METHODS TO REDUCE METALS CONTENT OF BLEACHED PULP WHILE REDUCING BLEACHING COST IN A CHEMICAL PULPING PROCESS BACKGROUND

The majority of corrugated boxes, paper grocery bags, fine papers, and market pulps are produced by a sulfate pulping process known as "Kraft" pulping. In the typical Kraft process, wood chips are added to an aqueous medium. In general, the liquor in which the wood chips are cooked comprises a mixture of black and white liquor, the black liquor being liquor added back to the cooking vessel, or digester, from a prior batch of wood chips and the white liquor being a freshly prepared alkaline solution as described below. Black liquor varies considerably among different mills depending on the white liquor used, the wood employed, and the method of cooking. Typical white liquor is a solution of sodium hydroxide, sodium carbonate, sodium sulfate, sodium sulfide and various inorganic materials. White liquor solubilizes the pulp and removes the lignin from the wood fibers as described below.

The largest part of the organic matter removed from the wood during cooking is combined chemically with sodium hydroxide in the form of sodium salts. Some of these compounds are resin soaps which account for the intense foaming properties of black liquor. In addition, organic sulfur compounds and mercaptans, which give the characteristic odor to the sulfate-containing black liquor, and small amounts of sodium sulfate, silica and other impurities such as lime, oxide, alumina, potash, and sodium chloride can be present in the black liquor.

Competing reactions are also in play. Calcium in the cooking liquor and in the wood (normally bound to the cellulose, but released upon contact with the alkali) form sticky precipitates with fatty and resin acids, swelling to block flow channels. Excess calcium can form precipitates with lignin, and hemicellulose among others. Such precipitates can present many difficulties in later stages. In high heat transfer areas, calcium cations form tenacious scales, reducing flow and heat transfer. In addition to calcium, certain other metals can catalyze the hydrolysis of wood sugars, hemicellulose, and cellulose, and can interfere in certain oxidation/reduction reactions. Moreover, aluminum, calcium, barium, magnesium, and transition metals (especially manganese, copper, and iron) can interfere with bleaching as well as other processes. The reaction conditions present during the cook, or digestion, cause lignin, the amorphous polymeric binder found in wood fibers, to be hydrolyzed. Ideally, wood chips are digested only long enough to dissolve sufficient lignin to free the cellulosic wood fibers but maintain sufficient lignin intact to provide added strength to the paper. The pulping process attempts to maximize pulp yield, which is defined as the dry weight of pulp produced per unit dry weight of wood consumed.

As the pulping process continues, the rate of cellulose dissolution increases to the point where it exceeds the rate of lignin dissolution. As a result, the pulping process must be stopped and more compounds that more selectively dissolve lignin must be added. This is termed "bleaching," and results in a whiter, brighter paper. Bleaching typically involves contacting the pulp with an oxidizer, such as a chlorine compound, for example, chlorine dioxide, or with an oxygen compound, such as ozone, oxygen, peroxide, or the like. The effectiveness of bleaching is highly pH dependent. The respective pH levels can be adjusted downwards as necessary by adding thereto pH adjusters such as acids or materials that will form acids in aqueous solution, such as sulfur dioxide, sulfuric acid, hydrochloric acid, or the like.

However, such pH adjusters can form insoluble sulfates, such as calcium sulfate, barium sulfate, and the like which are insoluble and unwashable. The metals and other impurities tend to remain associated with the fiber and are not washed out of the stock so that as the pH changes through the remaining processing, they tend to form scales on washer facewires, piping, and other associated equipment. Furthermore, such pH adjusters can also convert transition metals such as iron and manganese to their highly colored sulfate form, resulting in brightness reversion and the need for additional bleaching to compensate for such brightness reversion. The costs associated with removing scaling and maintaining bleaching can be significant.

As such, a need exists for a method whereby metals and other impurities can be removed from pulp. It would be particularly beneficial if such a method could reduce brightness reversion in bleached pulps caused by oxidation of certain metals to their highly colored states. It would also be desirable if such method could minimize the amount of oxidizer utilized in a bleaching process. SUMMARY

In accordance with certain embodiments of the present disclosure, a method of decreasing the amount of oxidizer required by a pulping or papermaking process is provided. The method comprises adding to a process stream or solution of the pulping or papermaking process an effective amount of urea hydrochloride to reduce the amount of oxidizer required by the pulping or papermaking process.

