Abstract:
An improved method of making binderless zeolite sieve extrudates. After extruding a mixture containing about 40-60% zeolites with the remainder being a metakaolin clay that has mixed with it about 90-120% of the stoichiometric amount of sodium hydroxide to convert the clay to a zeolite, the extruded particles are allowed to dry age under dry, bulk conditions in a container during which time autogeneous heat develops and most of the clay is converted to the zeolite. A further heat treatment in a dilute caustic aqueous solution may be used to insure complete crystallization with subsequent washing and calcination to produce an adsorbent product.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The basic invention upon which this improvement is based is the invention disclosed and claimed in the Nozemack, Chi and Schwonke U.S. application Ser. No. 225,075, filed Jan. 14, 1981 herewith. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a process for producing binderless zeolite sieve extrudates by extruding a mixture containing about equal amounts of a zeolite powder, preferably Type A zeolite, a metakaolin clay and a near stoichiometric caustic solution. The clay in the extrudate crystallizes to form a coherent particle that is essentially all zeolite. 
     2. Description of the Prior Art 
     Zeolite sieve formates have been made by mixing zeolite particles such as a 4A zeolite with a binder. Conventional formulations contain about 80% zeolite and 20% clay binder. This mixture is then formed into shapes such as beads or extrudates. The problem with this type of forming method is that 20% of the weight of the product is an inert binder which does not contribute to any absorption. 
     U.S. Pat. No. 3,119,659 to Taggart et al. discloses a process for producing molecular sieve bodies in which a reactive kaolin-type clay is reacted with an alkali metal hydroxide such as sodium hydroxide to form a preformed body which is then reacted with further sodium hydroxide to form a complete zeolite. Taggart et al also illustrate a method where relatively small amounts of a previously formed zeolite are added. However, in the preferred embodiments the Taggart et al. method only adds enough caustic to provide a molar ratio of Na 2  O/SiO 2  in the range of 0.1 to 0.3. Because they do not initially provide enough caustic in the extrudate for complete conversion of the clay, their method requires a post extrusion treatment in which the extrudates are soaked in caustic liquors in both a digestion step and a heated crystallization step. 
     The Taggart et al. method requires significant time for the digestion step as well as for a crystallization step, both conducted in their examples in the presence of a sodium hydroxide solution having a concentration of sodium hydroxide significantly greater than the 3% sodium hydroxide solution used in the present process. Because all of the clay has not reacted before the extrudate is placed in the digestion liquor, there may be some deterioration in structural rigidity of the extrudates when they are placed in the digestion liquid. The method also requires the extrudates to be dried so the moisture content is less than about 30 weight percent before they can undergo the further sodium hydroxide treatment. 
     3. Objects of the Invention 
     It is an object of this invention to produce formed zeolite molecular sieves which contain essentially 100% zeolite with negligible inert binder which have a higher absorption capacity, a faster adsorption rate, and improved selectivity and which are in the form of hard nodules with improved attrition resistant. 
     It is a further object to produce a zeolite extrudate with a lower dust content. 
     It is a further object of this invention to form zeolite molecular sieves from a formate containing a substantial amount of metakaolin clay without requiring the extrudates to be immersed in a concentrated caustic solution in a digestion step. 
     It is a further object to age the clay containing extrudates under dry bulk storage in a container to permit autogeneous heat to develop to crystallize the clay. 
     It is a further object to produce a small zeolite extrudate having a diameter of about 1/16 to 1/8 inch which contains essentially no inert material. 
     It is a further object of the invention to obtain calcium exchanged formed zeolites having a nominal pore size of 5 Angstrom units. 
     These and further objects will become apparent as the description of the invention proceeds. 
     SUMMARY OF THE INVENTION 
     Binderless zeolite extrudates have been obtained from a mixture of metakaolin clay and a zeolite such as 4A sieve powder to which has been added a relatively strong caustic solution which contains about the stoichiometric amount or slightly more of caustic required to convert the metakaolin to a 4A zeolite. This is described by Nozemack, Chi and Schwonke in the copending U.S. application Ser. No. 225,075 filed Jan. 14, 1981, the entire contents of which is incorporated herein by reference. In these mixtures there is a substantial amount of clay present which comprises 40-60% of the total mixture. These materials are dry blended together. In one embodiment, part of the water used to dilute the caustic solution is added in the form of ice to chill the reaction mixture. After adding a lubricant to aid in extrusion, the mixture is passed through a forming device such as a pellet mill or an extruder having a surrounding jacket which is chilled with cooling water. 
