Evaluation of Solid-Liquid Separation Technologies to
Remove Sludge and Monosodium Titanate from SRS High Level Waste

Michael R. Poirier
Westinghouse Savannah River Company
Aiken, SC 29808

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The author conducted a review of solid-liquid separation technologies as possible replacements for the MOTT crossflow filters in the crystalline silicotitanate (CST) ion exchange and solvent extraction flowsheets. The review used the Tanks Focus Area (TFA) funded solid-liquid separation study conducted in 1995 reviewing the technical literature as a starting point. The review also included discussions with vendors, as well as soliciting guidance from researchers at the Savannah River Technology Center (SRTC) and within the DOE complex who possess extensive experience in solid-liquid separation. Finally, the author coordinated a workshop with representatives from SRTC, Savannah River Site (SRS) High Level Waste, SRS Solid Waste, and the academic community on the specific application of interest.

Based on the findings, SRTC recommends the following work to evaluate alternative solid-liquid separation processes for removing sludge and MST from high level waste salt solution.

• Focus on identifying flocculants and additives that will improve the performance of the crossflow filters.
• Investigate settling and decanting followed by polishing filtration (both crossflow and dead-end). This testing needs to use a large volume of continuous fresh feed. It should also examine improvements that could be achieved by the addition of flocculants and additives.
• If flocculation with crossflow filtration proves ineffective, investigate high shear filtration (using a centrifugal filter or VSEP filter).
• If flocculation and crossflow filtration proves ineffective, investigate flocculation in combination with centrifugation.

Keywords: filtration, Alt Salt, solid-liquid separation, sludge

The Salt Disposition Systems Engineering Team selected three cesium removal technologies for further development to replace the In Tank Precipitation (ITP) process: small tank tetraphenyl-borate (TPB) precipitation, crystalline silicotitanate (CST) ion exchange, and caustic solvent extraction.

As a pretreatment step for the CST and solvent extraction flowsheets, the incoming salt solution that contains entrained sludge is contacted with monosodium titanate (MST) to adsorb strontium and plutonium. The resulting slurry is filtered to remove the sludge and MST. The filtrate is either contacted with CST in an ion exchange column or processed through a solvent extraction system to remove cesium.

The high level waste salt solution that feeds this process is approximately 5.6 M sodium and contains small levels of insoluble sludge (up to 600 mg/L).¹ The sludge particles are micron/submicron sized. The mean particle size in a sludge sample from Tank 41H was 15 m. The mean particle size in samples of simulated sludge have varied from 3 – 14 m. The MST concentration in the filter feed will be ~ 0.55 g/L and its mean particle size is ~ 10 m. The specification for MST is < 1% less than 1 m and < 1% greater than 35 m.² The expected viscosity of the supernate is 2.3 – 2.7 cp. at 30° C.³ The goal of this solid-liquid separation is to remove insoluble solids from the waste stream in order to meet the Z-area waste acceptance criteria for alpha contamination, to prevent the insoluble solids from plugging the ion exchange columns, and to prevent insoluble particles from reducing the solvent extraction process efficiency.

Testing performed by SRTC and the University of South Carolina showed the filtration rates were less than desired for simulated salt solution containing various concentrations of MST and sludge solids (0.02 – 0.08 gpm/ft² versus a target of 0.25 gpm/ft²).^4,5,6 To achieve the desired production rates, the current design has a 3000 ft² crossflow filter and a 5000 gpm filter feed pump. The large filter and pump needed for the process will significantly increase the size of the shielded cell needed.

HLW-PE requested SRTC to investigate methods to improve the separation of sludge and MST solids from high level waste salt solution.⁷ This work includes investigating flocculants and additives, changing filter operating parameters, and investigating alternate solid-liquid separation technologies. This report describes the evaluation of alternate solid –liquid separation technologies. The other tasks will be described in separate reports.