In still other aspects of the present disclosure, a method of reducing one or more metals, impurities, or combinations thereof from a pulping or papermaking process is provided. The method comprises adding to a process stream or solution containing pulp, metals, impurities, or combinations thereof of the pulping or papermaking process an effective amount of urea hydrochloride to reduce metals, impurities, or combinations thereof from the pulping or papermaking process.

Other features and aspects of the present disclosure are discussed in greater detail below.

DETAILED DESCRIPTION

Reference now will be made in detail to various embodiments of the disclosure, one or more examples of which are set forth below. Each example is provided by way of explanation of the disclosure, not limitation of the disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the disclosure. For instance, features illustrated or described as part of one embodiment, can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present disclosure covers such modifications and variations as come within the scope of the appended claims and their equivalents.

The present disclosure is directed to methods to reduce scaling and bleaching cost in a chemical pulping process. The present disclosure further describes a method to remove metals and other impurities from the bleached pulp. The methods include utilizing urea hydrochloride in place of and/or in addition to certain pH adjusters in a pulping process.

The use of urea hydrochloride in paper and pulp processing has been previously described in U.S. Patent No. 7,029,553 to Williams et al., which is incorporated by reference herein. Williams et al. describes that urea hydrochloride can be used to adjust the pH of a process stream and reduce the amount of sulfuric acid used in a papermaking process. Williams et al. also describes that urea hydrochloride forms calcium chloride when it comes into contact with solutions to which calcium hydroxide has been added.

However, Williams et al. fails to teach or suggest the removal of metals and/or other impurities from a pulping process. In addition, Williams et al. fails to recognize that a reduction in oxidizer can be realized by the use of urea hydrochloride. The present disclosure overcomes the shortcomings of the prior art in that the methods disclosed herein result in lower processing costs by reducing scaling and bleaching costs. Specifically, the present disclosure recognizes that urea hydrochloride can be utilized to remove metals and other impurities from pulp. The use of urea hydrochloride can also reduce brightness reversion in bleached pulps caused by oxidation of certain metals to their highly colored states. Furthermore, urea hydrochloride can minimize the amount of oxidizer used for bleaching.

In accordance with the present disclosure, as digestion proceeds, certain metals and/or other impurities are prevented from adhering to process equipment as scale.

Sealants such as calcium carbonate, calcium sulfate, calcium phosphate, calcium oxalate, barium sulfate, and the like, are controlled. Also, other metals are controlled, preventing them from interfering with oxidation/reduction reactions and from catalyzing the hydrolysis of sugars, hemicelluloses, and cellulose. Such metals can be found in the ash of wood chips in sufficient quantity to cause the abovementioned problems.

The methods of the present disclosure utilize urea hydrochloride in place of and/or in addition to certain pH adjusters in a pulping process, particularly a bleaching process. Such pH adjusters typically include acids or materials that will form acids in aqueous solution, such as sulfur dioxide, sulfuric acid, hydrochloric acid, or the like. However, urea hydrochloride can be used in addition to or as a substitution for any suitable pH adjuster as would be known in the art.

The urea hydrochloride used in accordance with the present disclosure can be formed from any desired ratio of urea and hydrochloric acid that performs the desired function of metal removal. Urea hydrochloride suitable for use in the present invention can be prepared by mixing urea with hydrochloric acid at the desired ratio. A suitable method for preparing a 1 :1 molar ratio urea hydrochloride salt is described in Example 1 of U.S. Pat. No. 5,672,279, the entire contents of which are hereby incorporated by reference. Suitable urea hydro chloride compositions are also commercially available. One such composition is NOVOC A-Cl (Peach State Labs, Inc., Rome, Ga.), which is a 1 :1 molar ratio aqueous urea sulfate solution containing 0.25% of a quaternary amine corrosion inhibitor.

The methods of the present disclosure are described as being used in a Kraft pulping process. The present disclosure, however, is not to be so limited. Any of the various equivalent wood cooking processes having the production of paper as its ultimate goal may also be employed. However, the Kraft process is described in more detail as follows.

