BRINE PRIMARY PURIFICATION
The saturated brine coming from Brine Saturator is treated by means of appropriate chemicals to perform primary brine purification in order to precipitate most of the raw salt contained impurities.
The main impurities normally contained in the saturated brine that must be removed because they can affect the membrane performance are the following;
1. Sulphates, which are precipitated as barium sulphate by reaction with barium carbonate.
SO4-- + BaCO3 ------------> BaSO4 + CO3--
2. Calcium, which is precipitated as calcium carbonate by reaction with sodium carbonate.
Ca++ + Na2CO3 ------------> CaCO3 + 2Na+
3. Magnesium, which is precipitated as magnesium hydroxide by reaction with caustic soda.
Mg++ + 2NaOH ------------> Mg(OH)2 + 2Na+
4. Strontium, which is precipitated mainly as strontium carbonate.
Sr++ + CO3 ------------> SrCO3
The saturated brine from the Brine Storage Pit is pumped to the reactors A and B connected in series.
Here chemicals are added in order to precipitate the impurities; barium carbonate is added in the first reactor, while light soda ash is added in the second reactor.
About 8% w/w barium carbonate suspension is prepared with water in stirred tank and fed to the reactor by dosing pump. The flow rate is adjusted by means of the flow meter.
About 8% w/w light soda ash solution is prepared with dilute brine in stirred tank. Barium carbonate added in the first reactor precipitates barium sulphate giving simultaneously sodium carbonate.
Precipitation is completed in the second reactor by adding light soda ash solution.
Barium carbonate should be dosed on the basis of the sulphates which are really present in the brine and introduced with the raw salt, in order to bring down sodium sulphate concentration in the brine feeding electrolysers 7-8 gpl.
Light soda ash shall be dosed so as to react with all the calcium and magnesium introduced with the raw salt.
Mechanical agitators are provided in the purifiers in order to disperse the reagents in to the brine, to keep the formed solids in suspension and also to make easier the agglomeration of the small solid particles in larger ones.
The insoluble compounds formed following the reactions reported above are removed from brine first by settling. To promote the settling of the solids, a flocculating agent (accofloc) is added to the brine. The brine over flowing from the second purifier is mixed with flocculating agent prepared in separate tank and injected by means of metering pump.
Tests must be done, taking brine sample directly on site, to adjust flocculent concentration in order to obtain the best performance. It should be noted that some substances which are flocculants can also act as flotation agents if they are added in large amounts. A concentration of 1-2 ppm in brine is normally effective. The solids settled in the conical bottom of the clarifier are removed to discharge by a slow-moving rake. If foe any reason, the rake is subjected to high torque, an alarm signalization will appear on control board. An automatic switch stops the mechanism for very high torque to prevent the mechanical damage. The sludge from the settler is discharged discontinuously to the sludge pit. The discharge is automatically operated by opening the on-off drain valve placed at the bottom of the clarifier and allowing the sludge to flow until clear brine is observed in the discharge line. The opening time of the valve and the drain frequency can be varied according to the impurities contents in the raw salt, but in any case, frequency should not be less than 3 times per day.
Brine overflowing from the settler/clarifier is sent to storage tank by passing gravity filters.
BRINE PRIMARY FILTRATION
Brine filters are required to remove the suspended solids overflowing with the brine from the settler/clarifier. The plant is provided with five flowing gravity sand filters operating in parallel. The filter design consists of a multilayered sand filtering bed and one upper layer of anthracite coal.
Clarified brine coming from settler is distributed by gravity to the battery of sand filters through individual hand control valves. The brine percolates the filtering bed from the top, leaving the filter from the bottom through the flow indicator. Filtered brine is collected in Secondary Brine Filter Feed Tank. Each filter is provided with high and very high alarms, it must be excluded from service for cleaning. In case of very high alarm, the filter is automatically cut out of service, by closing the relevant inlet and outlet valves.
The backwashing is performed first by means of air scrubbing and then by filtered brine coming from primary filters.
The filters operation and the cleaning system is performed from a local control panel where all manual operations, controls and alarm signalizations are placed. The operation of each filter is made by five remote controlled on-off valves (HV) operated by a selector switch and by a set of push buttons associated to the cleaning sequences of the filters and placed on the aforementioned panel. Backwashing brine and sludge from filters are sent by gravity to the sludge pit.
The filtered brine collected in the Filtered Brine Storage Tank is pumped either to existing Mercury Plant under flow control or to Secondary Brine Filtration Section for treating the flow required to the New Membrane Plant.
SECONDARY BRINE FILTRATION
The Secondary Brine Filtration (or brine polishing filters) are vertical tubular backwash filters that utilize a cellulose fibre precoat to achieve the required level of filtration. Primary brine typically contains < 10 ppm suspended solids (mostly CaCO3 and Mg(OH)2). The secondary brine filters reduce the concentration to < 1 ppm suspended solids with particle size < 0.5 micron. This level of filtration is required to protect the ion exchange system from suspended solids which would otherwise plug the column.