     The resulting extrudates can be aged from a relatively short period of time, for example about 1/2 hour, before being lowered into a drum conditioner for length reduction in those instances where a long length extrudate was initially produced. Then the extrudates are aged. In the preferred embodiment of this improvement invention the extrudates are aged under dry, bulk storage in a container such as a 55 gallon steel drum, a 20 gallon container or a large hopper. As the caustic reacts with the metakaolin clay, the container retains the autogenous heat which develops in the central part of the bulk to gradually increase the temperature sufficient to initiate the crystallization process at which point there will result a substantial conversion of the remaining clay to the crystalline zeolite form. 
     After the aging has taken place the extrudates may be treated with a heated solution of very dilute sodium hydroxide solution, such as, for example, about a 3% sodium hydroxide, to further insure complete crystallization. 
     The crystallized extrudates can then be washed to remove excess unreacted sodium, and then they are dried and calcined for activation. Alternatively, the washed, crystallized extrudates can be treated before drying with a calcium containing solution to exchange the sodium for calcium to produce a zeolite with a pore diameter of nominal 5 Angstrom units or the extrudates can be treated with a potassium containing solution to produce a zeolite with a pore diameter of nominal 3 Angstrom units. After the optional calcium or potassium exchange treatment, the treated extrudates are dried and then calcined to activate the sieves. 
     According to the present procedure the blend ratio of the clay to zeolite and amount of the caustic usage are optimized to prevent impurity formation while generating an extrudate of high strength for the subsequent wet crystallization step in a dilute caustic solution. This results in a superior sieve product. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One of the unique features of the extrusion step is the production of a zeolite extrudate by using, in addition to a zeolite powder, a relatively large amount of metakaolin clay that is reacted in situ with caustic to form additional zeolite so that essentially the entire extrudate is made of zeolite material. The clay content of 40-60% appears to be optimum for controlling the reaction rate and heat generation so as to enable the extrusion of even 1/16&#34; extrudates with relative ease. When adding caustic to convert the metakaolin clay to the zeolite, a preferred amount of caustic to be used is about 110% of the theoretical amount required since this is near the optimum for complete reaction and for preventing impurity formation. A lower amount can be used such as the stoichiometric amount or even as low as about 90% of the stoichiometric amount. However, when these smaller amounts are used, a longer time is required for the chemical reaction to occur and the quality also suffers. It is also possible to increase the caustic level slightly such as up to 120% of the theoretical amount. However, as the amount of caustic is further increased beyond this level, there is more tendency to form impurities such as zeolite Z-6 (sodalite). 
     When a conventional extruder having a relatively long extrusion zone is employed, it is preferred to use a chilled caustic solution when formulating the clay, zeolite and caustic mixture prior to extrusion. It is desirable to have the reaction mixture at a relatively low temperature to counteract the heat given off by the exothermic reaction when the caustic reacts with the mixed clay and the frictional heat produced during the extrusion operation. Without this initial chilling the temperature would rise too fast and the reaction will take off, preventing further extrusion. 
     It has also been found helpful in the embodiment utilizing an auger type extruder to have a cooling chamber around the extrusion area through which chilled water flows. This chamber further removes heat from the reaction mixture as it is being extruded through the auger type extruder. When using the more preferred embodiment of a pellet mill for the extrudate formation, the caustic solution temperature control is not as essential since the hold up time of the mixture in the machine is very short. 
     After the material has been extruded, the extrudates are permitted to dry age. Contrary to the strong caustic solution digestion step requirement of the examples of the Taggart et al. patent, it is not necessary to place the extrudates in the stronger caustic liquid solution used in the Taggart examples to now supply a substantial portion of the caustic reactant to convert the metakaolin in the extrudate to a zeolite, since each of the extrudates has had thoroughly mixed therein almost all of the necessary caustic in the process of the present invention. 