Initially, suitable trees are harvested, debarked and then chipped into suitable size flakes or chips. The wood chips that can be processed into pulp using the chemical pulping process of the present disclosure can be either hardwoods, softwoods or mixtures thereof. Such wood chips are sorted with the small and the large chips being removed. The remaining suitable wood chips are then moved to a digester. The digester is a vessel for holding the chips and a digesting composition.

In a batch type digester, wood chips and a mixture of "black liquor," the spent liquor from a previous digester cook, and "white liquor," typically a solution of sodium hydroxide, sodium carbonate, sodium sulfate, sodium sulfide and various inorganic materials are pumped into the digester. In the cooking process, lignin, which binds the wood fiber together, is dissolved in the white liquor forming pulp and black liquor.

The digester is sealed and the digester composition is heated to a suitable cook temperature under high pressure. After an allotted cooking time at a particular temperature and pressure in the digester, the digester contents (pulp and black liquor) are transferred to a holding tank. The pulp in the holding tank is transferred to the brown stock washers while the liquid (black liquor formed in the digester) is sent to the black liquor recovery area. The black liquor is evaporated to a high solids content in evaporators. The Kraft cook is highly alkaline, usually having a pH of 10 to 14, more particularly 12 to 14.

A Kappa number corresponds directly to the amount of lignin remaining in the pulp. Generally, the higher the Kappa number, the more lignin present in the pulp and, therefore, the higher the pulp yield. The Kappa number generally decreases as the digestion time is increased or the alkalinity of the cooking liquor is increased. The goal in such Kraft papermaking processes is to retain as much lignin as possible in order to enhance strength and to reduce the cost, while maintaining the uniformity of the cook. More uniform cooks result in a decreased percentage of rejects and, thereby, reduce costs for running paper mills.

Cooking, or digestion, of the pulp may be terminated when the amount of rejects in the pulp is reduced to an acceptable level. Substantial yield and quality advantages are achieved if the wood chips are cooked to a higher lignin content. As a result, an increase in a Kappa number target by the use of thinner chips can result in a substantial cost savings. However, the thickness of chips obtainable on a commercial scale is always variable, A major portion of the total rejects frequently originate from a relatively small fraction of the chips having the greatest thickness. The objective in every pulping process is to achieve a lower percentage of rej ects.

After one or more washing steps, the pulp is subjected to bleaching treatments. Bleaching results in a whiter, brighter paper. Bleaching typically involves contacting the pulp with an oxidizer, such as a chlorine compound, for example, chlorine dioxide, or with an oxygen compound, such as ozone, oxygen, hydrogen peroxide, or the like. The effectiveness of bleaching is highly pH dependent. The respective pH levels can be adjusted downwards as necessary by adding thereto pH adjusters as described herein.

Since most mills have abandoned bleaching with elemental chlorine, the simplest bleach plants contemplate three stages, most typically chlorine dioxide operating at about pH 2 to about pH4 for one hour retention time, with washing, followed by caustic extraction where the liquid contains sodium hydroxide, hydrogen peroxide or oxygen, and another washing stage, completed by a second chlorine dioxide stage with washing.

Shorthand for this configuration is D-Ep-D. Others will add another Ep-D, to form D-Ep- D-Ep-D. There are many other configurations possible, including acidification and enzyme treatments, and the methods of the present disclosure have application in all of them.

The present inventor has determined that pH adjusters, such as sulfuric acid, convert calcium, barium, and other metals present in the pulp to a sulfate form which is insoluble and unwashable. The pH adjusters also convert iron and manganese to their highly colored sulfate form. These materials tend to remain associated with the fiber and are not washed out of the stock, so as pH changes through the remaining process steps, calcium carbonate, calcium sulfate, barium sulfate, and iron and manganese oxides all form scales on washer face wires and in piping or associated equipment. Iron, manganese, copper, and the like, all catalyze the decomposition of the oxidizers and cellulose in the presence of oxygen. In accordance with the present disclosure, urea hydrochloride is utilized in place of and/or in addition to the pH adjuster and can remove a significant amount of these metals as well as other impurities, such as silica. Such removal reduces the material available for scaling, or that interfere with bleaching. The method of the present disclosure is measurable, consistent, and results in pulp with significantly less metal and/or impurity contaminant. For instance, in certain embodiments, between about 5% and 75% of one or more of metals or other impurities are removed. In certain embodiments, between about 25% and 70% of one or more of metals or other impurities are removed while in certain embodiments, between about 50% and 60% of one or more of metals or other impurities are removed. The resulting pulp has a metals content of from about 30 mg/kg to about 200 mg/kg.