The secondary brine filters are tubular backwash filter vessels with tubes suspended vertically from a tube sheet. The conical shaped bottom facilitates the backwash by directing the filtered solids cake to the dump (drain) connection during the backwash. The dished top serves as a reservoir of filtered brine which is used to backwash the filter. The filtered vessels are constructed of rubber lined steel to minimise contamination of the brine with metal impurities. The filter tubes are constructed of CPVC with a polypropylene sleeve and are covered with a seamless polypropylene sock type covering. All other internal hardware (fasteners etc.) is constructed of titanium or plastic to minimise metals contamination of the brine. Gaskets are constructed of EPDM.
Incoming brine enters the filter through the bottom conical section and is evenly distributed throughout the filter chamber. Filtration is from the outside of the tubes to the inside. As the brine passes through the cake and tubes, suspended solids are deposited on the outside diameter of the tubes. The collection of these suspended solids forms a cake layer on the tubes. The formation of this cake layer results in increased pressure drop and eventually necessitates backwashing of the filtered brine. Filtered brine flows up through the inside diameter of the tubes and enters the top head of the filter through the openings in the tube sheet. Filtered brine is then discharged from the top head through the main filtered brine outlet nozzle.
The top dome of filter serves as a reservoir of filtered brine which is used to backwash the filter. The design of the vessel is such that a small amount of air is trapped in the top dome during the filtration cycle. This trapped air provides the motive force for the backwashing action during the cleaning cycle. At the end of the filtration cycle, the main filtered brine outlet valve is closed. Incoming brine causes the fluid level in the dome to rise, compressing the air cap until the pressure equals brine feed pump dead head pressure. The main brine inlet valve is then closed, trapping the pressure within the filter vessel. A quick opening drain (dump) valve at the bottom of the filter is then opened, causing the pressure within the vessel to release with nearly explosive force. In turn, the filter socks expand slightly, cracking the filter cake along the entire length of the tube. Filtered brine is then forced backward through the tubes, carrying the accumulated solids and backwash liquid to blow down waste tank in a matter of seconds. Typically, blow down waste is sent to a filter press to separate the solid matter from the liquid. Solids from the filter press are typically sent off site for disposal, usually by landfills.
The secondary brine filtration system consists of two filters arranged in parallel. One in operation and the other is stand-by. Two filters are required because they are operated batch wise. The filters are operated with a constant flow rate of brine. During the filtration process a filter cake forms on the filter tubes. The formation of this cake layer results in increased pressure drop and eventually necessitates back washing of the filter at some pre-determined maximum allowable pressure drop (typically 2.1-2.5 kg/cm2). The filter is then taken offline, back washed and then placed in stand-by mode. The other (stand-by) filter is pre-coated and then put into service before the other filter is taken offline so that the flow of the brine to the ion exchange system is not disrupted.
The control system of the filters normally includes the following;
1. A flow control valve which maintains a constant brine flow rate by compensating for the increased pressure drop resulting from the cake formation and
2. A pressure drop switch to indicate when the filter is ready for back wash.
The secondary brine filters are pre-coated with a cellulose fibre material prior to being put into service. Using a pre-coat improves the achievable level of filtration, allowing particles as small as 0.5 micron to be removed from the brine stream. The pre-coat is applied by circulating a brine solution containing the pre-coat material through the filter until a uniform layer of pre-coat is applied. The pre-coat mix tank is used for mixing pre-coat material and brine. The tank is an open top FRP vessel with a hinged lid. An agitator is provided to ensure a homogenous mixture. Pre-coat material in fibre form is manually dumped into the top of the tank. The amount (kgs) of pre-coat material required depends on the filtration area (m2). During the pre-coat cycle, brine from the pre-coat mix tank is recycled through the filter. The brine turns from milky white to nearly clear as the pre-coat material forms a thin cake layer on the filter tubes. Visual observation of the filter tubes through the sight glass is used to confirm that the pre-coat has been applied uniformly.
If the brine feed stream contains a large percentage of fine (small) or slimy solids, then the pressure drop across the filter may rise rapidly, giving short cycle times. This is often the case when the brine has a poor Ca to Mg ratio (less than 2:1). Slimy and very fine solids form a dense, impermeable cake very quickly, which plugs the pre-coat layer and filter media. Practical filtration of such solids requires that the porosity of the cake be increased to permit passage of the brine. This can be accomplished by adding body feed to the brine just upstream of the filters. Eltech’s basic engineering design includes the provision for continuous addition of body feed. Included are the body feed tank, body feed tank agitator and body feed metering pumps.