SRTC conducted a meeting on June 29, 2000 with Professor Baki Yarar (Colorado School of Mines), Professor Vince Van Brunt (University of South Carolina), and representatives from the SRS High Level Waste Division, the SRS Solid Waste Division, and SRTC to discuss alternative solid-liquid separation technologies to replace for the Mott crossflow filters which are used in the current design bases for the ion exchange and solvent extraction flowsheets.⁸

The recommendations from the meeting were the following:

• One should not filter without prior settling and decanting.
• Without flocculation, solid-liquid separation options are limited.
• The filtration rate can be improved by making the solid particles hydrophobic.
• If these recommendations are followed, other solid-liquid separation technologies are viable.
• If these recommendations are not followed, solid-liquid separation is limited to membrane filtration.

The remainder of this report discusses alternative solid-liquid separation technologies that could be employed to separate MST and sludge from SRS high level waste salt solution. The technologies are grouped into the following categories:

• Crossflow filtration
• Dead-end filtration
• Gravity settling
• Centrifugal methods
• Other methods

The evaluation was conducted in the following manner: Previous SRTC studies of solid-liquid separation processes were reviewed. The author conducted a literature search, and contacted vendors and colleagues at DOE sites to identify plausible solid-liquid separation processes.

The Tanks Focus Area (TFA) funded an investigation of solid-liquid separation technologies to recommend the ones that would be most applicable to separating solids from DOE Site high level waste streams.⁹ The study found that although many solid-liquid separation techniques are available (i.e., centrifuges, settling, dead-end filters, depth filters, etc.), crossflow filtration has a number of advantages over these technologies for use in solid-liquid separations in DOE Site waste:

• Crossflow filters have minimal maintenance requirements compared with centrifuges.
• Crossflow filters generally do not require additives that increase waste volume and change process chemistry. Dead-end filters and depth filters generally require additives.
• Crossflow filters can rapidly concentrate slurries to high insoluble solids levels. Settling and clarification typically require long times.
• Crossflow filters can be employed for continuous washing of slurries.
• Crossflow filtration space requirements are typically less than other solid-liquid separation technologies.

The study reviewed a number of solid-liquid separation tests conducted by the DOE complex with simulated and actual DOE site waste. The results from those studies will be discussed later.

During the review and literature search, SRTC identified the following potential alternatives to the 0.5 m Mott crossflow filter for removing insoluble solids from SRS high level waste:

Filters separate solids from liquid with a semi-permeable barrier. The barrier contains pores which allow liquids and dissolved solids to pass, but which block insoluble solids that are larger than the pore. As the filter rejects particles, they can accumulate on the surface forming a filter cake. The filter cake provides an additional layer that can remove insoluble particles and increases the removal efficiency of the filter. The filter cake also increases the resistance of the filter. The filter flux can be described by equation [1]

where J is flux, DP is the pressure differential or driving force, and R_f is the filter resistance. With a filter cake, the flux is described by equation [2]

where R_c is the filter cake resistance. The filter cake can be removed and the cake resistance reduced by periodic backpulsing.¹⁰ If the filter cake thickness can be reduced, the filter flux will increase. The shear generated by crossflow filtration sweeps particles away from the filter, reduces the cake thickness, and increases filter flux.

Fine particles that are smaller than the pore opening can become trapped in the filter pores. These particles would decrease the porosity of the filter and filter flux. Pore fouling is generally not alleviated by backpulsing. Filter flux can be increased by reducing the number of particles that become trapped in the filter pores.

In addition to the 0.5 m porous metal, crossflow filter, Mott manufactures 0.1 m and 0.2 m porous metal, crossflow filters. By having a smaller pore size filter, small particles are less likely to become trapped within the filter pores. If filter fouling by particles becoming trapped within the filter pores could be reduced, the overall filter flux might be increased. Charles Nash, a researcher associated with the River Protection Program research effort for treating Hanford waste, indicates that a 0.1 m Mott porous metal crossflow filter is the baseline solid-liquid separation technology for BNFL’s program to treat high level waste at the DOE’s Hanford Site.

The smaller pore size would increase the membrane resistance and could reduce filter flux. The effect of pore size on filter flux can be modeled with a modified Hagen Poiseuille equation

where J is filter flux, e is porosity, d is pore diameter, DP is differential pressure, m is viscosity, and l is pore length.¹⁰ If all solid particles are stopped by the filter, filter flux should decrease with pore size.

