Definitions

• the present invention relates to a process for preparing alumina trihydrate having significantly greater whiteness and brightness than commonly available alumina trihydrate. More particularly, the present invention relates to a process for increasing the G. E. brightness and ASTM whiteness level of alumina trihydrate to levels of at least about 98% and 99%, respectively, and with a thin, platelet structure suitable for use as a paper filler or coating material for whitening and brightening purposes in a papermaking process.
• the solution is sometimes referred to as a "Bayer process liquor,” and the alumina serves as a purifying agent to remove impurities from the liquor so that alumina trihydrates that are precipitated from the purified solution have improved whiteness as compared with precipitates from untreated liquor, from carbonaceous filter treated liquor, and from alpha-methyl cellulose treated liquor.
• the product resulting from the disclosed process had improved whiteness over pre-existing approaches, even higher whiteness levels are desirable.
• a patent that discloses a process for preparing an alumina monohydrate for use as a pigment or filler in papermaking processes is U.S. Patent No. 4,946,666, which issued on August 7, 1990, to Neil Brown.
• the product of that process is a flat, crystalline boehmite, the crystals having an hexagonal shape and a particle size of between 0.2 and 0.8 microns.
• the process includes an oxidation step that requires autoclaving of the Bayer liquor under vigorous agitation to induce nucleation of crystalline boehmite upon cooling.
• an improved process for significantly increasing the brightness and whiteness of alumina trihydrate.
• the process includes rapidly flash activating an initial white alumina trihydrate and reducing the particle size of the activated material to less than about 6 ⁇ m to provide an activated alumina seed capable of being rehydrated to an alumina trihydrate.
• the activated alumina seed is rehydrated by admixing with a purified sodium aluminate process liquor having a ratio of aluminum oxide to caustic soda (expressed as equivalent sodium carbonate) of about 0.5 and is agitated for about 48 hours at ambient temperature in a vessel to form a slurry.
• the solids are subsequently separated from the liquid component of the slurry, and the solids are washed and dried to provide the resulting ultra-white alumina hydrate product having a G.E. brightness value of at least about 98%.
• an alumina trihydrate product prepared in accordance with the process is provided as an additive for imparting whiteness.
• an alumina trihydrate product prepared in accordance with the process is a component of a pigment mixture of titanium dioxide and the alumina trihydrate product.
• Figure 1 is a flow chart showing the process steps for one process for the dissolution of alumina trihydrate in caustic to 0.6 A/C, along with subsequent filtration, for making an alumina trihydrate starting material that is suitable for use in the present invention.
• Figure 2 is a flow chart showing the process steps for an alumina hydrate activation process for providing an activated alumina seed material capable of being rehydrated to alumina trihydrate for a subsequent precipitation step in accordance with the present invention.
• Figure 3 is a flow chart showing the process steps for an alumina trihydrate precipitation process in accordance with the present invention for obtaining an alumina trihydrate product that has the desired high G.E. brightness and ASTM whiteness properties, as well as the desired particle shape and size distribution.
• Figure 4 is a photomicrograph showing in enlarged form at a magnification of 10,000X the morphology of the product made in accordance with the process of the present invention, wherein the black bar in the upper right hand corner of the Figure represents a length of 1 ⁇ m.
• the present invention utilizes an initial white alumina trihydrate material as a feed stock to be used in a process from which the ultimate, ultra-white alumina trihydrate product is obtained.
• the starting white trihydrate is a relatively coarse material, with a particle size such that as much as 30%-40% of the material is ⁇ 325 mesh (44 ⁇ m) . If the starting alumina trihydrate that is subsequently activated is substantially finer than the particle size specified, having a fineness level of about 9 to 10 ⁇ m, for example, it will not result in an activated Al 2 0 3 product that is capable of being rehydrated to the desirable physical form after flash activation in the manner hereinafter described.
• the desirable form for the final, rehydrated material is very thin platelets, and the crystal phase is preferably gibbsite, or at least predominately gibbsite, along with bayerite or nordstrandite, all of which are the desirable trihydrate form.
• the finer particles will result in a rehydrated product of predominately pseudoboehmite, with very little of the desired trihydrate phases present under any rehydration conditions, and with an unacceptably low brightness value of less than about 95%.
• the starting material has a Hunter "b" value of less than 0.3%, an ASTM whiteness index of greater than 97%, an ASTM yellow index of less than 0.15%, and a G.E. brightness of greater than 97.5%.
