equivalent of lime and one equivalent of soda.

`Mgso_4+Na_2Co_3+Ca{left(ohright)}_2rightarrow
Caco_3+Mg{left(ohright)}_2+Na_2so_4`
`Mgcl_2+Na_2Co_3+Ca{left(ohright)}_2rightarrow
Caco_3+Mg{left(ohright)}_2+2Nacl`

(v) Lime reacts with HCl, `H_2so_4`, `Co_2`, `H_2s`, salts of iron,aluminium, etc. Accordingly, their respective equivalents must be considered for calculating the lime requirement.

`Caleft(ohright)_2+2hclrightarrow Cacl_2+2H_2o`
`Caleft(ohright)_2+H_2so4rightarrow Caso_4+2H_2o`
`Caleft(ohright)_2+Co_2rightarrow Caco_3+H_2o`
`Caleft(ohright)_2+H_2srightarrow Cas+2H_2o`
`Caleft(ohright)_2+Feso_4rightarrow Caso_4+Feleft(ohright)_2`
`2Feleft(ohright)_2+H_2o +½ o_2rightarrow2Feleft(ohright)_3`
`2Caleft(ohright)_2+Al_2left(so_4right)_3rightarrow2Alleft(ohright)_3+3Caso_4`

(vi) Lime, while reacting with HCl, `H_2so_4`, `Mgso_4`,`Mgcl_2`,

`Mgleft(no_3right)_2`, salts of Fe, Al etc., generates the corresponding quantities of calcium permanent hardness. Accordingly, these constituents also should be considered while calculating the soda requirement.

(vii) Two equivalents of `Hco_3` reacts with two equivalents of lime as follows:

`2Hco_3+Caoh_2rightarrow Caco_3+2H_2o +Co_3^-2`
It is evident that in the above reaction, 2 equivalents of `Co_3^-2` are generated. Thus for every one equivalent of `HCo_3` present, the corresponding reduction in the dose of soda has to be made in the
calculations for soda requirement.

For solving numerical problems on lime-soda requirements for softening of hard water, the following steps may be followed:

  • The units in which the impurities analysed are expressed i.e., ppm (or mg/l), grains per gallon (or degrees Clark), etc., are to be noted.
  • Substances which do not contribute towards hardness (e.g., KCl, NaCl,`Sio_2`,`Na_2So_4`, `Fe_2o_3`,`K_2So_4`, etc.) should be ignored while calculating lime and soda requirements. This fact should be explicitly stated.
  • All the substances causing hardness should be converted into their respective `Caco_4` equivalent, as a matter of convention and convenience.

CaCo3 equivalent=Weight of the impurityChemical equivalent weight of the impurity×50

(since chemical equivalent weight of `Caco_3` = 50).

For instance, 136 parts by weight of `Caso_4` would contain the same amount of Ca as that of 100 parts by weight of `Caco_3`. Hence, in order to convert the weight of `Caso_4` as its `Caco_3` equivalent,
the weight of `Caso_4` should be multiplied by a factor of `frac100 136` or `frac50 68`.

Conclusion

Choosing the correct Cold Lime-Soda Softener is essential for successful water softening and preserving the functionality of water-using devices. Taking into account factors like water hardness, flow rate, space needed, operational expenses, automation capabilities, environmental effects, and vendor credibility allows for a well-informed decision that suits individual water treatment requirements. Purchase a top-notch Cold Lime-Soda Softener to guarantee a consistent flow of softened water, supporting the durability of plumbing and appliances while aiding in sustainability efforts.

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