SRTC tested 0.2, 0.5, and 2.0 m porous metal Mott filters for the ITP process. The 0.5 m filter gave adequate decontamination and had a higher flux than the 0.2 m filter.⁹ However, the ITP feed is different from the feed for this process in that it contains tetraphenylborate solids known to improve filter performance relative to that observed for slurries of sludge and MST. Previous filter testing for the SRS Effluent Treatment Facility found 100,000 nominal molecular weight cutoff (NMWC) ultrafilters (~ 0.05 m) performed better than 0.2 m ceramic microfilters.¹¹ That feed was different from the feed in this process (50 mg/L insoluble solids, 1500 mg/L dissolved solids). If the pore fouling can be reduced, the smaller pore size Mott filter may still produce high filter flow rates. Based on previous SRTC testing of a 0.1 m Graver filter that is discussed below, no testing of the 0.1 m Mott filter is recommended.

The Graver Separation Systems produces a combination ceramic/stainless-steel filter. The ceramic is composed of titania and is bonded to the stainless-steel substrate by sintering. A pore size of 0.1 m is available. The filter has a very fine pore at the filter surface to block small particles, but has a more open structure within the filter to reduce its resistance. The smaller pore size will reduce pore fouling. The more open structure will reduce filter resistance and the decrease in filter flux from the smaller pores at the surface. Since some forms of titania extract strontium and actinides from alkaline solutions, this phenomenon would need to be evaluated before placing the filters in radioactive service.

SRTC tested the 0.1 m Graver filter and the 0.5 m Mott filter with Hanford and Oak Ridge simulated sludge.^9,12,13 The mean particle size was 1.9 – 6.5 m. With the Mott filter, the axial velocity varied from 2 – 12 ft/sec, and the transmembrane pressure varied from 5 – 45 psi. With the Graver filter, the axial velocity varied from 2 – 9 ft/sec, and the transmembrane pressure varied from 10 – 65 psi. The measured filter flux varied as a function of axial velocity, transmembrane pressure, and insoluble solids concentration. The Graver filter performed slightly better (~ 20%) with 0.1 wt.% sludge, while the 0.5 m Mott filter performed better (~100%) with 5 wt.% sludge.

The 20% improvement in filter flux observed is much less than what is needed for this process. Similar filtration results would be expected with the 0.1 m Mott crossflow filter. No tesing is recommended.

The centrifugal system combines centrifugation with membrane filtration. Solids are removed from the liquid at the membrane surface, and the centrifugal force acts to keep the surface clean, minimizing the formation of a polarization layer. The centrifugal force is used to slough off any buildup on the surface, rather than to separate the solids from the liquid.

The centrifugal filter could be combined with most commercially available filter media (i.e., it could be equipped with 0.1, 0.2, or 0.5 m porous metal filter sheets that are similar to the Mott crossflow filters in the current design bases). The centrifugal motion increases shear at the filter surface and reduces cake buildup. The effect is the same as increasing the axial velocity without increasing system pressure requirements.

SRTC tested a centrifugal filter as a replacement to the ceramic microfilters at the Effluent Treatment Facility.¹¹ The filter ran for over 10 hours and showed no significant fouling. That feed stream was different than the feed stream for this process, and it contained a low concentration (43 mg/L) of small, colloidal particles. Centrifugal filters are commercially available (Spintek, Pall) and have been used in radioactive service at LANL.

The manufacturer’s experience with commercial units shows they require regular maintenance to balance the rotor. Frequent maintenance is undesirable for solid-liquid separation equipment in the Alternative Salt Disposition Process.

The centrifugal filter should be considered as a backup to flocculation combined with crossflow filtration and settling/decanting combined with polishing filtration. This filter is likely to achieve the desired filter flux rates. Issues of maintenance and reliability need to be addressed if further development is warranted.

The VSEP filter, manufactured by New Logic, is similar to a plate and frame or disk stack filter. It could be fitted with a variety of filter elements. The filter pack consists of parallel disks. The feed moves slowly between the disks. A pressure differential forces fluid through the filters. The filter elements vibrate vigorously to create shear. The shear is equivalent to 200 Gs.

