CONCENTRATIONS IN A PULP MILL CHEMICAL RECOVERY SYSTEM

Field of the Invention This invention pertains to the production of bleached or unbleached cellulosic fibrous pulp. The invention is concerned more particularly with a method by which chloride and potassium can be removed from the pulping chemicals used in the pulp production process. Chloride and potassium are common

contaminants found in pulp mill chemical recovery cycles. Background of the Invention

In a primarily closed-cycle pulp mill, wood in the form of chips or sawdust is processed with a combination of pulping chemicals to produce cellulosic fibrous pulp. The pulping liquor used in a Kraft mill is known as white liquor, and comprises sodium sulfide (Na ₂ S), sodium hydroxide (NaOH), sodium carbonate (Na ₂ C0 ₃ ), and impurities. In alternate pulping processes other chemicals can be used, such as alkaline peroxide mechanical pulp (APMP) mills which use sodium hydroxide and hydrogen peroxide (H ₂ 0 ₂ ).

After separation from the cellulosic fibres, the spent pulping chemicals are recovered in the pulp mill chemical recovery cycle. The first step in the chemical recovery cycle is evaporation, where spent liquor is concentrated by a multi-stage evaporation and concentration process. The concentrated liquor is then burned in a recovery boiler. The recovery boiler has two purposes:

generating steam for the pulping process, and converting spent chemicals to useful pulping chemicals. Spent chemicals are recovered by dissolving the smelt from the recovery boiler. In a Kraft process the solution comprises mainly dissolved sodium sulfide and sodium carbonate and is known as green liquor, while in zero-effluent APMP mills it is mostly sodium carbonate.

Sodium carbonate is converted to sodium hydroxide in a recausticizing plant. In recausticizing, the sodium carbonate reacts with calcium oxide (CaO) in a causticizing reactor to form sodium hydroxide. The calcium oxide is converted to calcium carbonate (CaC0 ₃ ) which is separated from the slurry by

clarification or filtration and burned in a lime kiln to reform calcium oxide. The clarified slurry is reused as pulping chemical.

In a mill chemical recovery cycle, potassium, chloride and other impurities entering with the wood and input chemicals tend to build up to a steady state concentration in the pulping liquors. Chloride and potassium need to be controlled to low levels because high concentrations can cause operating problems such as recovery boiler plugging or corrosion (Tran, TAPPI Journal, November 1986). Typical methods of removing potassium and chloride are to dispose of a portion of the recovery boiler electrostatic precipitator (ESP) catch (ash) or to process the ESP catch in a separation system which separates the chloride and potassium from sulfate and carbonate pulping chemicals.

Existing separation systems that separate based on the solubility differences between potassium chloride and sodium sulfate suffer from high chemical losses of sodium sulfate and sodium carbonate, which are sewered alongside the by-product potassium chloride in a saturated solution. This increases make-up chemical requirements and the conductivity of the effluent.

Amphoteric resins used for chloride separation systems are only capable of selectively removing chloride, while cation exchange resins used for the separation of potassium suffer from poor potassium removal unless large quantities of chemicals are used to regenerate the resin. It is well known that chloride and potassium become enriched relative to sodium sulfate in the flue gas dust retained by the electrostatic precipitator in the recovery boiler. Several prior art processes have taken advantage of this enrichment to facilitate removal of chloride impurities from the chemical recovery cycle by treating the precipitator catch to separate potassium and chloride from Na ₂ S0 ₄ and Na ₂ C0 ₃ . Examples in the patent literature include US 4,007,082 (Fuller), US 5,567,293 (Paleologou et al.), US 5,91 1 ,854

(Lindman), US 5,922, 171 (Paleologou et al.), and US 7,553,394 (Furusho et al.). The main technologies used today for separation of chloride from sulfate and carbonate include leaching, evaporation-crystallization, freeze crystallization and ion exchange (Honghi Tran and Paul F. Earl, "Chloride and Potassium Removal Processes for Kraft Pulp Mills: Λ Technical Review", Pulp & Paper Center and Department of Chemical Engineering & Applied Chemistry, University of Toronto, 2004).

