PARTIALLY CONDUCTIVE CHLOR-ALKALI DIAPHRAGMS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application claims the benefit of priority of U.S. provisional application No. 63/745,939 filed Jan. 16, 2025, which is incorporated herein by reference in its entirety.

FIELD

[0002]The present disclosure generally relates to non-asbestos diaphragms used in chlor-alkali electrochemical processes.

BACKGROUND

[0003]Many commercial manufacturing processes utilize electrochemistry. For example, the chlor-alkali process electrolyzes aqueous sodium chloride or potassium chloride to form valuable commodity materials, such as chlorine gas, sodium hydroxide (caustic) or potassium hydroxide, and hydrogen gas. Water is electrolyzed to produce hydrogen gas and oxygen gas. Other electrochemical processes are used to prepare a variety of commodity chemicals and intermediates for the chemical and pharmaceutical industries.

[0004]Commercial chlor-alkali electrochemical processes generally utilize a diaphragm as a permeable barrier that separates two compartments in a diaphragm cell that results in chlorine, caustic soda and hydrogen. Historically such diaphragms have been made of asbestos; however, asbestos is being regulated out, so there is a need for a cost-effective alternative to commercialized non-asbestos alternatives. In non-asbestos and asbestos diaphragms of the prior art, partially or fully fluorinated polymers are used as both binders and/or as the primary fiber content of the diaphragm. These fluorinated materials are known to be associated with per- and polyfluoroalkyl substances (PFAS) and/or perfluorooctanoic acid (PFOA), fluorinated surfactants, which are known to cause health issues and possibly will be subjected to similar regulation as asbestos in the near future. It has also been found that alkali-compatible thermoplastics, such as polyethylene (HDPE=High Density Polyethylene) can be used as binders and fibers in a diaphragm, but these are subject to attack by hypochlorite and hypochlorous acid components of the anolyte. With this slow chemical attack, diaphragms made with non-fluorinated binders become soft over time and, especially in the absence of inorganic fibers such as asbestos, these diaphragms develop holes and lose integrity.

[0005]The publication M. S. A. Rahaman, A. F. Ismail, A. Mustafa, “A review of heat treatment of a polyacrylonitrile fiber.” Polymer Degradation and Stability 2007; 92 (8): 1421-1432, is incorporated herein by reference in its entirety.

SUMMARY OF THE INVENTION

[0006]To answer these deficiencies and need for an improved non-asbestos diaphragm that minimizes or eliminates fluorinated components, alternative materials were investigated. As a first iteration, conductive graphite fibers were incorporated in the diaphragm as the fiber component using HDPE as binder and talc as inorganic filler. When this diaphragm was operated in brine service as a chlor-alkali cell, hypochlorite in the cell liquor dropped to non-detectible levels, and diaphragms made with HDPE binders unexpectedly operated for several months without loss of integrity. Examination of these diaphragms after use revealed that HDPE binder remained part of the diaphragm. A new challenge was revealed after the chlorine produced from cells with graphite fiber was contaminated with hydrogen in significant amounts. Hydrogen in chlorine is potentially dangerous because it can concentrate as the chlorine is purified and liquified, especially when it exceeds the explosive limit in the range of 3% to 7%, such that it is preferable to maintain the concentration of hydrogen in chlorine gas below about 4%. It is believed that partially carbonized fibers offer electronic conductivity to suppress parasitic cathodic hypochlorite formation within the diaphragm while avoiding the formation of highly conductive pathways characteristic of fully carbonized or graphitic fibers that promote hydrogen evolution.

[0007]An unexpected solution to the combined problems of hydrogen formation and hypochlorite reduction was discovered in the construction of diaphragms with limited amounts of carbon fibers or with partially-carbonized fibers that had been processed at a temperature below 1200° C. but higher than 600° C. The processed carbon fibers or partially-carbonized fibers are electronically conductive and convey electrical resistance to a diaphragm, including new diaphragm constructions, restored diaphragm constructions, renewed diaphragm constructions and refurbished diaphragm constructions, of at least 20 ohm measured over an about 10 cm² area of the dry diaphragm. Diaphragms with as much as 1000 ohm were found to also operate with little or no hypochlorite (substantially free of hypochlorite), while simultaneously generating zero hydrogen in chlorine. Various examples include the use of partially-carbonized fibers obtained from thermally treating partially oxidized non-fluorinated organic polymer precursors under inert atmosphere.