ION EXCHANGE FEED TANK AND BRINE CROSS XCHANGE
Filtered brine from the secondary brine filters flows into the Ion Exchange Feed Tank. The brine is then fed to the brine ion exchange system by level control. The ion exchange feed tank provides minimal residence time (typically 15 minutes at NLL = 50%), based on the normal brine flow rate. The tank is typically constructed of FRP. Brine exiting the ion exchange feed tank is pumped through the brine cross exchanger which is located upstream of the ion exchange system. The brine cross exchanger is designed to exchange thermal energy between the brine feed stream to the ion exchange system and the dechlorinated brine stream from the vacuum dechlorination tower. This cross exchange recovers the heat that would otherwise dissipate (lost) to the atmosphere in the brine saturation and primary brine treatment areas. The brine cross exchanger is a plate and frame heat exchanger constructed of titanium plates. It has two separate sections;
1. The first section exchanges heat between the two brine streams, and
2. The second section is used to steam heat the brine feed to the brine ion exchange system as required to maintain the temperature in the range of 60 to 70ºC. The steam heating section is required because the dechlorinated brine stream is not hot enough to provide for all of the required duty (and may not be available at all during start-up). It is typically sized to provide 100% of the heating duty for times when heat from the dechlorinated brine is not available.
BRINE ION EXCHANGE SYSTEM
Ion exchange system is used to further reduce the calcium and magnesium concentrations in the brine stream to those levels required to operate and sustain good overall performance of a chloralkali membrane cell electrolyser. Total hardness includes both calcium and magnesium ions but for simplicity only calcium is mentioned in discussing and expressing total hardness. The ion exchange system consists of three columns with associated piping, valves and instruments. The calcium contents of brine passing through the ion exchange system are lowered from 3 to 5 ppm to less than 20 ppb. Brine from the ion exchange system will therefore be referred to as ultra-pure (UP) brine. The characteristics reactions are shown below:
Service:
2RCH2NHCH2PO3Na2 + Ca+2 ------> (RCH2NHCH2PO3)2CaNa2 + 2Na+
Regeneration to Hydrogen Form:
(RCH2NHCH2PO3)2CaNa2+4HCl--->2RCH2NHCH2PO3H2+CaCl2+2NaCl
Conversion to Sodium Form:
RCH2NHCH2PO3H2 + 2NaOH ---------> RCH2NHCH2PO3Na2 + 2H2O
The recommended ion exchange resin type is a chelating resin of micro porous structure with polystyrene matrix cross-linked with di-vinyl benzene substituted with weakly acidic aminophosphonic active groups. Eltech resin IER8-630 is one such resin. The chemical structure of this resin facilitates the formation of complexes with metallic ions such as calcium and magnesium. The relative affinities for metals in an alkaline brine environment are as follows;
Mg+2 > Ca+2 > Sr+2 > Ba+2
The brine which is fed to the ion exchange system will have already been filtered by the secondary brine filters. Efficient brine filtration is crucial to the proper operation of the ion exchange system. Solids will foul the resin, increase the differential pressure and possibly damage the resin itself. Proper operation of the secondary brine filters is critical to successful ion exchange system operation.
The brine ion exchanges system must be located downstream of any mercury removal system. Mercury will be removed from the brine by the IER8-630 resin, but it is not stripped from the resin during regeneration. Thus, any mercury removed from the IER8-630 resin will permanently reduce the capacity of the resin, thereby shortening cycle time (i.e. shortening the length of time between regenerations). It is therefore crucial that any upstream mercury removal system eliminate as much of the mercury as possible.
Brine feed to the ion exchange system must not have any free chlorine present. Free chlorine will oxidize the resin which destroys the resin’s ion exchange capacity. The feed brine temperature should be maintained in the range of 60 to 70ºC and the pH should be maintained in the range of 9.5-11.0. The brine flow rate is typically controlled in the range of 10 to 30 BV per hour.
Note: BV = bed (resin) volume
During normal operation, brine flows through two columns which operate in series (one primary column and one secondary column). Alkaline brine enters the top of the column and flows downward through the resin bed. As the brine contacts the resin, calcium ions in solution are “exchanged” for sodium ions in the resin. Two sodium ions are exchanged for each calcium ion. The resin bed becomes “exhausted” where there is too few sodium ions left to exchange with the calcium ions, resulting in the “break-through” of calcium ions in concentrations exceeding 20 ppb in the exiting brine. Lab analysis of the brine downstream of the primary calcium every 8 hours is used to determine when break-through has occurred indicating the need to regenerate the primary column. Calcium, magnesium, strontium and barium should be monitored for break-through. In some plants, the need to regenerate is based upon the barium or strontium levels because they break-through first (before the calcium and magnesium).