Leaching, evaporation-crystallization and freeze crystallization capitalize on the solubility differences of potassium chloride and sodium sulfate. The potassium chloride remains in solution while the sodium sulfate precipitates out. The solid precipitate is separated from the solution and returned to the recovery cycle while the solution is sewered. Sulfate and carbonate losses from these systems are typically 15-35%. Chloride and potassium removal effectiveness of these systems are typically 70-90%, with greater removal effectiveness corresponding to greater chemical losses.

High chemical losses are of particular concern in newer boilers which operate at increased temperatures generating higher levels of carbonate in the ash. When carbonate concentrations exceed 6 to 7 wt%, ash slurries become difficult to separate, reducing the sodium sulfate recovery and the removal efficiency of potassium chloride (Goncalves and Tran, "Factors Affecting Chloride and Potassium Removal Efficiency of a Recovery Boiler Precipitator Ash Treatment System", International Chemical Recovery Conference, 2007).

The conventional method to reduce the carbonate concentration in the ash is to add sulfuric acid to neutralize the sodium carbonate and form sodium sulfate. The drawback of adding sulfuric acid is that the addition of a sulfur-containing compound upsets a mill sodium sulfur balance, causing an unwanted rise in liquor sulfidity or a need to purge streams that are enriched in sulfur (i.e. C10 ₂ generator salt cake, precipitator catch and/or product). Some mills are thus incapable of adding sulfuric acid and must operate chloride and potassium removal systems with reduced efficiencies.

Sulfate and carbonate losses are minimized with the use of ion-exchange technology. However, amphoteric resin used for the removal of chloride does not have any selectivity of potassium over sodium, limiting potassium removal. Cation exchange resin technology used for the removal of potassium requires large quantities of regenerant chemical to achieve a high level of potassium removal due to the very low potassium to sodium ratio that is typically present in the ash feed. The low potassium to sodium ratio is especially prevalent in newer boilers where low potassium levels in the ash are required to minimize superheater corrosion concerns.

Using sodium hydroxide as a regenerant with cation exchange resin has been proposed to create a closed system where the regenerant is subsequently used as a make-up caustic source in the bleach plant (US 7,553,394, Furusho et al.). However, the caustic requirements in the bleach plant are typically not large enough to achieve both a closed chemical system and a high level of potassium removal. The system would either have to be supplemented with additional chemical that would have to be sewered, or operated at poor potassium removal efficiencies.

There remains a need for an improved method for removing chloride and potassium from pulping chemicals. Summary of the Invention

According to one aspect of the invention, there is provided a method of reducing chloride and potassium ion concentrations in a pulp mill chemical recovery cycle, in which electrostatic precipitator ash is produced in a recovery boiler. At least a portion of the ash is mixed with water to form a mixture that is treated in a chloride and potassium removal system to separate chloride and potassium from sodium sulfate and sodium carbonate based on the solubility differences between potassium chloride and sodium sulfate, producing a first process stream enriched in chloride and potassium and a second process stream having reduced chloride and potassium. Optionally, the first stream is adjusted in temperature and/or diluted with water, if required. At least a portion of this first stream is treated in an amphoteric resin bed to remove chloride, producing a third process stream having reduced chloride. This third stream is treated in a cation exchange resin bed to remove potassium, producing a fourth process stream having reduced potassium. The amphoteric resin is regenerated by flowing a fluid comprising water through the resin bed and producing a fifth process stream enriched in chloride. The cation exchange resin is regenerated by flowing a fluid through the cation exchange resin bed and producing a sixth process stream enriched in potassium. In some aspects, the positions of the amphoteric resin bed and the cation exchange resin bed are reversed, such that the first stream is treated first in the cation exchange resin bed.