It is assumed that reactions with the chloride ion are stoichiometric, where 2 moles of chloride can react with one mole of the divalent metal to form a soluble salt. However, the laboratory example described herein in Table 1 describes that about 200 ppm of urea hydrochloride removes greater than 200 ppm of calcium, magnesium, manganese, iron, and aluminum with an approximate ratio of 1 :1. In addition, about 1200 ppm of sodium is removed, compared with about 600 ppm with H ₂ SO ₄ .

Metals can include aluminum, calcium, barium, magnesium, potassium, sodium, and zinc as well as transition metals (especially manganese, copper, and iron transition metals) or combinations thereof. Impurities can include one or more of sodium sulfate, silica, lime, oxide, alumina, potash, sodium chloride or combinations thereof.

An effective amount of urea hydrochloride is employed in a bleaching step of a chemical pulping process to improve the efficiencies of the chemical pulping processes.

The effective amount depends on the particular urea hydrochloride employed and other factors including, but not limited to, wood type, the digester composition, the operating conditions of the digester, the mode of addition of the compounds including any additional compounds added, as well as other factors and conditions known to those of ordinary skill in the art.

For instance, in certain aspects of the present disclosure, about 1-4 lbs. of UC1 per ton of pulp provides the maximum removal of metals. In certain embodiments of the present disclosure, about 1.5 lbs. per ton of UCL was utilized in only one of the C1O ₂ stages. In the interest of keeping total chemical cost as low as possible, pH control can be managed by use of sulfuric acid, as is the current practice. Addition of the UC1 displaces sulfuric acid within the existing control schemes.

As a result of the urea hydrochloride being added in a bleaching step, the amount of oxidizer utilized in the process can be reduced when compared to a traditional chemical pulping process that does not utilize such urea hydrochloride. The amount of oxidizer can be reduced by about 10% when compared to traditional processes. In certain embodiments, the amount of oxidizer can be reduced by about 15% and in still other embodiments, the amount of oxidizer can be reduced by about 20%. In addition, brightness can improve from about 88% to about 90% when compared to traditional processes.

Hydrochloric acid is known to react with chlorate (CIO ₃ ) to form chlorine dioxide (C1O ₂ ) in acidic environments. Chlorate is normally present in the bleach stage filtrate, so this is presumed to be responsible for some of the immediate reduction in bleaching chemical requirements. For instance, the amount of oxidizer added is from about 35 to 60 lbs. per ton C1O ₂ , and about 15 to 20 lbs. per ton H ₂ O ₂ . As described above, oxidizer can include chlorine compound, for example, chlorine dioxide, or with an oxygen compound, such as ozone, oxygen, hydrogen peroxide, or the like. The effectiveness of bleaching is highly pH dependent.

In some embodiments, other additives can be added to the alkaline aqueous mixture in the extraction stages. Typical additives include, but are not limited to, conventional additives known for use in a chemical pulping process.

In some embodiments of the present disclosure, the methods of the present disclosure can reduce the formation of scaling in the pulp washers, bleach plants, and evaporators. The methods of the present disclosure can remove metals and/or other impurities, thereby improving the bleach chemical efficiency. The methods of the present disclosure can also reduce brightness reversion in bleached pulps caused by oxidation of certain metals to their highly colored states.

The following examples are meant to illustrate the disclosure described herein and are not intended to limit the scope of this disclosure.

EXAMPLES

Example 1 :

Laboratory evaluation using semi-bleached pulp (one stage with C1O ₂ , one alkaline extraction stage Eop was washed with one liter of deionized water; alkaline extraction stage assisted by oxygen and peroxide, an Eop stage). The others were washed with either sulfuric or urea hydrochloride as noted. Samples were analyzed by ICP Spectrometer.

In the present study, there is a reduction in aluminum, barium, copper, iron, and manganese with 3 pH UC1 versus 2 pH H2SO4. The sulfate compounds are substantive to cellulose fiber at low pH (aluminum sulfate or alum is used as a sizing and retention aid in papermaking). As pH drops to 2 with the UCL, calcium continues to reduce, The drop in silica was unexpected, but is likely to be important in making low-ash pulps. The removal of the transition metals was also unexpected, but led to removal of those metals from the pulp to reduce bleaching costs in peroxide stages, and to reduce decomposition of CIO2 in later bleaching stages.

