WO1991004948A1

WO1991004948A1 - Highly selective zeolites for removal of ammonium from a wastewater

Info

Publication number: WO1991004948A1
Application number: PCT/US1990/005425
Authority: WO
Inventors: Robert P. Carnahan; Charles B. Knotts
Original assignee: Mww Limited Partnership
Priority date: 1989-09-27
Filing date: 1990-09-27
Publication date: 1991-04-18

Abstract

A process for enhancing the selectivity of a zeolite, such as clinoptilolite, for ammonium ion versus other cations such as calcium. The zeolite is heated to a temperature and for a period of time sufficient to increase the selectivity of the zeolite for ammonium ion without substantially reducing the capacity of the zeolite for ammonium ion. The process is preferably carried out at a temperature of 600 and 800 °C for at least one hour and preferably, for between one and two hours. A process for selectively removing ammonium from a wastewater using the above-described zeolite is also provided.

Description

HIGHLY SELECTIVE ZEOLITES FOR REMOVAL OF AMMONIUM FROM A ASTE ATER

BACKGROUND OF THE INVENTION:

The present invention relates to zeolites used in ion exchange processes and, more particularly, to a process for enhancing the selectivity of a zeolite such as clinoptilolite for ammonium ions versus other ions such as calcium.

Clinoptilolite is a naturally occurring zeolite with substantial ion exchange properties. The exchange capacity is normally measured at 1.0-2.0 milliequivalents/gram, depending upon the source of the material. The practical capacity of the zeolite depends upon the solution composition of given cations in addition to the total electrolyte concentration. All cations are adsorbed to some extent by the zeolite and the selectivity exhibited for the removal of a particular cation is uniquely determined by the solution composition. The general order of selectivity for several cations is given as follows: K+ > NH₄ ⁺ > Na⁺ > Ca⁺⁺

This selectivity is quite different from that normally observed with synthetic organic resins where the general order of ion selectivity is: trivalent > divalent > monovalent i.e., calcium is adsorbed more extensively than ammonium and sodium.

It has thus been observed that zeolites such as clinoptilolite have a high affinity for ammonium ions and would therefore be quite effective as adsorbents for removing the ions from wastewater solutions. Since clinoptilolites have a limited affinity for ammonia, the ammonium ions initially adsorbed onto the zeolite could be desorbed merely by increasing the pH enough to convert the bound ammonium ions to unbound gaseous ammonia. Such a process would prove highly desirable from both an economic and environmental viewpoint since the bulk of the ammonium ions would be removed from the wastewater and since the ammonia subsequently released is not only a commercially .valuable material as is, but also may be converted relatively easily to a host of other commercially valuable materials such as ammonium phosphate.

Despite the above advantages attendant with the use of zeolites to adsorb ammonium from a wastewater, a number of problems have been encountered. Thus, even though the selectivity exhibited by clinoptilolite produces greater adsorption for ammonium than for calcium, the selectivity coefficient is such that substantial amounts of calcium are also adsorbed, especially when high levels of calcium are present. This is a serious problem when clinoptilolite is used for removal of ammonium from wastewater where the normal levels of ammonium are in the range of 10-20 parts per million (ppm) and calcium and magnesium levels may reach 150-200 ppm. Under these conditions, clinoptilolite is as effective for the removal of calcium as it is for the removal of ammonia.

Problems have also been encountered with respect to the regeneration of the clinoptilolite. Specifically, although it is possible to use the adsorbent material for many hundreds of cycles, the use of clinoptilolite, having a relatively low selectivity for removal of ammonium, in repetitive cyclic operation results in (1) increased use of regenerant, normally sodium chloride, to remove both ammonium and calcium, (2) larger system components to accommodate increased flows, (3) precipitation of calcium and magnesium in the system as regeneration is normally conducted at an alkaline pH and

(4) increased transfer of sodium into the treated water during the adsorption phase. All of these factors translate into increased operating and capital costs as well as system complexity. Thus, while facilities have been built to remove ammonia from water by the ion exchange method, the process has not proven to be economically viable in view of the lack of ion selectivity by the zeolite.

Not surprisingly, in its use of various zeolites in ion exchange processes for the removal of ammonium from a wastewater, the art has typically had to strike a compromise between zeolites having both adequate cation exchange capacity and adequate selectivity for the ammonium cation in the presence of alkali and alkaline earth metal cations. For example, as indicated above clinoptilolite is regarded as one of the zeolites having desirable selectivity characteristics, but relatively low cation exchange capacity in terms of equivalents per unit weight.