According to another aspect of the invention, there is provided an apparatus for reducing chloride and potassium ion concentration in a pulp mill recovery cycle. The apparatus has means for mixing electrostatic precipitator ash with water to produce a mixture and a chloride and potassium removal system for receiving the mixture and separating chloride and potassium from sodium sulfate and sodium carbonate, based on solubility differences between sodium chloride and sodium sulfate, to produce a first process stream enriched in chloride and potassium and a second process stream having reduced chloride and potassium. The apparatus includes an amphoteric resin bed for receiving the first process stream and removing chloride to produce a third process stream having reduced chloride. There is a cation exchange resin bed for receiving the third process stream and removing potassium, to produce a fourth process stream having reduced potassium. The apparatus includes means for regenerating the amphoteric resin and producing a fifth process stream enriched in chloride, and means for regenerating the cation exchange resin and producing a sixth process stream enriched in potassium. In some other aspects, the positions of the amphoteric resin bed and the cation exchange resin bed are reversed.

Further aspects of the invention and features of specific embodiments are described below.

Brief Description of the Drawings

Figure 1 is a schematic diagram of an embodiment of the process and apparatus of the invention.

Figure 2 is a schematic diagram of a leaching system according to Example 1.

Figure 3 is a schematic diagram of an ion exchange system according to Example 1.

Figure 4 is a schematic diagram of the process of the invention, according to Example 1. Figure 5 is a schematic diagram of a further embodiment of the process and apparatus of the invention, in which the positions of the amphoteric resin bed and the cation exchange resin bed are reversed.

Detailed Description of the Invention Referring to Figure 1 , the ash (catch) 10 from the electrostatic precipitator of a recovery boiler is first treated in a primary separation system 15 of a type that separates potassium chloride from sodium sulfate based on solubility differences between these salts. In the illustrated embodiment, the separation system 15 is a leaching system comprising a leaching tank 14 and a centrifuge 18. The precipitator ash 10 is mixed with water 12 in the leaching tank to form an ash slurry 16. The slurry 16 passes to the centrifuge 18, in which the solids, primarily sulfate, is separated from the leachate, rich in chloride and potassium.

In other embodiments, the primary separation system may comprise an evaporation-crystallization system, or a freeze crystallization system. Such systems and leaching systems, and their modes of operation, are well known in the art.

The primary separation system 15 produces a first process stream 20 enriched in chloride and potassium, and a second process stream 22 having reduced chloride and potassium, enriched in sodium sulfate. Optionally, a portion of the first process stream 20 may be recycled (stream 27) as make-up water to minimize chemical losses of the leaching system and reduce the size of the ion- exchange equipment required. The second process stream 22 may optionally be returned to the recovery cycle (stream 23) by dissolving in black or green liquor, or optionally be further processed to achieve greater removal of chloride and potassium (stream 25), for example by dissolving the stream in water and treating it in by ion exchange. The first process stream 20, enriched in chloride and potassium, is adjusted in temperature and/or diluted with water 24, if required, to ensure the stream is within allowable operating temperatures of the ion exchange resin and below the solubility limit. The solubility limit is the concentration at which salts begin precipitating. If temperature adjustment is required, this is done by passing the first process stream 20 through a heat exchanger 29. This first process stream, which may be a diluted and/or temperature-adjusted stream 26, is passed to an amphoteric resin bed 28 for treatment to selectively remove chloride. An example of a suitable amphoteric resin is marketed by Mitsubishi Chemical Products as the DIAION™ AMP series, which contains a quaternary ammonium group and a carboxyl group incorporated on a cross-linked polystyrene frame.