A VSEP filter vibrating at 1 inch peak-to-peak displacement and 60 Hz produces a shear rate of 150,000 s^-1 which is about four times the shear rate attainable with crossflow filters.¹⁴ The system is suitable for concentrating submicron particles and colloids.

The VSEP filter is commercially available, but has not been demonstrated in radioactive service. The manufacturer recently sold a unit for use in low level radioactive service. If one of these systems were to be used in high level radioactive service, SRS would need to evaluate the system parts for radioactive service and minimize the maintenance needed.

The vendor could test this filter on SRS simulated waste at their facility for approximately $1200. SRS could rent a pilot unit for approximately $6000/month or procure a pilot unit for $90,000.

The VSEP filter should be considered as a backup to flocculation combined with crossflow filtration and settling/decanting combined with polishing filtration. This filter is likely to achieve the desired filter flux rates. Issues of maintenance, reliability, and use in radioactive service need to be addressed if further development is warranted.

The VACCO filter is another crossflow filter. It is composed of a series of stacked disks. The disks contain micro-channels or pores. As the fluid flows through the disks, a differential pressure drives liquid and soluble solids through the pores. It has a more structured packing than the Mott or Graver filters, but the smallest pore size available is 3 m. It was previously tested by SRTC with 3 wt.% ORNL Radiochemical Engineering Development Center (REDC) simulant and fouled very rapidly.¹⁵ The filter flux was about an order of magnitude less than the filter flux with a 0.5 m Mott crossflow filter using the same simulant. No testing is recommended unless he manufacturer can produce a filter with a smaller pore size.

Another plausible technology for removing sludge and MST from high level waste is dead-end porous metal filters (e.g., such as those manufactured by Fundabac or Pall). The filter surface would be similar to the filter surface of the Mott crossflow filters, but it would be the outside surface of a cylinder approximately 2 inches in diameter and 10 – 50 inches long. The fluid would flow from the outside to the inside of the filter at a constant flow rate. As the solids are rejected by the filter, they form a filter cake and increase the pressure drop across the filter. When the pressure drop reaches a certain value, the filter is back washed. If the time between back washes is long, this filter is a viable option. If the time between backwashes is short, the dead-end filter is not desirable.

Previous SRTC testing evaluated a Pall porous metal filter as a replacement for the ceramic crossflow filters at the ETF.¹¹ During testing, the filter fouled vary rapidly and the time between back-washes was typically 5-6 minutes and about 50% of the filtrate was needed to back-wash the filter. The filter had a pore size of 5 m and was fouled by small, colloidal particles. If a filter with smaller pore size could be found, it might operate longer between back-washes.

The performance of the dead-end filter might be improved with the addition of a filter aid.⁹ Diatomaceous earth is commonly used, but would not be suitable for this waste stream. Any filter aid would need to be evaluated for compatibility with high pH, high ionic strength, radioactive stream, as well as compatibility with down stream processes (e.g., DWPF).

In the previous TFA investigation of solid-liquid separation technologies, the author found dead-end filtration to work best with low concentrations of large particles. In a study to treat Hanford Cladding Removal Waste, the authors investigated crossflow and dead-end filtration. The simulated waste contained 1000 – 2100 ppm solids with a mean particle size of 1.2 m. The 0.5 m Mott crossflow filter performed better than the 0.5 m dead-end filter tested.

This type of filter should be examined in combination with settling and decanting.

With this technique, the insoluble solids would settle, and the supernate would be decanted and processed through the ion exchange or solvent extraction systems without any additional treatment.

In theory, the sludge solids in this waste stream settle very slowly (i.e., they did not settle out in the waste tanks and were carried forward with the salt solution). Very long settling times could be required to achieve the solids removal required. Every day of settling time required adds 25,000 gallons of storage capacity to the facility, which will increase the footprint and cost of the building dramatically. SRTC measured settling rates of insoluble solids in an actual Tank 41H sample.¹⁶ Tank 41H was to feed the ITP process, so it should be similar to the feed for the this process. Table 1 shows the particle size measured and Table 2 shows the settling rate measured. The mean particle size is ~ 15 microns. The measured settling rates for the smallest particles (< 4 m) are less than 4 in/day. If the particles in the feed to this process have similar settling rates, settling and decanting is unlikely to be effective at removing a significant fraction of particles. The settling rates could be improved by the addition of flocculants and additives.