     The major improvement of this invention involves storing the extrudates under dry, bulk conditions in a container such as a steel drum to permit the heat of reaction that is produced as the caustic reacts with the clay to be retained. The retained heat increases the temperature inside the container which in turn causes the reaction to proceed faster and produce more heat to further speed up the reaction. As a result the aging period can be less than 8 hours with periods of 2-8 hours ordinarily being suitable. During this aging period the excess caustic which is reacted with the metakaolin clay causes a very substantial amount of metakaolin to crystallize. The resulting product has sufficient integrity to withstand a further wet crystallization treatment. 
     Contrary to the Taggart procedure where the newly formed extrudates are heated prior to wet crystallization the present process does not require an extra heating step once the extrudates have been formed. Thus this process saves the heating energy required in the Taggart process where Taggart must heat the extrudates by supplying external process heat before they are given any further treatment. 
     By utilizing the dry aging crystallization technique with the high initial excess caustic according to the present invention, a substantial amount of the metakaolin clay is converted to zeolite. To insure near complete conversion, the aged zeolites can be heated in an aqueous solution. It is preferred to add a small amount of caustic to the aqueous solution to prevent the caustic already present in the extrudate from leaching out into the solution. Thus, treating the aged extrudates in a relatively dilute solution of about 3% caustic heated to near the boiling temperature will insure the final crystallization of almost all of the metakaolin clay to the zeolite. This post treatment with a low caustic solution is a significant production advantage since raw material usage is reduced because less caustic is required than would be necessary if a concentrated digestion solution were required. 
     An advantage of this invention of dry aging in a container is that if the product is allowed to age for a slightly longer period of about 16-40 hours, there is substantially enough crystallization occurring so that it is not necessary to use a wet crystallization step which is typically conducted at about 200° F. Instead only a warm water wash step conducted at about 110° F. is employed so there can be a substantial savings in the process energy requirements. 
     A further advantage of using the high caustic concentration technique in forming the initial extrudate is that the aging times are reduced. If less caustic were used then it would take a longer time for aging to enhance crystallization. There are further problems involving product quality when a relatively small amount of caustic is added in relation to the clay present. After the initial reaction has taken place there will still be some unreacted clay present if less caustic were present. When these extrudates are subsequently added to the caustic solution some of the unreacted clay is leached out which results in weaker products. 
    
    
     Having described above the basic aspects of the process and resulting product of this invention, the following examples are given to illustrate specific embodiments thereof. 
     EXAMPLE 1 
     This example relates to the production of 1/8&#34; extrudates made with a 50--50 mixture on a dry basis which have been aged in a drum container. 
     On a basis of 150 pounds of dry blend, 75 pounds of Hi-Opaque metakaolin clay (obtained from Freeport Kaolin) and 96 pounds of 4A zeolite powder, containing about 20% moisture, were charged to a sigma mixer and 1.5 pounds of Sterotex powder was added and the resulting mixture was blended together for 15 minutes. 
     A caustic solution was prepared from 60 pounds of 50% caustic and 42 pounds of ice. 
     To facilitate the mixing of the dry powder and liquid, a paste was made from about 70% of the blended powder added to all of the chilled caustic solution. The paste was mixed intensively for about 10 minutes and then the remaining 30% of the blended powder was added and admixed for an additional 15 minutes. 
     This material was then fed into an auger type extruder having a barrel surrounding the extruder through which chilled water was passed. This was done by forming a mixture of ice, water and rock salt and the resulting water solution at a temperature below 32° F. was continuously recirculated around the barrel with a small pump. 
     The extrudates obtained had good integrity and did not generate a significant amount of heat. They were placed in a 55 gallon drum where they immediately started to heat up. Samples were taken after various times and conditions set forth in Table 1 below. In each case they were washed to remove the caustic and the amount of crystallinity measured. They then were given a crystallization treatment in a 3% caustic solution for 4 hours at 200° F. and again the amount of crystallinity was measured. In none of the cases was there any impurity found such as Z-6 or Z-14. 