[0008]One example includes starting with oxidized polyacrylonitrile (PAN) fibers, such as PANOX™ fibers from SGL Carbon SE (Wiesbaden, Germany). Another example includes starting with polyacrylonitrile polymer and oxidize it at about 300° C. in air to form a preferred oxidized precursor. This material is unique in that a further thermal treatment at a temperature greater than about 600° C. in nitrogen or other inert atmosphere imparts thermal and chemical resistance on the material that allows it to be a useful material in diaphragm applications while also imparting the advantage of low hypochlorite in the produced cell liquor-which is advantageous to the life of the diaphragm as well as advantageous in production. In regards to this low hypochlorite production, not only can common diaphragm binders and filler of the art be used, specifically fluoropolymer binders like Kynar™ polyvinylidene fluoride (PVDF) (Arkema S. A., Colombes, France) and Teflon™ polytetrafluoroethylene (PTFE) (The Chemours Company, Wilmington, Delaware (U.S.A.)) and fillers like talc, attapulgite clay, zirconia, but also surprisingly binders that are not fluorinated, like high density polypropylene, which previously have been minor components in the art, if present at all.

[0009]Generally, partially carbonized and partially conductive organic fibers having an electrical conductance of greater than 0 mhos and up to 1000 mhos, as measured across a dry, compressed material, that provide an improved non-asbestos diaphragm when such fibers are incorporated therein. Other features and iterations of the invention are described in more detail herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a table (Table 1) showing an exemplary thermal treatment protocol for oxidized polyacrylonitrile organic fibers.

[0011]FIG. 2 is a table (Table 2) showing exemplary resistance measurements and conductivity of oxidized PAN fibers thermally treated under inert atmosphere to the identified temperature.

[0012]FIG. 3 is a table (Table 3) showing exemplary resistance measurements of non-asbestos diaphragms comprising 2 mm oxidized PAN fibers thermally treated under inert atmosphere to the identified temperature, highly fibrillated high-density polyethylene (HDPE) as binder, and talc as inorganic filler.

[0013]FIG. 4 is a table (Table 4) showing exemplary values of hypochlorite concentration in caustic and % hydrogen in chlorine monitoring of diaphragms over certain days of life (D.O.L.) prepared with thermally treated oxidized PAN fibers under inert atmosphere at various temperatures in one embodiment.

[0014]FIG. 5 is a table (Table 5) showing exemplary values of caustic strength, hypochlorite concentration in caustic and % hydrogen in chlorine monitoring of diaphragms over certain days of life (D.O.L.) prepared with thermally treated oxidized PAN fibers under inert atmosphere with HDPE as binder in combination with one or more fillers.

[0015]FIG. 6 is a table (Table 6) showing product descriptions of thermally treated under inert atmosphere oxidized PAN fibers utilized, their respective thermal treatment protocols, and measured conductivities utilized in embodiments.

[0016]FIG. 7 is a table (Table 7) showing product descriptions of fillers utilized in embodiments.

[0017]FIG. 8 is a table (Table 8) showing a summary of caustic GPLs (Grams per Liter) before and after treatment of the four (4) hole cells with a non-asbestos commercially available diaphragm on a series of bipolar cells treated with a slurry including thermally treated under inert atmosphere non-crimped oxidized polyacrylonitrile fibers.

[0018]FIG. 9 is a table (Table 9) showing a summary of hypo GPLs before and after treatment of four (4) hole cells with a non-asbestos commercially available diaphragm on a series of bipolar cells treated with a slurry including thermally treated under inert atmosphere non-crimped oxidized polyacrylonitrile fibers.

[0019]FIG. 10 is a graph illustrating caustic GPL sampling profile of bipolar cell series taken before the treating trial showing four (4) hole cells with a non-asbestos commercially available diaphragm highlighted with boxes.

[0020]FIG. 11 is a graph illustrating caustic GPL sampling profile of bipolar cell series taken after the treating trial showing four (4) hole cells with a non-asbestos commercially available diaphragm highlighted with boxes.

[0021]FIG. 12 is table (Table 10) showing a summary of caustic GPLs before and after treatment of five (5) hole cells with an asbestos diaphragm on a series of bipolar cells treated with a slurry including thermally treated under inert atmosphere non-crimped oxidized polyacrylonitrile fibers.