A number of patents disclosing zeolite adsorption of ammonium ions from a wastewater have issued wherein it has been attempted to prepare synthetic zeolites exhibiting both high selectivity and a high cation exchange capacity. Thus, in U.S. Patent No. 3,723,308, there is disclosed a process for the removal of ammonia from wastewater streams. There was then proposed as a solution to the problem the use of a zeolite F, which is a synthetic crystalline aluminosilicate having a silica to alumina oxide mol ratio of about 2 and derived from a potassium rich reaction mixture.

There are also patents which have issued relating to special types of zeolites which are said to exhibit the desired combination of selectivity and high exchange capacity. Thus, U.S. Patent No. 4,344,851 relates to a zeolitic cation exchange of ammonium ions from aqueous solutions containing calcium cations. Specifically, a method for the zeolitic cation exchange removal of ammonium ions from an aqueous solution containing calcium cations utilizing a zeolite possessing both high cation exchange capacity and excellent selectivity for the ammonium ion etc. is said to be obtained by employing a natural or synthetic zeolitic crystalline aluminosilicate of the phillipsitegismondite type.

Also described are processes wherein a certain type of zeolite is subjected to various process manipulations so as to impart thereto the desired ion exchange characteristics. Thus, U.S. Patent No. 4,130,484 discloses a process for removing water, ammonia and/or primary amines from a dehydration sensitive compound, e.g. , a secondary or tertiary alcohol using a base treated molecular sieve which can be any suitable natural or synthetic crystalline zeolite material but, in general, will be a synthetic Type A zeolite molecular sieve zeolite. In general, the molecular sieve material is contacted with an aqueous solution of the base at a suitable temperature and pressure, for example a temperature in the range of about 10-200°C at a pressure in the range of 0.1 to 6.9 MPa to reduce the dehydration characteristic of the molecular sieve material to the desired extent. The thus treated molecular sieve material can then be activated by heating to a temperature in the range of 500-1200°C for a period of time in the range of about 15 minutes to about 24 hours.

U.S. Patent No. 3,929,600 discloses a process for removing ammoniacal nitrogen from wastewater using a zeolite. Specifically, after the adsorption of the ammonia nitrogen by zeolite, the ammonia nitrogen is eluted by treating the ion-exchange substance with a treating liquid containing mainly alkali metal chlorides to regenerate the ion-exchange substance. The treating liquid containing the ammonia nitrogen is then electrolyzed to produce active chlorine which is used to decompose oxidatively the ammonia nitrogen. In the examples, natural zeolites such as clinoptilolite can be employed.

Processes have also been developed wherein the various treating fluids are subject to specific process manipulations so as to increase the efficiency of the ion exchange process. In U.S. Patent No. 4,522,727, there is disclosed a process for the continuous removal of ammoniacal nitrogen from aqueous streams using a particulate zeolitic ion exchange material that is continuously regenerated by heating in the presence of an oxygen-containing gas. Naturally occurring as well as synthetic zeolitic ion exchange materials that sort ammoniacal nitrogen can be used. In similar fashion, U.S. Patent No. 4,695,387 discloses a process for the removal of ammonium from wastewater wherein the process efficiency is greatly enhanced by employing a number of process manipulations in a special ion exchange apparatus.

From the above, it is apparent that the art has long struggled with the seemingly contradictory goals of obtaining a zeolite ion exchange material which is both highly selective for ammonium yet also has a high cation exchange capacity. To this end, there have been developed new synthetic zeolites, new processes for chemically or physically modifying a natural or synthetic zeolite, and processes wherein the treating fluids themselves used in the process are subjected to various types of manipulations. Nonetheless, in view of the highly desirable properties of zeolites such as clinoptilolite for the removal of ammonium from wastewaters, the art continues to search for ways to more efficiently utilize zeolites in such processes.

SUMMARY AND OBJECTS OF THE INVENTION: In view of the foregoing, it should be apparent that there exists a need in the art for zeolites for the removal of ammonium from a wastewater which exhibit both a high selectivity for ammonium and a high cation exchange capacity. It is, therefore, a primary objective of the present invention to fulfill that need by providing a process for activating clinoptilolite which increases the selectivity of the clinoptilolite for ammonium while decreasing its selectivity for cations other than ammonium which are often found in a wastewater together with ammonium such as calcium. More particularly, it is an object of the invention to provide a process for activating clinoptilolite which, for all practical purposes, does not adsorb calcium when contacted with a wastewater containing both ammonium and calcium ions thus enables selective adsorption of ammonium, at a high rate, from the wastewater.