• the desired starting alumina trihydrate material can be prepared by a number of different processes, one process that has been found to provide an alumina trihydrate that has the necessary properties for successful use in the process of the present invention is shown in flow chart form in Figure 1.
• the process illustrated in Figure 1 utilizes as one of the starting materials a sodium aluminate liquor having an alumina to caustic soda ratio of about 0.7, based upon the sodium carbonate equivalent of the caustic soda present in solution.
• An especially preferred solution for use in the process illustrated in Figure 1 has a caustic content of 168.6 g/kg of solution, expressed as Na 2 C0 3 , a total soda content of 216.8 g/kg, expressed as Na 2 C0 3 , an Al 2 0 3 content of 117.5 g/kg, and a specific gravity of 1.31. Additional information relating to the preparation of alumina hydrates in general can be found in applicant's earlier patent, U.S. Patent No. 3,268,295, which issued on August 23, 1966, the disclosure of which is hereby incorporated by reference to the same extent as if fully rewritten herein.
• the starting trihydrate produced in accordance with the process illustrated in Figure 1 is flash activated at a temperature of about 900°F for about 2 seconds or less in the process shown in Figure 2. It is essential that a white trihydrate be employed as the feed material for activation, because although normal Bayer trihydrate that is activated and rehydrated in accordance with the method steps disclosed herein will produce the desired thin, flat, platelet structure in the final product, the brightness and whiteness of that final product will not conform with the desired level for a starting alumina hydrate material in accordance with the present invention, and it will, therefore, not be acceptable.
• the flash activated material is ground to a particle size of about 4 to 6 ⁇ m.
• a ceramic lined, fluid energy mill has been found to provide the desired particle size, although an impact mill may also be used.
• an impact mill will probably be more costly to operate and will require greater maintenance because of the abrasive qualities of the activated alumina.
• the particle size of the activated alumina that is to be rehydrated is important because it influences both the particle size of the final product as well as its morphology.
• a relatively coarse activated material 80% of the particles having a particle size greater than 44 ⁇ m, when rehydrated in NaOH will provide a rehydrated product having an LOI of 30% to 31%, which will be predominately gibbsite, will have about a 5 ⁇ m average particle diameter, and a G.E. brightness of about 98%.
• the morphology of the product will be in the form of long, thin rods having an hexagonal cross section and with an L/D ratio of about 2 to 5.
• Such a material could be ground to a desired particle size of 1 ⁇ m in a wet attrition mill, which would enhance the brightness level to between 98.5% to 98.75%, and which would make the product acceptable for use in a paper pigment, but it also would make the product more expensive because of the need for the attrition mill grinding step.
• the present invention provides size reduction by chemical means. It was discovered that the particle size of rehydrated activated alumina was significantly less than that of the initial, relatively coarse particle size activated material that was introduced into a caustic solution for rehydration.
• the ground, activated alumina is added as a seed material to a sodium aluminate solution contained in a vessel.
• the sodium aluminate solution preferably has an L to P (liquor to precipitation) of 160 g/kg of caustic, 86 g/kg of alumina, and an alumina to caustic ratio of 0.54.
• L to P liquid to precipitation
• Such a solution can be obtained by mixing suitable parts of the green liquor, the recycled spent liquor, and the wash filtrate, to dilute the A/C ratio of the original green liquor.
• the blend can include the following constituents by weight to provide a final L to P liquor having an A/C ratio of 0.54:
• the final liquor is cooled to a temperature of about 25°C, which is an important criterion for the present invention, in that higher temperatures will result in larger particle sizes for the final product. For example, if seeding occurs with the final liquor at about 50°C, the particle size of the final white product will be about two times larger than when the final liquor is at 25°C, and the brightness of the final product will be reduced by about 0.5 to about 0.6 percentage points.
• the solution is maintained at a temperature of about 25°C, and the mixture is subjected to strong agitation for a period of about 48 hours, during which time the activated material is being rehydrated and some A1 2 0 3 -3H 2 0 is being precipitated.
• the resulting slurry has a solids content of about 10% and is filtered, followed by three displacement washes of hot water at a temperature of about 90°C.
• the resulting filter cake is dried at 105°C and provides a product having substantially the following properties:
• the final rehydrated alumina in the form of alumina trihydrate should be approximately equal in amount to the amount precipitated from the liquor as gibbsite.
• the final product is not merely a rehydrated alumina, it is both rehydrated and precipitated.
• a ratio in the final product of rehydrated alumina trihydrate to precipitated alumina trihydrate of from about 0.8 to about 1.2, preferably from about 0.95 to about 1.05, is necessary to result in a final product having good brightness and good platelet formation.