The Colorado Mineral Research Institute evaluated a counter-current de-cantation system for use in SRS high level waste sludge processing.¹⁷ They performed settling studies on simulated Purex and Hanford sludge. Without the addition of flocculants, the settling rates were very low (0.17 – 2.2 in/h). With the addition of flocculants (e.g., Alcar W23, Alcar 662, Alcar 600, and Percol 600), the settling rates increased dramatically (to as high as 92 in/h).

The settling tank size is related to particle settling velocity by equation [4]

where A is the tank cross section area, Q is the desired processing rate, v_s is the particle settling rate, and F_b is a fraction of the bulk settling rate (0.5 is commonly used with circular clarifiers of good design).^18,19 Equation 4 can be solved for v_s to determine the required settling rate as a function of tank diameter. Table 3 shows the estimated required settling velocity as a function of tank diameter.

The tank diameter values in Table 3 approximate the expected values for the facility. The table shows that even with the addition of flocculants, settling and decanting without a polishing step is unlikely to perform the solids removal needed for this process.

In this technique, the insoluble solids (sludge and MST) would settle and the supernate would be decanted and filtered. This technique was tested at the University of South Carolina’s Filtration Research Engineering Demonstration (FRED) in 1998.⁶ After settling for two days and decanting, the filter flux with the decanted supernate was 1.3 – 2.1 gpm/ft² depending on operating conditions and approached the clean water flux (2.25 gpm/ft²). When settling and decanting was not used as a pretreatment, filter flux varied between 0.02 – 0.12 gpm/ft². Because of the small batch size, the decanted supernate may not have had enough solids to significantly foul the filter. In a full-scale process, the decanted supernate could contain more very fine particles that could foul the filter more severely than the simulated sludge feeds. Additionally, the process would need to be designed so the settled solids could be re-suspended. Testing performed by ORNL in 1999 to evaluate re-suspension of settled sludge and MST showed this could be difficult.²⁰

SRTC should investigate settling and decanting, followed by polishing with a crossflow filter to treat this waste stream. This technique needs to be evaluated at the pilot-scale with a large volume of continuous fresh feed. This work should also include flocculation/additive addition to improve the settling step.

In this technique, the insoluble solids would settle and the supernate would be decanted and filtered. By using settling and decantation as a pretreatment step to the dead-end filter, the solids loading on the filter will be decreased which should lead to a longer operating time between back-pulses. If 90% of the solid particles could be removed by settling, the improvement in operating time could be as much as 10X. If 99% of the solid particles could be removed by settling, the operating time between back-pulses could be as much as 100X. Additionally, the process would need to be designed so the settled solids could be re-suspended.

SRTC should investigate settling and decanting, followed by polishing with a dead-end filter to treat this waste stream. This technique needs to be evaluated at the pilot-scale with a large volume of continuous fresh feed. This work should also include flocculation/additive addition to improve the settling step.

The centrifuge relies on centrifugal force to exaggerate the density difference between the particles in a liquid, so the solids will "settle" more quickly. Thus, the centrifuge can theoretically be expected to completely remove even small, colloidal solids, given a long enough period of operation. There is no separation by a barrier, and therefore, no place for solids to become trapped. Centrifuges work best with fast settling solids.

The particle settling velocity can be estimated from the following equation

where V_s is the settling velocity, Dr is the density difference between the particle and the fluid, m is viscosity, g is the gravitational constant, d is particle diameter, W_b is the rotational speed of the centrifuge bowl, and R_b is the bowl radius. The required settling rate is described by

where V_s,req is the required settling rate, h is the distance between internal surfaces of the centrifuge, L is the centrifuge length, Q is flow rate, and A is cross-sectional area of the centrifuge. Combining these equations gives the following expression for centrifuge flow rate

where V_s(1g) is settling rate under gravity settling, and R_av is the average radius of the bowl and the pool.²¹

Using the above equations, the throughput of a centrifuge can be estimated. Table 4 shows the results.

