The use of technical salt (mineral halite) in the regeneration technology of modern automatic Na-softening filters

Ivan Tikhonov

The consumption of salt for the regeneration of Na-softening filters is one of the most expensive items in the operation of water treatment equipment.

Modern water softening systems are fully automated. The operator only needs to add salt to the feeder (salt solvent tank) after it has been consumed. Modern valves automatic control of flows in the softening systems are complex technical devices with lots of transitions, openings, orifices, injectors, plungers, etc. The use of solutions from crude technical salt (halite) in such devices can lead to clogging of the device and its failure.

Manufacturers of flow control valves recommend using tableted salt for resin regeneration. Tableted salt is a salt that has been pre-cleaned of various contaminants. The solution obtained from such a salt is transparent and an insoluble precipitate is not formed at the bottom of the feeder. The content of NaCl in the tableted salt should be at least 99.0%. For regeneration of resin of softening systems the amount of the sodium chloride in the salt should be at least 97 %.

The main disadvantage of the tableted salt is its cost. Currently, the wholesale purchases of 1 kg of the tableted salt costs from 0,25 $. As a cheaper analogue a mineral concentrate halite (technical salt) of the highest category, which contains 97-98% NaCl can be considered.

Before the advent to the Russian market of tablet salt halite has been used widely for the purpose of water treatment. The cost of 1 ton of premium halite is an average of $ 60, i.e. 1 kg of halite is $ 0.06. Halite is cheaper than tableted salt on average 4.5 times. There is a natural question about the possibility of using halite in modern water softening systems, which are equipped with automatic flow control valves.

When the technical salt is dissolved directly in the feeder (salt-solvent tank), a large number of contaminants initially contained in the salt is formed. The formation of a stable film on the inner surface of the feeder and in the salt mine is observed. There is the formation of a large number of suspended solids and insoluble precipitate. All this leads to failure of flow control valves. It also has a negative effect on the ion exchange quality of the resin.

Thus, taking into account these circumstances, a scheme for the use of technical salt (halite “top grade”) in automatic water softening plants was developed. For water treatment purposes only the highest grade of halite with NaCl content of more than 97% is applicable.

The Figure.1 presents the schematic circuit of the use of technical salt in the automatic installation of water softening consisting of an automatic flow control valve (CLACK, FLECK), fiberglass housing with resin, and feeder with a salt intake device. This installation is standard for the vast majority of modern water softening systems of medium and low productivity.

Principle of operation:

The initial hard water enters the control valve and is redirected to the filtration through the resin in the filter housing from top to bottom. The initial hard water enters the control valve and is redirected to the filtration through the resin in the filter housing from top to bottom. The resin requires regeneration after exhaustion of ion exchange resin for sodium ions. Resin regeneration is carried out by a working solution of tablet salt concentration of 8-10%. The working salt solution is obtained by mixing the saturated salt solution and the source water. Mixing of flows takes place in the control valve, and then the working solution enters the filter. The saturated solution has a concentration of about 26%. The working solution is of 8-10%. A saturated solution is prepared previously in the feeder (the tank). The operator pours the tableted salt into the feeder. Water enters the feeder automatically for a clearly defined time after each automatic regeneration of the unit. The amount of water supplied to the feeder is a very important parameter, which largely determines the efficiency of salt consumption for resin regeneration.

The volume of water in the feeder is determined by two conditions:

1. Tank fill time;
2. Emergency float valve, which regulates the maximum water level in the feeder.

Both conditions do not always allow to ensure the right amount of saturated salt solution in the feeder for optimal management of the regeneration process. For example, 1 liter of resin usually requires 120 g of table salt for medium hardness water. If the filter is filled with 100 liters of resin, the regeneration will require 12 kg of salt or 40 liters of saturated 25% solution. Manufacturers of water treatment equipment indicate that the volume of undissolved salt in the feeder should be slightly above the water level. It is worth remembering that the insoluble salt occupies a significant volume in the feeder and the volume of water supplied to the feeder in the automatic mode may not be enough to obtain the necessary volume of the saturated solution. Increasing the time of water supply to the feeder in this case will not correct the situation, because the float valve will block the water supply line when the volume of water will increase to the maximum level, despite the fact that most of the internal volume of the feeder is occupied by undissolved tableted salt.