Example 2:

UC1 was introduced into the first chlorine dioxide bleaching stage, along with the sulfuric acid normally used for bleaching pH chemical control, and chlorine dioxide. This was a mill system utilizing a bleaching sequence of 3 chlorine dioxide stages, with two Extraction/Peroxide stages in between. The test was only 24 hours duration, so the system had not stabilized in the later stages. Also there was an upset in the brown stock washer system that resulted in a 15% higher conductivity.

Paper mills use the conductivity measurement to indicate the additional load of dissolved but unwashed lignin coming into the system, This would normally result in increased C1O ₂ demand in the first stage, but in accordance with the methods of the present disclosure, first stage demand actually dropped by 5% (17.8 lbs/ton to 16.9).

Final pulp brightness was improved, and would have allowed further chemical cuts. Insofar as pH control, 1.5 lbs per ton of the UCL displaced 0.7 lbs per ton of H2SO4 despite the increased brownstock conductivity (5.3 lbs/ton to 4.6). Brightness improved from 87.8 to 89.2. This brightness measurement is the most common in the United States and is the one described in TAPPI T 452 Brightness of Pulp, Paper and Paperboard (Directional Reflectance at 457 nm), incorporated by reference herein,

Finally, peroxide usage dropped 4.4% (18.2 lbs/ton to 17,4),

The laboratory work is most representative of the final washing stage prior to bleaching. TABLE II illustrates that C1O ₂ , H ₂ O ₂ , and H ₂ SO ₄ were reduced by 2.2 LBS/ton, .8 LBS/ton, and .7 LBS/ton, respectively with the addition of urea hydrochloride. The relative costs on these chemicals are approximately CIO ₂ ($0.40/LB), H ₂ O ₂ ($0.44) and H ₂ SO ₄ ($0.37). Using these metrics a cost reduction of $ 1.49/bleached ton of pulp is calculated.

TABLE II further shows that bleaching costs were lower and brightness was higher using urea hydrochloride at kappa numbers that are the same or relatively close, The C1O ₂ /kappa and the ClO ₂ /brightness point are all favorable.

Further, when urea hydrochloride was added the C1O ₂ was cut in response to the brightness on the front end increasing. This occurred even as the tonnage rate was going up. The C1O ₂ leveled out and then went up very quickly when the urea hydrochloride was taken out of the system.

Example 3:

Table III lists results from two trials of a mill demonstration. All "best practices" were used to remove metals, and then a portion of the sulfuric acid was replaced with urea hydrochloride. Best practices included increasing washing, increasing the quantity of sewered water, and reducing pH on the pulp dryer. Final pH was controlled to 2.8 in all cases by adjusting sulfuric acid feed. Calcium content is reduced by about 50-60%, as are the transition metals, sodium, and silica.

Transition metals are known to speed the decomposition of bleaching chemicals like chlorine dioxide and hydrogen peroxide. Mills often use mixtures of chelants (EDTA, DTPA, and the like) to control these materials. Others use mixtures including magnesium to form a floe with iron and manganese. In another short evaluation in a mill pulping hardwoods, 1.5 lbs per ton urea hydrochloride was used to replace a portion of sulfuric acid. pH was controlled at 2.8 with the sulfuric acid. Chlorine dioxide usage was reduced by 17% and pulp was brighter at comparable incoming kappa numbers, indicating that significant savings could be realized. Sulfuric acid was reduced as was hydrogen peroxide.

In the interests of brevity and conciseness, any ranges of values set forth in this specification are to be construed as written description support for claims reciting any subranges having endpoints which are whole number values within the specified range in question. By way of a hypothetical illustrative example, a disclosure in this specification of a range of 1-5 shall be considered to support claims to any of the following sub-ranges; 1- 4; 1-3; 1-2; 2-5; 2-4; 2-3; 3-5; 3-4; and 4-5.

These and other modifications and variations to the present disclosure can be practiced by those of ordinary skill in the art, without departing from the spirit and scope of the present disclosure, which is more particularly set forth in the appended claims. In addition, it should be understood that aspects of the various embodiments can be interchanged both in whole or in part. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the disclosure.