[0022]FIG. 13 is a table (Table 11) showing a summary of hypo GPLs before and after treatment of five (5) hole cells with an asbestos diaphragm on a series of bipolar cells treated with a slurry including thermally treated under inert atmosphere non-crimped oxidized polyacrylonitrile fibers.

[0023]FIG. 14 is a graph illustrating caustic GPL sampling profile of bipolar cell series taken before the treating trial showing five (5) hole cells with an asbestos diaphragm highlighted in boxes.

[0024]FIG. 15 is a graph illustrating caustic GPL sampling profile of bipolar cell series taken after the treating trial showing five (5) hole cells with an asbestos diaphragm highlighted in boxes.

DETAILED DESCRIPTION

[0025]When introducing elements of the embodiments described herein, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.

[0026]In exemplary diaphragm compositions a combination of partially-carbonized oxidized polyacrylonitrile fibers, such as heat treated PANOX™ fibers, inorganic particles, and a binder are used. Inorganic particles such as zirconium dioxide (zirconia), attapulgite clay, or talc may be used to produce the diaphragm, and in some embodiment it is preferable for these particles to have a particle diameter less than about 5 microns and be both hydrophilic and chemically resistant to alkali. In other embodiments, inorganic particle size can range up to 10, 20, 30, 40 and over 50 microns. As an example of the varying particle sizes that can be utilized in various embodiments, zirconia CF10 particle size can be 7 μm at D50, ranging from submicron at D10 to 35 μm at D90. A binder is also used to provide improved mechanical integrity and is made in some examples from polytetrafluoroethylene (PTFE), in other examples from partially fluorinated polymer such as polyvinylidene fluoride (PVDF), and in other further examples from high density polyethylene or polypropylene. Binders are preferably resistant to alkali and modestly resistant to hypochlorite. The steps needed to make a diaphragm with said materials include mixing the materials in water with a thickening agent (as known in the art), depositing the slurry onto a cathode, and baking the resultant diaphragm to fuse together.

[0027]As used herein, “partially carbonized’ means fibers thermally treated under inert atmosphere to a temperature sufficient to impart measurable electronic conductivity while retaining non-graphitic carbon structure, as distinguished from fully carbonized or graphitized fibers.

[0028]Optional components in a diaphragm are netting that can be applied to the cathode prior to depositing a diaphragm. Thickening agents added in some examples include xanthan gum or diutan gum. In the event that a netting is applied to the cathode prior to depositing the cathode, this netting may optionally be made of a conductive material so as to make electrical contact with the diaphragm and may be coated with a catalyst to reduce the hydrogen overvoltage and thus the overall voltage of the cell.

[0029]Partially-carbonized fibers of different fiber lengths are preferably used that preferably range in length from about 20 microns to about 65 mm, and in some embodiments up to 100 mm, such as for online treatment. For example, milled carbon fibers of varying lengths from about 20 to about 500 microns, chopped fibers from about 2 mm length up to about 6 mm, and in some examples up to about 65 mm length are also used. These fibers can be blended together to achieve desired diaphragm porosity and ability to retain fine inorganic particles and binder. The fibers of this invention in exemplary examples are prepared by spinning a precursor polymer that forms carbon on heating, such as polyacrylonitrile, rayon, pitch, or polyvinyl pyrrolidone, preferred oxidized polyacrylonitrile (such as PANOX™ fibers). After initial oxidation, the partially carbonized fibers are heated to form partially conductive fibers, preferably at a final temperature of about 600° C. to about 1200° C. for a time necessary to achieve desired carbonization.

[0030]PANOX™ fibers that are thermally treated under inert atmosphere as used in embodiments may have an initial diameter in the range of 12-13 microns, which is not intended to be limiting. Fibers from MiniFIBERS, Inc. (Johnson City, TN) (see Fiber IDs F7, F8 and F9 in FIG. 6) that are thermally treated under inert atmosphere as used in embodiments may have an initial diameter of 12.5 microns, which is not intended to be limiting. It will be appreciated that the diameters of fibers may shrink during such thermal treatments under inert atmosphere.