In a first aspect, the present invention relates to a process for enhancing the selectivity of clinoptilolite for ammonium ion in a wastewater comprising heating the clinoptilolite to a temperature and for a period of time sufficient to increase the selectivity of the clinoptilolite for ammonium ion in the wastewater without substantially reducing the cation exchange capacity of the clinoptilolite for ammonium ion, preferably at a temperature between about 600 and 800°C.

In a second aspect, the present invention relates to a process for removing ammonium ion from a wastewater comprising contacting the wastewater with the above- described clinoptilolite. In a third aspect, the present invention relates to a clinoptilolite produced by the above-described process.

With the foregoing and other objects, advantages, and features of the invention that will become hereinafter apparent, the nature of the invention may be more clearly understood by reference to the following detailed description of the invention and to the appended claims.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS;

The zeolites employed in accordance with the present invention include the naturally occurring clinoptilolites as well as chabazite, mordenite, phillipsite, and erionite. The particle size of the zeolite must be large enough such that containment of the bed can be properly controlled but not so large such that the reduced total sorbent surface area restricts the overall adsorption capacity. In this regard, a particle size ranging from -20 + 35 mesh and preferably from -30 + 50 mesh can be employed. It is noted that in tests performed with larger clinoptilolite particles (-20+35 mesh) and with powders demonstrated that the finer material provides both a higher ammonium removal capacity and increased selectivity for ammonium over calcium. A particularly preferred zeolite is clinoptilolite obtained from Armagossa Valley in western Nevada. Although virgin clinoptilolite can be subjected to the thermal treatment of the invention, the clinoptilolite may in certain instances be pretreated without an adverse effect on the activation process. For example, washing virgin clinoptilolite with solutions such as potassium chloride, potassium sulfate, ammonium chloride, ammonium sulfate, sodium chloride, and sodium sulfate at concentrations in the range of 1000 ppm followed by the optimum thermal treatment yields zeolites of similar activity and selectivity. However, the washing of the clinoptilolites with solutions containing in excess of 25 grams per liter reduces the sorbent activity for ammonia adsorption. Thus, the salt solution should have a concentration of less than 25 grams per liter. Similarly, washing with calcium chloride solution prior to heat treatment has no significant effect on the activity.

The virgin clinoptilolite or, alternatively, the clinoptilolite which has been subjected to the above- described pretreatment, is then activated so as to render such clinoptilolite highly selective for ammonium over calcium such that virtually no calcium salts are adsorbed thereby. The activation process is essentially thermal in character and involves heating the clinoptilolite to a temperature and for a period of time sufficient to increase the selectivity of the clinoptilolite for ammonium ion without substantially reducing the capacity of the clinoptilolite for ammonium ion.

The activation typically involves heating the virgin clinoptilolite to elevated temperatures of between 600 and 800 °C for at least an hour. The degree of selectivity improvement is greatest at temperatures between 675 and 775°C. A thermal activation period between one and two hours is preferred for the purpose of convenience and reliability. Activation is also possible at temperatures below about 600^βC but such temperatures are less preferred due to the longer heating times which result. Furthermore, temperatures in excess of about 800 " C are less preferred since such high temperatures can significantly reduce the capacity of the zeolite for ammonia and, furthermore, would require the use of excess amounts of sorbent to carry out the process.

The thermal activation can be carried out in a closed but not sealed electric furnace by both placement in a pre-heated furnace and in a cold furnace. The material remains in the oven during the cool-down period before removal. It is not necessary to control or alter the humidity and atmosphere during the heating. To demonstrate the beneficial effect which the thermal activation has on the selectivity of clinoptilolite for ammonium, a batch study was carried out. In that study, 5.0 gram samples of clinoptilolite were exposed to 100 milliliters of solution with and initial ammonium concentration of 20 ppm and a calcium concentration of 50 ppm. The chemical pretreatment consisted of washing the clinoptilolite with 1.9 grams/liter potassium followed by washing with 1.3 grams/liter of ammonium chloride. The final ammonium and calcium levels in the solution were as follows:

TABLE 1 CHEMICAL THERMAL FINAL SOLUTION FINAL SOLUTION PRETREATMENT TREATMENT NH₄ ⁺,ppm CA⁺⁺,ppm

None None 0.53 2.27

KCI/NH4CI 770°C,2 hrs. 1.1 36

None 770^βC,2 hrs. 1.1 36

Related studies were conducted with continuous water flows utilizing 2-inch (5.08 cm) diameter columns of clinoptilolite with a sorbent depth of 8 inches (20.32 cm) . Four columns were used in series for adsorption to provide a total mass transfer zone length of 32 inches (81.28 cm) . The specific data presented in Table 2 below are for a clinoptilolite sample having a particle of

(-20+35 mesh) treated at 725°C for two hours. Initial concentrations of ammonia and calcium were 31 ppm and 58 ppm respectively. The data presented are for eluants from the first column.

TABLE 2

BED VOLUMES FINAL AMMONIUM FINALCALCIUM TREATED NH₄ ⁺,ppm CA⁺⁺ ppm

15 0.22 35.9

33 0.68 38.9

48 0.83 39.1

93 3.55 39.5

183 8.8 39.9

275 15.2 42.1

370 17.5 44

473 17.9 44.2

Data for eluants from columns 1,2,3, and 4 are given below, i.e., C_{l r} C₂, C₃, and C₄ in Table 3.

TABLE 3

BED VOLUMES FINAL AMMONIUM FINAL CALCIUM TREATED NH₄ ⁺,ppm CA⁺⁺,ppm

33 0.68 0.25 0.25 0.34 38.9 36.9 41.2 28.6

38 0.83 0.25 0.23 0.3 39.1 37.4 38.3 43

433 17.9 6.1 <1 <1 44.2 38.9 38.3 38.1

The above data indicate that the mass transfer zone length is less than 3 columns, i.e., less than 24 inches (60.96cm) . All of the calcium appeared to be removed in column 1, along with the ammonium. Successive columns remove little or no calcium beyond that removed in column 1.

Studies were also performed with finer material, which was found to provide results with both a higher ammonium removal capacity and increased selectivity for ammonium over calcium. In those studies, each test involved the exposure of 5.0 grams of large particle zeolite and 0.5 grams of powdered zeolite to a standard solution (100 milliliters) of 20 ppm ammonium and 50 ppm calcium. The final solution of ammonium and calcium were as follows:

TABLE 4

ZEOLITE THERMAL FINAL SOLUTION FINAL SOLUTIO

PARTICLE SIZE TREATMENT NH₄ ^"t',ppm Ca ++ ppm Powder 770" C,2 hrs. 7.8 51.45 Powder None 2.0 12.14

-20+35 mesh 770^βC,2 hrs. 1.1 36.34 Thus, the present invention, by virtue of thermal activation of clinoptilolite in the manner disclosed and claimed, enables selective removal of ammonium from a solution of ammonium and calcium, even in the presence of large amounts of calcium. This significantly reduces the complexity and the cost of the process.

Although only preferred embodiments are specifically illustrated and described herein, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.

Claims

CLAIMS :

1. A process for enhancing the selectivity of a zeolite for ammonium ion in a wastewater comprising heating said clinoptilolite to a temperature and for a period of time sufficient to increase the selectivity of said clinoptilolite for ammonium ion in said wastewater without substantially reducing the cation exchange capacity of the clinoptilolite for ammonium ion.

2. The process of Claim 1 wherein said zeolite is clinoptilolite.

3. The process of Claim 2 wherein the clinoptilolite is heated at a temperature between about 600 and 800°C.

4. The process of Claim 3 wherein the clinoptilolite is heated at a temperature between about 675 and 775^PC.

5. The process of Claim 2 wherein the clinoptilolite is heated for at least one hour.

6. The process of Claim 5 wherein the clinoptilolite is heated for between about one and two hours.

7. The process of Claim 2 wherein said clinoptilolite is virgin clinoptilolite.

8. The process of Claim 1 further including treating said zeolite with a salt solution having a concentration of less than about 25 g per liter prior to said heating step.

9. The process of Claim 8 wherein said salt solution has a concentration of about 1000 ppm.

10. A process for removing ammonium ion from a wastewater comprising contacting said wastewater with the zeolite prepared by the process of Claim 1.

11. The process of Claim 10 wherein said wastewater includes cations other than ammonium.

12. The process of Claim 11 wherein said wastewater includes calcium cations.

13. A zeolite produced by the process of Claim 1.