The process stream 30 exiting the amphoteric resin bed 28 constitutes a third process stream comprising a solution principally of sodium, potassium, sulfate and carbonate. This third process stream 30 is passed to a cation exchange resin bed 32, in which potassium ions are adsorbed by the sodium-based cation exchange resin. This produces a fourth process stream 34 having reduced potassium. An example of a suitable sodium-based cation exchange resin is a strong acid cation resin marketed by Mitsubishi Chemical Products as the DIAION™ SK series, which contains a sulfonic acid group in a cross-linked styrene frame. The fourth process stream 34, enriched in sodium sulfate and sodium carbonate, is optionally sent to the evaporator set of the chemical recovery cycle (stream 36); or optionally it may be returned to the primary separation system 15 for the purpose of improving the overall removal of chloride and potassium (stream 38). The resin in the amphoteric resin bed 28 is regenerated by flowing water

(stream 40) through the bed. This produces a fifth process stream 42 enriched in chloride. Amphoteric resin has a slight preference to remove sodium relative to potassium, so the stream 42 is enriched in both sodium and chloride. This may be sewered (stream 43).

The resin in the cation exchange resin bed 32 is regenerated by a sodium-based regenerant. This elutes the potassium from the bed in a sixth process stream 46, 48. The regenerant may comprise one or more of a sodium chloride brine solution (stream 44), a sodium hydroxide solution (stream 50), and the effluent stream 42 from regeneration of the amphoteric resin bed 28, with water (stream 49).

In bleached pulp mills, where a source of make-up caustic is required in the bleach plant, the amphoteric resin effluent stream 42 and/or sodium hydroxide 50 are preferentially used to regenerate the cation exchange resin, with the sodium and potassium hydroxide regenerant stream 48 used subsequently in the bleach plant. If greater potassium removal is required, sodium chloride (stream 44) may also be used as an additional regenerant chemical. In unbleached pulp mills, where there is not a need for a sodium and potassium hydroxide regenerant stream, the amphoteric resin effluent stream 42 and/or sodium chloride (stream 44) are preferentially used to regenerate the resin. The effluent stream 46 may be sewered.

In a further embodiment of the invention, the positions of the amphoteric resin bed and the cation exchange resin bed are reversed. Referring to Figure 5, the system is the same as discussed above in respect of Figure 1 up to the point of treatment of the first process stream 20. In the embodiment of Figure 5, this first process stream 20, which may be a diluted and/or temperature-adjusted stream 26, is passed to the cation exchange resin bed 32, in which potassium ions are adsorbed by the sodium-based cation exchange resin. This produces a third process stream 52 having reduced potassium. This third process stream 52 is passed to the amphoteric resin bed 28 for treatment to selectively remove chloride. The process stream 54 exiting the amphoteric resin bed constitutes a fourth process stream, enriched in sodium sulfate and sodium carbonate. It is optionally sent to the evaporator set of the chemical recovery cycle (stream 56); or it may optionally be returned to the primary separation system 15 for the purpose of improving the overall removal of chloride and potassium (stream 38). The regeneration of the resins in the amphoteric resin bed 28 and the cation exchange resin bed 32, and the use or disposition of the effluents, are done in the same manner as described above for the embodiment of Figure 1.

Examples The following Examples compare the results of treatment of ESP ash by the process of the invention versus treatment by leaching alone and by ion exchange alone, with respect to chloride and potassium removal and the loss of sulfate and carbonate.

Example 1 A representative ESP ash composition typical for modern pulp mills is the following: potassium - 2 wt%; chloride - 2 wt%; sodium - 35 wt%; carbonate - 20 wt%; sulfate - 41 wt%.

When such ash is treated by means of a leaching system to reduce the chloride and potassium levels, the performance suffers due to the high carbonate levels of the ash. Figure 2 shows the results of the treatment of 200 tons/day of ash by a leaching system. The treatment removes 65 wt% of the potassium, 65 wt% of the chloride, and resulted in a loss of 25 wt% of sulfate and 40 wt% loss of carbonate. This would be the expected performance of a single stage leaching system treating high carbonate ash, based on known data from operating plants. Considering next the treatment of the ESP ash directly by chloride and potassium ion exchange equipment, with no leaching or other primary separation system, the ash is dissolved in water in a dissolving tank 52, and the ash slurry 54 is passed to an amphoteric resin bed 28. The effluent from that bed passes to a cation exchange resin bed 32 (Figure 3). Where the demand for caustic in the bleach plant is 30 tons/day NaOH, the caustic used for

regeneration of the cation exchange resin is not sufficient to remove a substantial amount of potassium. Performance of the amphoteric resin is based on known data from operating plants, while an equilibrium-based calculation routine was used to calculate the potassium removal efficiency. The results of the treatment are shown in the process flow diagram of Figure 3. If additional potassium removal were desired, additional brine or other chemicals would be required, which would have an associated cost and environmental impact.