[0031]The baking of partially carbonized fibers above about 600° C., and preferably not exceeding 1200° C. (and more preferably not exceeding 1000° C.) results in preferred partial conductivity of the fibers that are subsequently used in new, restored, renewed and refurbished diaphragm constructions used for chlor-alkali electrochemical processes. Generally, partially carbonized and partially conductive organic fibers having a conductance greater than 0 mhos and up to 1000 mhos to provide an improved non-asbestos diaphragm when such fibers are incorporated therein. By comparison, fluorinated organic fibers (PTFE) tested in accordance with the conductance testing processes disclosed herein show almost no electrical conductance of 0 mhos (40M ohms) and carbon fibers tested in accordance with the conductance testing processes disclosed herein show high electrical conductance of 25,000 mhos (0.4 ohms).

[0032]In addition to these partially-carbonized fibers, either monodispersed in length or of mixed lengths, diaphragm formulations in various embodiments also comprise a binder and inorganic particles. In some examples, zirconia fiber may be added, including where zirconia is substituted for talc. In some embodiments the zirconia CF10 particle size is 7 μm at D50, ranging from submicron at D10 to 35 μm at D35. Attapulgite clay (attagel) may also be substituted for talc in other examples. In some examples hydrophilic components can be used, including of talc, zirconia, and attapulgite clay. In certain non-limiting embodiments, at least about 50% hydrophilic components may be used.

[0033]In some examples, a non-asbestos diaphragm unit includes described components in a single layer. In other examples, a diaphragm unit may include multiple layers that include a combination of described components.

[0034]In some examples, a diaphragm unit may be manufactured as a new unit wherein the entire unit is produced and subsequently installed. In other examples, a pre-existing diaphragm may be coated with compositions of described components, such as physically reinforcing a diaphragm that was previously in operation and would benefit from coating or integrating such pre-existing diaphragm with an inventive composition set forth herein to provide the resulting benefits of such composition to the diaphragm unit.

[0035]The conductivity or resistivity of a diaphragm produced by example methods and recipes can be measured as described in the illustrative examples.

Illustrative Examples

Example 1

[0036]Oxidized polyacrylonitrile organic fibers of various lengths and linear densities, as set forth in FIG. 6 (Table 6), were thermally treated under nitrogen to the processing temperature identified in Table 6 for a duration of at least four hours. The thermal treatment was carried out in a high-temperature furnace capable of delivering up to approximately 10 kW of power; in this one non-limiting example, a 10 KW, 14-AMP Lucifer furnace model 5AM-K12 equipped with an internal chamber measuring 12 inches×12 inches×12 inches was employed. Prior to heating to the designated temperature, the internal chamber of the furnace was purged with inert gas until oxygen concentration within the chamber was reduced to less than approximately 0.5% and preferably less than 0.1%. The furnace temperature was then increased to the designated processing temperature. Post-firing, the furnace was allowed to cool without intervention from peak temperature. The thermal treatment protocol shown in FIG. 1 (Table 1) is provided for illustrative purposes and is not intended to be limiting, as variations in furnace configuration, heating profile, dwell time, atmosphere composition, and cooling conditions may be employed without departing from the scope.

[0037]In other embodiments, the thermal treatment is applied using a production-scale furnace capable of processing batch quantities of up to or exceeding approximately 400 kg of material. While oven parameters such as configuration, heating profile, dwell time, and cooling conditions may be selected as appropriate for the batch size, the furnace is similarly operated under an inert atmosphere and for a minimum of four hours at peak temperature, consistent with the laboratory-scale protocol in the foregoing description.

[0038]The method for determining the conductivity of fibers according to the invention is described herein. For comparative purposes, the method was also applied to other fiber types. A force gauge test stand equipped with a digital force-measuring device capable of measuring compressive force was utilized. Compressive force was applied manually using an adjustment wheel, although other embodiments may employ automated force application.

[0039]The test setup included a stationary, nonconductive sample chamber with an inner diameter of approximately 25 mm and a height of approximately 46 mm tall, and a nonconductive base having a thickness of at least about 8 mm. The sample chamber and base were preferably formed from clear polyvinyl chloride (PVC) material, although other nonconductive materials may also be used. A copper post was embedded at the center of the base of the sample chamber, the copper post flush with the inner chamber's base surface and having a diameter of approximately 6.2 mm. A non-stationary, nonconductive compression piece was positioned within the sample chamber and configured to move axially relative to the chamber. The compression piece had a thickness of approximately 8 mm and was preferably formed from the same clear PVC material as the sample chamber. The compression piece had a diameter of approximately 25 mm, such that it fit snugly within the sample chamber without restricting movement, and included an embedded copper post having the same diameter as the copper post in the base, approximately 6.2 mm.