     
                       TABLE 1______________________________________Time AfterWhich TheSample Was    % Crystallinity                          % CrystallinityTaken         Before      AfterRun  (Hours)       Crystallization                          Crystallization______________________________________1.    3            44          642.   16            72          803.   40            75          854.   64            78          835.   88 from       --          92top of drum6.   88 from       --          87bottom of drum______________________________________ 
    
     The results of the aging tests in Table 1 show the longer the period of aging the higher the crystallinity and that a period of 16-40 hours provides an adequate level of crystallinity. In other words, by dry aging the extrudates for this period, it is not necessary to undertake a subsequent wet crystallization step. This is advantageous since it saves time required on process equipment and it also saves heat energy because instead of required temperatures of about 200° F. for wet crystallization one only uses warm water at about 110° F. for the final washing step. 
     Typically for many small scale runs the crystallinity after crystallization is on the order of 95%. 
     EXAMPLE 2 
     This example relates to the production of 1/8&#34; extrudates from a round ring pellet mill. 
     The same formulation of Example 1 was made and mixed for 30 minutes to an Ohaus of 34.5-35.0%. The material was then fed to a ring roller pellet mill with openings which made 1/8&#34; extrudates. The cutter permitted easy regulation of the length with length to diameter ratios, L/D, of 2 or 3 to 1. The extrudates were screened and stored in a drum. The extrudates heated up in the drum with a temperature of 190° F. being reached in less than 3 hours. After aging for 64 hours the material crystallized in a 35 caustic solution for 4 hours at 200° F., some of the extrudates were then dried and activated. A crystalline analysis made by X-ray diffraction shows nearly complete conversion to zeolite A with no other phases present. 
     Some of the extrudates from the 4 hour treatment with 3% caustic were then given a calcium exchange by treating about 400 g. of wet beads with 3 batch exchanges for 1 hour each in a solution of 135 g. of calcium chloride in 700 ml. of water maintained at a temperature of 140° F. These treated extrudates were then placed overnight in a dryer and then calcined at 1000° F. for 2 hours. A crystalline analysis made by X-ray diffraction shows nearly complete conversion to zeolite A with no other phases present. The resulting extrudates had a total volatile content when measured at 1750° F. of 2.74% and when the extrudates were placed in an atmosphere of 10% relative humidity they adsorbed 19.77% water which demonstrates a very high conversion to zeolite A. 
     Zeolites with pores of 5 Angstrom units produced by calcium exchange are able to separate normal paraffins from branched paraffins. These calcium exchanged extrudates exhibit this property since they adsorbed 11.59 g. normal butane from the gas at 1 atmosphere and 22° C. while only adsorbing 0.12 g. of isobutane under the same conditions. This shows the extrudates are highly crystalline and highly selective. 
     EXAMPLE 3 
     This example illustrates the production of 3/32&#34; extrudates using a pellet mill. 
     The same procedure of Example 2 was followed to prepare the material for extrusion. The ring roller pellet mill was used with die openings that produced 3/32&#34; extrudates. The material hardened in 5-10 minutes and the material was stored in a drum where it heated to 190° F. in less than 3 hours. After 3 hours a sample was crystallized using a 3% caustic solution at 200° F. for 4 hours. A crystalline analysis made by X-ray diffraction shows nearly complete conversion to zeolite A with no other phases present. This example shows that after only 3 hours of aging the extruded product is ready for wet crystallization solution. 
     These extrudates were then given a calcium exchange using the same procedure set forth in Example 2 including placing the treated extrudates overnight in a dryer and calcining them at 1000° F. for 2 hours. A crystalline analysis made by X-ray diffraction shows nearly complete conversion to zeolite A with no other phases present. The resulting extrudates had a total volatile content when measured at 1750° F. of 2.69% and when the extrudates were placed in an atmosphere of 10% relative humidity they adsorbed 18.5% water which demonstrates a very high conversion to zeolite A. 
     Zeolites with pores of 5 Angstrom units produced by calcium exchange are able to separate normal paraffins from branched paraffins. These calcium exchanged extrudates exhibit this property since they adsorbed 11.64 g. normal butane from the gas at 1 atmosphere and 22° C. while only adsorbing 0.16 g. of isobutane under the same conditions. This shows the extrudates are highly crystalline and highly selective.