In the present invention, the ion exchange system is used to treat the leach effluent rather than to treat the ash directly. With the same demand for caustic in the bleach plant (30 tons/day), a marked improvement is seen in the overall potassium removal. Again, an equilibrium-based calculation routine was used to calculation potassium removal efficiency. The calculation routine determined what would be expected, that a marked improvement in removal is seen when the potassium to sodium ratio in the feed increases. In this case, the overall removal of potassium and chloride is close to that of the initial leaching system, but the chemical losses are greatly reduced. The results of the treatment are shown in the process flow diagram of Figure 4.

A comparison between the three systems is shown in the Table 1 below: Table 1

For all configurations, the contaminant removal and chemical losses were calculated using the equation:

Removal (Losses) = (Quantity leaving in kidney streams) / (Quantity in feed ash) x 100%

For the present invention, chloride removal would be:

Chloride Removal = (Chloride in Amphoteric Resin Effluent + Chloride in Bleach Plant Caustic) / (Chloride in Ash) x 100% = (2.34 + 0) / 4 x 100% = 59%

Potassium removal would be:

Potassium Removal = (Potassium in Amphoteric Resin Effluent + Potassium in Bleach Plant Caustic) / (Potassium in Ash) x 100% = (0.3 + 1.9) / 4 x 100% = 55%

Sulfate losses would be:

Sulfate Losses = (Sulfate in Amphoteric Resin Effluent + Sulfate in Bleach Plant Caustic) / (Sulfate in Ash) x 100% = (2.0 + 0.9) / 81.9 x 100% = 4%

Carbonate losses would be:

Carbonate Losses = (Carbonate in Amphoteric Resin Effluent + Carbonate in Bleach Plant Caustic) / (Carbonate in Ash) x 100% = (1.6 + 0.7) / 40 x 100% - 6% The chemical losses from the use of the present invention are lower than those of either leaching alone or ion exchange alone. The potassium removal by the present invention is greater than that by the ion exchange alone due to the enrichment of the potassium relative to sodium that occurs in the primary leach treatment. Importantly, the size of the ion exchange equipment required when treating the leach waste is much smaller than when ion exchange equipment is used to treat the entire ash stream.

Example 2

A second representative ESP ash composition typical for modern pulp mills is the following: potassium - 2 wt%; chloride - 2 wt%; sodium - 32 wt%;

carbonate - 5 wt%; sulfate - 59 wt%.

When such ash is treated by means of a leaching system, higher performance is achieved due to the lower carbonate levels in the ash. The performance would be equivalent to the performance that could be achieved in the leach system of Example 1 if the ash were treated with sulfuric acid to neutralize the carbonate. This example again assumes there is only 30 t/d demand for NaOH in the bleach plant.

A comparison between the three systems, leaching alone, ion exchange alone, and leaching followed by ion exchange, to treat the second ash composition is shown in Table 2 below:

Table 2

Again, the chemical losses of the present invention are the lowest when compared to leaching alone and to ion exchange alone. The potassium removal of the present invention is greater than removal by the ion exchange alone due to the enrichment of the potassium relative to sodium that occurs in the primary leach treatment.

Throughout the foregoing description and the drawings, in which corresponding and like parts are identified by the same reference characters, specific details have been set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described to avoid unnecessarily obscuring the disclosure, e.g. various conduits, valves, pumps, controllers, etc. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense. As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the scope thereof. Accordingly, the scope of the invention is to be constructed in accordance with the following claims.