[0040]A fiber sample was loaded into the sample chamber such that, upon compression, the sample formed a compressed bed having a height of greater than about 12.7 mm, preferably 19.1 mm. Compressive force was applied to the sample through the movable compression piece, with the applied force measured in pounds-force (lbf). Electrical resistance across the compressed sample was measured using a voltmeter electrically connected to the copper posts.

[0041]Resistance measurements were recorded at successive, incremental compression forces, as summarized in FIG. 2 (Table 2), for the defined fiber types. Any weight loss observed for the fiber samples during thermal treatment was also recorded and included in Table 2. Notably, weight loss increased with increasing thermal treatment temperatures. Conductivity was calculated at each compression condition using Equation 1, shown below. The defined conductivity value for each sample was determined based on the slope and intercept of a best-fit line generated from resistance measurements spanning 50-100 lbf, evaluated at 80 lbf. Conductivity of thermally treated oxidized polyacrylonitrile fibers also increased with increasing temperatures.

$\begin{array}{cc}\mathrm{Conductivity},\mathrm{mhos}=\frac{1,000}{\mathrm{Resistance}\left(\mathrm{in}\mathrm{ohms},\Omega \right)}.& \mathrm{Equation}1\end{array}$

[0042]FIG. 2 (Table 2) details resistance measurements and conductivity of oxidized PAN fibers thermally treated under inert atmosphere to the identified temperature.

[0043]Oxidized polyacrylonitrile 2 mm non-crimped staple organic fibers (from SGL Carbon SE, product #S2-1.7/1.37-A110) were thermally treated under nitrogen to the processing temperature identified in FIG. 3 (Table 3) for a duration of at least four hours.

[0048]The resulting dispersion, after having been stirred 30 minutes, was filtered under vacuum over a basic perforated steel cathode measuring about 5″×6″ and exhibiting openings measuring about 2-3 mm. Partial vacuum is established to achieve flow through the cathode and then opened fully to achieve a value greater than or equal to −24 inHg, which is maintained for at least 30 minutes or until the deposited material appears dry. Note, if the full contents of dispersion cannot be poured all at once, then the dispersion was added in stages as space allowed. Further, any dispersion pulled through the cathode structure was recycled and poured over again to ensure all material was deposited.

[0049]The resulting diaphragm deposited on the cathode structure was then dried at least 2 hours at 90° C. and then sintered at a temperature greater than the softening point of the binder polymer, which is about 135° C. for HDPE.

[0050]To determine the resistivity of the diaphragm in the direction perpendicular to the plane of the cathode an ohmmeter was used. To improve the consistency of the resistance measurements and confirm uniformity across the diaphragm a copper disk with a diameter of about 3.5 cm was moved across the diaphragm's surface, and resistance measurements in ohms were taken by attaching one lead to the cathode post (below the diaphragm) and the other lead to the copper disk. For the purposes of these measurements, no downward pressure was applied to the copper disk apart from gravity. Resistivity results are shown in FIG. 3 (Table 3).

[0051]Targeting a conductive diaphragm formulation, diaphragms comprising in part with thermally treated oxidized organic fiber precursors prepared using treatment protocols 3-6 were chosen for short-term operation is a lab cell to monitor hypochlorite production in caustic and hydrogen gas in chlorine gas. Table 4 shows the measured values. Hypochlorite production in caustic was observed to significantly decrease as the treatment protocol peak temperature increases. However, hydrogen gas was observed in chlorine production when diaphragms contain thermally processed organic fibers treated at 900° C., and presumably, higher temperatures. Note, diaphragm lab cells are equipped with individual power supplies (rectifiers) and are operated at current densities around 0.7 kA/m2. The cathode compartment is constructed of acrylic plastic with the perforated carbon steel plates as cathodes. The anode compartment is constructed of grade 2 titanium with titanium expanded metal coated anodes. Filtered brine (310 GPL) is gravity fed to each test cell from head tanks and adjusted to maintain a constant brine level in the anolyte compartment of each cell. Brine temperature inside each cell, maintained at approximately 70° C., is controlled by a small immersion heated located in a thermowell.