Hobbs measured the settling rate of the insoluble solids in Tank 41H salt cake.¹⁶ The measured settling rates varied between 1 in/day and 25 in/day. The facility design requires a minimum flow rate, on average, of 21 gpm. If the settling rate is 1 in/day, a centrifuge would likely be impractical for this solid-liquid separation need.

Hanford evaluated centrifuges for separating solids in Purex sludge, Redox sludge, Cladding removal waste, and Neutralized Current Acid Waste streams. In simulant testing performed, the centrifuge was ineffective unless polymeric flocculants were added to the waste. In a test performed with actual NCAW, large volumes of water were required to removed the separated solids from the centrifuge bowl.⁹

Professor Yarar advised SRS against using centrifuges without flocculants.⁸

Centrifuges have been used successfully in the SRS Separations canyons. The centrifuges used there are standard milk centrifuges. The motors are remoted from the bowls so they can receive periodic maintenance. The bowls have not required replacement.

A centrifuge is the baseline technology for separating insoluble solids in Hanford K-basin. However, that design uses a centrifuge in combination with a polishing filter.

Centrifuge manufacturers have small portable centrifuges that can perform quick scoping tests. SRTC should coordinate one of these tests. If that test shows promising results, a centrifuge could be rented (~ $7500/3 weeks) to perform laboratory-scale tests with simulated salt solution. For centrifuges to be effective in this application, they will most likely need to be used in combination with flocculants and/or polishing filters.

Centrifugation should be considered as a backup technology if the desired processing rates cannot be achieved with flocculation combined with filtration or settling/decanting combined with polishing filtration. Centrifugation will likely require a flocculant to work effectively.

High gradient magnetic separation (HGMS) removes magnetic particles that cannot be separated by other traditional magnetic separation processes because of their lower paramagnetic properties and smaller size. The process consists of a fine ferromagnetic wire matrix inserted in the bore of a magnet, which is energized by an externally applied magnetic field. The external magnetic field creates large magnetic field gradients around the wires, thereby improving the removal efficiency of small and weakly magnetic particles. As the wires become loaded with particles, the magnet can be turned off and the particles drop off of the wires.

HGMS can only remove magnetic particles. Non-magnetic particles (e.g., MST) would need to be adsorbed onto magnetic particles in order to be removed by this process.

The process has been tested with simulated SRS high level waste sludge by Professor James Ritter at the University of South Carolina. In those tests, the removal efficiency was very good, but the solids loading was less than desirable. The system was only able to concentrate solids to 16 g/L insoluble solids.²² HLW-PE has stated a goal to concentrate this stream to 5 wt.% insoluble solids.

Additionalb testing would be needed to more thoroughly evaluate this technology, including testing with MST. No testing is recommended.

SRTC recommends the following work to further evaluate alternative solid-liquid separation processes for removing sludge and MST from high level waste salt solution:

• Focus on identifying flocculants and additives that will improve the performance of the crossflow filters.
• Investigate settling and decanting followed by polishing filtration (both crossflow and dead-end). This testing needs to use a large volume of continuous fresh feed. It should also examine improvements that could be achieved by the addition of flocculants and additives.
• If flocculation with crossflow filtration proves ineffective, investigate high shear filtration (using a centrifugal filter or VSEP filter).
• If flocculation and crossflow filtration proves ineffective, investigate flocculation in combination with centrifugation.

The author wishes to than the following people for participating in the Solid-Liquid Separation Technology workshop conducted on June 29, 2000: Professor Baki Yarar (CSM), Professor Vince Van Brunt (USC), Steve Subosits (HLW), Roy Jacobs HLW), Chi Leung (HLW), Skip Wiggins (SWD), Lee Dworjanyn (SRTC), Reid Peterson (SRTC), Charles Nash (SRTC), James Brooke (SRTC), Bill Van Pelt (SRTC), and Walt Tamosaitis (SRTC).

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