• That ratio is a calculated value based upon the LOI of the final product, A1 2 0 3 from activated material, Al 2 0 3 from precipitated material, and the theoretical LOI of precipitated gibbsite. If the material is outside the range of the ratios given above, the brightness and other optical properties of the final product are diminished, and it is believed that the morpholgy of the final product would be different from the platelet form shown and described herein.
• the mechanism of formation of the platelets in the final product is theorized to be that the platelets grow out of an activated particle as the latter rehydrates, and that the rehydrated particle acts as a seed to permit the platelet growth.
• the platelets are thought to grow to a certain size and then break off the rehydrated particle, and that other platelets then grow from the resulting smaller seed particle as the rehydration process continues. Neither precipitation alone nor rehydration alone will produce the desired final product morphology. Additionally, maintenance of the temperature of the materials undergoing rehydration and precipitation is very important to produce a final product having the desired particle size of less than 1.0 ⁇ m. It is also important that the LOI of the rehydrated phase be at least 30%, for product having an LOI of 27% or less will not provide the desired brightness levels.
• the preferred platelet structure is as shown in enlarged form in Figure 4, wherein the product is in the form of thin, flat, generally hexagonal platelets having a ratio of thickness to diameter (L/D) of substantially less than 1, preferably about 0.2.
• Ultra-white alumina trihydrate in accordance with the present invention can be obtained by carrying out the process described in the following example:
• the starting material for the process for preparing the desired fine, ultra-white alumina trihydrate is a dry, white, alumina trihydrate, having a G.E. brightness of about 97%.
• the starting material can be prepared by dissolving a conventional beige colored, Bayer process alumina trihydrate in caustic to an A/C of about 0.6 in any convenient digestion apparatus.
• the digestion liquor is filtered and is subjected to conventional Bayer precipitation techniques by using previously-prepared white alumina trihydrate as a seed material.
• the precipitated alumina trihydrate and Bayer-type spent liquor are filtered to provide a filter cake containing about 80% solids, which is then washed in two displacement washes and dried to provide a dry, white, alumina trihydrate initial material having a G.E. brightness of about 97%.
• the dry, white alumina trihydrate initial material can be flash activated at 900°C for 2 seconds and the activated product can be dry ground to a particle size of 5 ⁇ m in a fluid energy mill to provide a ground, white, flash activated product to serve as a seed in a subsequent precipitation step.
• the activated product can have an LOI of 6%.
• An L to P liquor can be provided having the following analysis:
• the liquor can be cooled to 25°C and can be deposited in a vessel along with the ground, white, flash activated alumina in the ratio of 1000 kg of liquor to 52 kg of activated alumina.
• the rehydration/precipitation can be carried out at
• the resulting slurry can have a solids content of 13.24% by weight, and can be filtered and washed with three displacement washes of distilled water at 90°C.
• the separated, spent sodium aluminate liquor can be at an A/C of 0.222.
• the filter cake can be dried at 105°C to provide an ultra-white alumina product composed of 50% rehydrated alumina and 50% precipitated alumina, the product having the following analysis:
• the product that results from the process described above can advantageously be employed in papermaking to improve the brightness and whiteness of the resulting paper product, and at a lower cost than would result from using solely Ti0 2 as the pigment material, which is generally considered to be the most effective additive for that purpose.
• commercially available alumina trihydrates have been used in the past as additives for fillers and coatings for paper.
• ultrafine, white alumina trihydrates could be substituted for up to about 15% to 20% of the Ti0 2 pigment normally used in paper filling applications, to thereby serve as a pigment extender.
• such a filler content of the paper can be prepared by adding the Ti0 2 and commercially available alumina trihydrate not having the properties of the product of this invention separately to the thick stock mix tank before the stock enters the suction side of the paper machine's fan pump.
• the final pigment content of the filled paper would have no more than 20% of the Ti0 2 replaced on a two part alumina trihydrate for one part Ti0 2 removed basis, without loss of G.E. brightness and opacity. Therefore, in a paper normally containing 10% by weight Ti0 2 pigment for G.E. brightness and opacity control, up to 2% by weight Ti0 2 could be replaced with 4% by weight of such a white alumina trihydrate to give a total pigment content of 12% by weight of the paper sheet without loss of optical properties.
• the product of the present invention has been found to be a better, more effective extender for Ti0 2 pigment, when considered relative to the optical properties of the resulting paper product, than are conventional, so- called "white trihydrates" that are commercially available.