[0052]FIG. 3 (Table 3) details resistance measurements of non-asbestos diaphragms comprising 2 mm oxidized PAN fibers thermally treated under inert atmosphere to the identified temperature, highly fibrillated high-density polyethylene (HDPE) as binder, and talc as inorganic filler.

[0053]FIG. 4 (Table 4) shows hypochlorite concentration in caustic and % hydrogen in chlorine monitoring of diaphragms over certain days of life (D.O.L.) prepared with thermally treated oxidized PAN fibers under inert atmosphere at various temperatures.

Example 2 (Counter Example)

[0058]The resulting diaphragm was similarly dried, sintered at 135° C., loaded into the diaphragm test cell and wetted out with Triton X-100 surfactant over the weekend, utilizing about 0.5 grams of surfactant per liter of water. The experiment was terminated after detecting high levels of hydrogen gas (3.8%) in chlorine after two weeks online. Standard in-situ treatments of the diaphragm known to the art while in operation failed to lower the hydrogen to safe levels. Notably, hypochlorite in caustic was non-detect (<1 ppm) after start-up.

Example 3

[0064]The resulting diaphragm deposited on the cathode structure was then dried at least 4 hours at 90° C. and then sintered at 140° C. for 1 hour followed by 168° C. for an additional 30 minutes. After cooling, the diaphragm was wetted out with Triton X-100 surfactant over the weekend. During start-up, the diaphragm was treated in situ with attagel (6 grams added to the anolyte chamber over the first week) to bring caustic strength up to >70 GPL. Following start-up, caustic was maintained around 100 GPL for >200 days, occasionally treated at a maximum with about 1-2 gram of attagel as needed, during which hypochlorite concentration was measured at minimum once per week and was always observed to be non-detect (<1 ppm). Similarly, hydrogen in chlorine was nearly always non-detect when measured: 0.03% at 21 DOL, 0.00% at 36 DOL, 0.00% at 86 DOL, 0.01% at 150 DOL, 0.00% at 219 DOL.

Example 4

[0065]Using the general methods described herein, a series of aqueous dispersions were prepared and evaluated with a variety of fillers commonly employed in the art to demonstrate the versatility of the partially carbonized and partially conductive organic fibers derived from a non-halogenated source thermally treated under inert atmosphere for use in diaphragms. The formulations prepared are detailed in FIG. 5 (Table 5), and the fiber and filler types listed in Table 5 are defined in FIG. 6 (Table 6) and FIG. 7 (Table 7), respectively. The fillers listed therein are provided for illustrative purposes only and are not intended to be limiting, as one of ordinary skill in the art will recognize that combinations of one or multiple fillers and binders may also be employed in conjunction with the fibers of the invention.

[0066]The specified dispersions were applied to perforated steel cathodes under vacuum. In certain embodiments, a small volume of dispersion is prepared and applied to the cathode surface, as detailed in Example 1. In other embodiments, a large volume is prepared, and the steel cathode is submerged while vacuum is applied through the cathode structure. In additional embodiments, even larger volumes of dispersion are prepared, and correspondingly larger cathode structures that approximate 100 m2 are utilized. The resistance of the diaphragm formed on the cathode can vary depending on the method of application, as different components of the dispersion are preferentially deposited onto the cathode structure depending on whether the dispersion is poured over the cathode or the cathode is submerged.

[0067]FIG. 5 (Table 5) details caustic strength, hypochlorite concentration in caustic and % hydrogen in chlorine monitoring of diaphragms over certain days of life (D.O.L.) prepared with thermally treated oxidized PAN fibers under inert atmosphere with HDPE as binder in combination with one or more fillers.

[0068]FIG. 6 (Table 6) details product descriptions of the thermally treated oxidized PAN fibers utilized, their respective thermal treatment protocols, and measured conductivities utilized in embodiments described.

[0069]FIG. 7 (Table 7) details product descriptions of the fillers utilized in embodiments described.

Example 5

[0070]Thermally treated under inert atmosphere oxidized polyacrylonitrile non-crimped stable organic fibers of various lengths are dispersed in a slurry for hole cell treatment purposes. Hole cell status of a bipolar electrolytic cell is determined by individual sampling. As used herein, a “hole cell” is identified when the caustic strength is roughly 50 GPL below the average of that for the series, frequently in combination with excessive hypochlorite concentrations of greater than 10 ppm.