In any case, this method of creating a saturated salt solution does not allow to fully control the process of water softening with optimal parameters for salt consumption. It takes a long time to set up the process of adding salt and the time of water supply to the feeder. The efficiency of this process can be tracked only after some time when calculating the relative salt consumption per 1 m3 of softened water. This can only be done using salt and water consumption data.

According to the scheme in Fig. 1 the saturated solution of technical salt is prepared in a separate tank with a volume much larger than the volume of the feeder. The volume of the obtained saturated salt solution in the tank should be enough for several regenerations. The “dirty” saturated solution is supplied to the clarification filter by means of a pump. The pump must be equipped with a nonreturn valve and a pressure switch, as well as a corner strainer at the inlet. The clarification filter is a fiberglass body of sizes 8-32 with a manual flow control valve. A filter loaded high-performance brightening download – Filter AG. Water flow through the filter should not exceed 0.4 m³/h. After clarification, the saturated solution enters the feeder. The level of filling the feeder with solution is controlled by a float valve. Thus, you can fill a certain amount of saturated solution required for resin regeneration into the feeder. If the pressure drop increases on the filter clarification more than 0.5 bar it is necessary to carry out washing out of the filter into the drain. For this purpose it is necessary to connect the pipeline with the source water to the supply line of the saturated solution.

Automation of the process consists in the installation of a microswitch in the valve of automatic control softening. The microswitch must be switched on (close contact) at the automatic output of the softening unit into regeneration mode. When the contact is closed, the solenoid valve (normally open) must be switched on (closed), thereby blocking the supply of saturated solution to the feeder. In the process of regeneration, the saturated solution is injected from the feeder and, mixing with the source water, enters the filter for the regeneration of the resin. The float valve in the intake salt device opens as the saturated solution in the feeder decreases, but the closed solenoid valve does not allow the solution to enter the feeder after the clarification filter. Thus, accurately measured amount of solution is spent for  the regeneration of the resin. After the regeneration is complete, the control valve automatically starts filling the feeder with the source water. You must turn off this stage or make it minimal (approx. 1 min). After completion of the regeneration and the “filling the feeder with water” stage, the microswitch will be opened and the solenoid valve will open. Filling the feeder with a saturated solution from the tank will begin. The feeder will be filled with saturated solution until the float valve of the salt intake device is activated. The cycle is completed.

This scheme is easy to operate and inexpensive. When it is used in boilers with a capacity of 1.5 m3/h of softened water, the additional purchased equipment pays off in less than six months by saving on cheaper technical salt. The payback period is even less than if the boiler capacity is more than 1.5 m3/h.

The example of calculation of economic effect from the use of technical salt in systems of water softener instead of tablet salt.

Source data

The steam boiler with installed water softening system which has a capacity of 1.5 m³/h. The daily amount of softened water is 36 m³.

The source water is a drinking city water supply. The total hardness of the source water is 4.0 mg-eq/l.

With such hardness of the source water, the specific consumption of salt per 1 m3 of the source water will be 0.45 kg, i.e. the daily consumption of salt will be 0.45*36=16.2 kg.

The cost per day for tablet salt – 16.2*0.27=4.37 $/day

The cost per day for technical salt – 16.2*0.06=0.97 $/day

0.27 – the cost of 1 kg of tableted salt, dollars

0.06 – the cost of 1 kg of technical salt, dollars

Savings using of technical salt will be 4.37-0.97=3.4 $/day.

The cost of additional installed equipment for the use of technical salt:

1. The tank of saturated solution (1000 liter) – 150 $.
2. The pump with pressure switch and check valve – 60 $.
3. The water clarification filter with manual control – 180 $.
4. The solenoid valve (n.о.) ½ “ – 45 $.
5. Pipelines, instrumentation and installation works – 200 $.

                                                  Total amount: 635 $.

The payback 635/3.4= 186 days

Conclusion: I believe that the use of technical salt instead of tableted in boilers with a capacity of softened water more than 1.0 m3 / hour is justified and creates a significant economic effect through the use of cheaper salt (halite). The equipment used in the modernization of the standard scheme pays off in less than a year. Commissioning of the softening unit using the proposed scheme becomes more efficient in the process of controlling of the salt consumption for resin regeneration, which will avoid unnecessary salt overruns.