• white trihydrates For example, in tests conducted on a pilot plant papermaking machine, when using pure compounds, it was found that only 11 lbs. of the product of the present invention per 100 lbs. of paper would produce a G. E. brightness (TAPPI method) of 88%. When the best commercially available white trihydrate was used, it was found that 11.6 lbs. of conventional white trihydrate per 100 lbs. paper, over 5% more, were required to provide the same brightness value. The difference was even more dramatic when a G. E.
• the G. E. brightness level rose to 90%.
• the addition to the base loaded stock of 5 lbs. of the leading commercially available trihydrate per 100 lbs. of paper only resulted in a G. E. brightness of 89%.
• Adding an additional 3.6 lbs. Ti0 2 per 100 lbs. paper to the base loaded stock only produced a G. E. brightness of 89.7%.
• the base loaded stock had an opacity of 91% and it would take 2.1 lbs. more Ti0 2 above the base loaded stock to achieve a 92% opacity. It required 3.5 lbs. of the product of this invention and 4.0 lbs. of the leading commercially available alumina trihydrate to achieve the same 92% opacity in the base loaded stock. Similarly, for a 93% opacity value, 4.2 lbs. more Ti0 2 would be required, and only 7 lbs. of the product of this invention would be necessary. By comparison, 10.2 lbs. of the leading commercially available alumina trihydrate would be required to obtain the same result, thereby showing the improved Ti0 2 extension obtained when using the product of the present invention as opposed to the leading commercially available alumina trihydrate.
• the improved alumina hydrate product in accordance with this invention has been shown to be a better extender for Ti0 2 than commercially available alumina trihydrates when the materials are added at the conventional point in the process (to the thick stock mix tank) , even more significant unexpected results were observed when the product of this invention and Ti0 2 were premixed prior to addition to either the thick stock mix tank or the machine chest upstream of the thick stock mix tank.
• the papermaking machine operating parameters remained as they were for the tests described above, except that 5 lbs. per ton alum were added to the internal size (Hercon 76) to help "set" the size in the paper.
• the Ph varied between 6.9 and 8.9, instead of between 7.9 and 8.1.
• Ti0 2 and the alumina hydrate product of the present invention were slurried together at the desired ratios before addition to the thick stock mix tank, rather than being slurried separately and then simultaneously added independently to the thick stock mix tank, as was done in the previously-described tests.
• the preblending was performed to provide both 15% total solids and 30% total solids (Ti0 2 + alumina trihydrate) in varying ratios of Ti0 2 :Al 2 0 3 .3H 2 0.
• Individual, pre-slurried blends were then added to the paper stock, either at the conventional point in the papermaking process (to the first thick stock mix tank ahead of the suction to the fan pump) , or directly to the beater thick stock in the machine chest, which is before the first thick stock mix tank.
• any diluent such as a pigment
• any diluent when added to paper, plastics, rubber, and the like, will result in a reduction of physical property values to a value less than that for pure cellulose, plastic, etc. , unless it happened to be a reinforcing pigment such as carbon in rubber wherein some physical or chemical bonding occurs. Therefore, it is totally unexpected to discover that the preblended combination of Ti0 2 and the product of this invention exhibits less of a diluent effect.
• Another variant of the process is to preblend the alumina trihydrate and Ti0 2 pigments at the point of manufacture to provide a single dry pigment to the user. Several different blends could be provided in this manner to suit the user's need. The user would need only to slurry a single blended pigment at 30% solids and add to the machine chest as described above.
• the disclosed invention provides a process for making alumina trihydrate having significantly higher G.E. brightness and significantly higher ASTM whiteness levels as compared with alumina trihydrate that is presently commercially available.
• the product can be used in papermaking processes to impart improved whiteness levels to the resulting paper. Additionally, the product can permit a given whiteness level to be achieved with less titanium dioxide pigment, by substituting the product of the present invention for a part of the titanium dioxide pigment normally used to achieve that whiteness level. Because the cost of the alumina trihydrate material is less than that of a corresponding quantity of titanium dioxide pigment, the cost to produce a paper product having a given whiteness level can therefore be reduced.

Landscapes

• Paper (AREA)
• Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)

Applications Claiming Priority (1)

Publications (2)

Family

ID=1340886

Family Applications (1)

Country Status (1)

Citations (1)

Family Cites Families (1)

• 1994
  • 1994-08-23 EP EP94926497A patent/EP0788571A4/de not_active Withdrawn

Patent Citations (1)

Non-Patent Citations (2)

Also Published As

Similar Documents

Legal Events