An exhausted column that has been taken off line must have the resin converted from the calcium form back to the sodium form. This conversion procedure is called regeneration. Unlike other ion exchange resins, the special micro-porous aminophosphonic chelating resin used for softening saturated NaCl brine solutions requires two step regeneration. It is not possible to go directly from the calcium to the sodium form as with a classical water softener resin. Sodium chloride is not used to place the resin in sodium form since the chelated functionality of the cation resin is not able to split a neutral salt.
The regeneration procedure involves the addition of certain chemicals to the resin bed. A general outline of the sequence for this chemical addition is as follows;
| Step # | Description | BV/hr | Time (min) | BV | Direction | Note |
| 1 | DI water rinse | 3.0 | 60 | 3.0 | Down flow | |
| 2 | DI water backwash | 8.8 | 30 | 4.4 | Up flow | |
| 3 | Acid regen. | 4.0 | 60 | 4.0 | Down flow | 4-5 wt % HCl |
| 4 | DI water rinse | 3.0 | 60 | 3.0 | Down flow | |
| 5 | Caustic conversion | 4.0 | 60 | 4.0 | Down flow | 4-5 wt % NaOH |
| 6 | DI water rinse | 3.0 | 60 | 3.0 | Down flow | |
| 7 | Brine rinse | 3.0 | 60 | 3.0 | Down flow | |
The flow rate and total volume for each regeneration step is based on the requirements listed in the resin datasheet. After this procedure has been completed, the regenerated column is placed in stand-by mode. When the primary column is taken offline for regeneration, the stand-by column is put into service as the secondary (polishing) column and the secondary column becomes the primary column. This cyclic operation has been described as “merry-go-round” fashion.
The strength of the acid and caustic used for regeneration should be 4-5% by weight HCl or NaOH respectively. The concentration of acid/caustic is used because it provides roughly the same molarity as the brine solution which reduces the likelihood that the resin will experience osmotic shock.
The design of the ion exchange system includes provision for recovering some of the regeneration rinse water and rinse brine. The streams of steps 1,2 and 7 may be recovered for reuse in the primary brine area (typically for dissolving salt). After exiting the ion exchange columns, these three streams are sent to the recovered water sump, and then pumped to the salt dissolving area.
The acid and caustic streams should be sent to the waste treatment section of the plant. Acid and caustic should not be reused in the brine stream because they contain impurities removed from the resin during the regeneration. After exiting the ion exchange columns, the streams for steps 3, 4, 5 and 6, are combined in the regenerated waste sump, and then pumped to the waste treatment area for further processing. Auxiliary equipment required for the ion exchange system regeneration includes;
HCl Measuring Tank (for 31% HCl)
NaOH Measuring Tank (for 32% NaOH)
HCl Metering Pump
NaOH Metering Pump
For regeneration of the ion exchange resin, weak acid and weak caustic are created by diluting strong acid and strong caustic with deionised water. The metering pumps are designed to provide the proper flow rates of strong chemicals so that the final (diluted) concentrations are 4-5 wt% HCl/NaOH.
Provisions must be made for adding sodium sulfite to the HCl Measuring Tank as required eliminating the chlorine. Free chlorine will damage the ion exchange resin permanently, causing reduced capacity. Also note that free chlorine can be generated by the reaction of HCl with sodium chlorate. It is therefore important to completely rinse the column of brine prior to the start of acid regeneration. Lastly, it is imperative that the deionised water used for regeneration be free of chlorates and free chlorine.
Ion exchange column regeneration is controlled automatically by a Programmable Logic Control (PLC). Once manually initiated, the PLC automatically sequences the proper valve open and closed as required to regenerate the column. Proper flow rates and contact time must be maintained in order for the regeneration to be successful. The PLC monitors these parameters closely to help ensure that the regeneration is complete. The flow rate for each regeneration step is controlled by the PLC. The duration of each regeneration step is based on totalised volume using the PLC to do the flow totalisation.
The ion exchange vessels are typically constructed of rubber lined steel to minimise impurities pick up by the passing brine. Internal piping is typically constructed of Titanium, Hastelloy C-276, or CPVC (plastic only for the top inlet distributor). External piping is typically constructed of CPVC or FRP. The vessel under-drain system must be constructed of metal (Titanium and/or Hastelloy C-276), and must be spiral wound wedge (well screen) design. Plastic is not allowed to be used for the bottom under-drain system, and cloth wrapped laterals are forbidden.
The ion exchange system must include a resin trap in the exiting brine line to prevent resin from passing downstream, in case the under-drain system ever fails.
The ion exchange system is typically supplied as a package unit, skid mounted system. Major equipments mounted to the skids includes the ion exchange columns (vessels), the HCl measuring tank and HCl metering pump and the NaOH measuring tank and NaOH metering pump. Piping, valves and instruments are typically pre-installed by the ion exchange system supplier. All wiring and pneumatic tubing within the boundary limits of the skids are typically pre-installed by the ion exchange system supplier.