[0071]A slurry must first be mixed before addition to the cells after hole cell confirmation. Sampling is performed a second time after treatment to determine slurry addition success. Successful slurry addition is shown by the increase of caustic GPL and/or decrease of hypochlorite (hypo) GPLs. This treatment methodology can be used while bipolar electrolytic series are online or offline.

[0079]The resulting slurry, after having been mixed until thermally treated under inert atmosphere non-crimped oxidized polyacrylonitrile fibers are fully dispersed and slurry was uniform were transferred to a transfer tote. Any residual slurry was rinsed from the mix tank into the transfer tote with extra water to ensure all material was used. Treating slurry was used to individually treat hole cells in bipolar electrolytic series.

[0080]After slurry preparation was complete four (4) individual hole cells on one series using a non-asbestos commercially available diaphragm were targeted for treating slurry addition. Treating was completed across each entire bipolar cell by use of the throat holes and downcomers. For treating slurry transfer directly into a bipolar cell, a 2″ diameter hose was used to connect the transfer tote to a 2″ diaphragm pump. A second 2″ hose connected the 2″ diaphragm pump to a treating wand. The treating wand was used to direct treating slurry into the correct throat hole or downcomer of the bipolar cell. The amount of treating slurry added to each hole cell was based on historical treating practices and operator expertise. After the addition of the treating slurry to the four (4) hole cells analysis was performed to check the caustic and hypo GPLs (g/L) of the cells. The caustic GPLs increased and the hypo GPLs decreased in all 4 cells after addition of treating slurry.

[0081]FIG. 8 (Table 8) provides a summary of caustic GPLs before and after treatment of the four (4) hole cells with a non-asbestos commercially available diaphragm on a series of bipolar cells treated with a slurry including thermally treated under inert atmosphere non-crimped oxidized polyacrylonitrile fibers.

[0082]FIG. 9 (Table 9) provides a summary of hypo GPLs before and after treatment of four (4) hole cells with a non-asbestos commercially available diaphragm on a series of bipolar cells treated with a slurry including thermally treated under inert atmosphere non-crimped oxidized polyacrylonitrile fibers.

[0083]FIG. 10 is a graph of caustic GPL sampling profile of bipolar cell series taken before the treating trial showing the four (4) hole cells with a non-asbestos commercially available diaphragm highlighted with boxes.

[0084]FIG. 11 is a graph of caustic GPL sampling profile of bipolar cell series taken after the treating trial showing the four (4) hole cells with a non-asbestos commercially available diaphragm highlighted with boxes.

Example 6

[0092]The resulting treating slurry was then transferred into five (5) individual hole cells on one series using an asbestos diaphragm that were targeted for treating slurry addition. The transfer process into the individual bipolar electrolytic cells identified as hole cells utilized the same transfer methodology as Example 5. The amount of treating slurry added to each hole cell was based on historical treating practices and operator expertise. After the addition of the treating slurry to the five (5) hole cells, analysis was performed to check the caustic and hypo GPLs of the cells. The caustic GPLs increased and the hypo GPLs decreased in all five (5) hole cells after addition of treating slurry.

[0093]FIG. 12 (Table 10) provides a summary of caustic GPLs before and after treatment of five (5) hole cells with an asbestos diaphragm on a series of bipolar cells treated with a slurry including thermally treated under inert atmosphere non-crimped oxidized polyacrylonitrile fibers.

[0094]FIG. 13 (Table 11) provides a summary of hypo GPLs before and after treatment of five (5) hole cells with an asbestos diaphragm on a series of bipolar cells treated with a slurry including thermally treated under inert atmosphere non-crimped oxidized polyacrylonitrile fibers.

[0095]FIG. 14 is a graph of caustic GPL sampling profile of bipolar cell series taken before the treating trial showing the five (5) hole cells with an asbestos diaphragm highlighted in boxes.

[0096]FIG. 15 is a graph of caustic GPL sampling profile of bipolar cell series taken after the treating trial showing the five (5) hole cells with an asbestos diaphragm highlighted in boxes.

[0097]Having described the invention in detail, it will be apparent that modifications and variations are possible without departing from the scope of the invention described herein.