Process for Separating Ammonia and Acid Gases From Waste Waters Containing Fixed Ammonia Salts

Aqueous solutions containing ammonium compounds and especially "fixed ammonia" and optionally acid gases are a common waste water which requries purification. Such a waste water is produced as a by-product from coal carbonization plants. The need for such purification has become increasingly important due to the emphasis today on clean air and clean water. If the waste water is to be discharged into a river or a stream, it is obviously necessary to reduce the noxious or toxic properties of the waste water. On the other hand, in a coal carbonization plant it is often desirable to reuse the waste water to "quench" the hot coke from the coke ovens. If these waste waters contain noxious or toxic materials, then there may be a serious air pollution problem.

A common way of removing impurities from these waste waters is by means of a two-step distillation process using distillation apparatus with a free ammonia and a fixed ammonia section. See "Industrial Chemistry", E. R. Riegel, Reinhold Publishing Corp., N.Y., 1942, pp. 265-268.

One of the serious problems of this two-step distillation process is the fouling of the fixed ammonia still section by deposits of solids formed in the waste waters upon the lime addition. The free ammonia portion of the distillation apparatus removes most of the free ammonia and/or acid gases present in the waste water. The waste water is then treated with lime to convert the fixed ammonia to free ammonia which is then removed in the fixed ammonia still portion of the distillation apparatus. Solids deposit on the trays in the fixed ammonia still so that periodic cleaning is necessary to allow efficient operation of the still. This is costly and time-consuming and also cuts down on the production possible from a given facility. A number of attempts have been made to overcome this problem. In U.S. Pat. No. 2,839,369 an attempt is made to remedy the problem by designing a special still which can be easily cleaned. A further approach is described in U.S. Pat. No. 3,278,423 wherein the waste waters are first treated in a free ammonia still to remove most of the free ammonia, then phenols and the like are removed by biochemical oxidation, then lime is added to the thus treated waste waters to convert the fixed ammonia to free ammonia, then substantially all of the solids are separated from the lime-containing waste water and then the thus treated waste water is steam distilled to remove the free ammonia thus produced. This process has the disadvantage of requiring substantial lime consumption and results in a large amount of sludge which creates serious handling and disposal problems. Furthermore, the biochemical oxidation step may have serious operating problems since many biochemical oxidation plants will not work if the waste water being treated contains more than about 500 parts per million by weight of total ammonia. Total ammonia includes both free ammonia and fixed ammonia. Thus, in cases where significant total ammonia is present, the waste waters would require considerable dilution for efficient operation of the biochemical oxidation part of the process, thus increasing the size of equipment and therefore the cost of such equipment.

The term "fixed ammonia salts" is used herein in its generally accepted meaning to cover those compounds of ammonia which are not decomposed by heat alone at the temperature employed in the ammonia still, but which in alkaline solution yield free ammonia. Generally, in coke-plant waste waters, the major portion of the fixed ammonia is present as ammonium chloride. Other such compounds are ammonium thiosulfate, ammonium thiocyanate, ammonium ferrocyanide and ammonium sulfate. The term "fixed ammonia" refers to the ammonia portion of the fixed ammonia salts.

The term "free ammonia salts" as used herein is meant to include ammonia itself and also those compounds present in waste waters from which ammonia is liberated by heat in the free ammonia still. Such compounds are ammonium carbonate, ammonium bicarbonate, ammonium sulfide, ammonium bisulfide, ammonium cyanide and ammonium carbamate. The term "free ammonia" refers to ammonia or the ammonia portion of the free ammonia salts.

The term "lime" is used herein to include any of the various chemical and physical forms of quicklime (calcium oxide), hydrated lime (calcium hydroxide), and hydraulic lime. The lime may include a number of impurities such as silica, calcium carbonate, magnesium oxide, iron oxide and aluminum oxide. Some of them may be insoluble when added to the waste water.

The term "acid gases" is used herein to include hydrogen cyanide, hydrogen sulfide and carbon dioxide. These gases may be present in the original waste water as weak ammonium salts that completely dissociate into ammonia and their respective acid gases upon steam distillation.

The waste water described herein are generally industrial waste waters produced by the high-temperature cracking of carbonaceous material. Often such carbonaceous materials are coal or petroleum products. Waste water may also be formed in water-scrubbing of coke-oven gases.

This process comprises adding lime to waste waters in amounts sufficient to react with the fixed ammonia salts therein, thereby allowing substantially all of the ammonia to be evolved upon distillation, adding a threshold amount of a scale inhibitor compound of the general formula ##STR1## R' is selected from the group consisting of R and --CH.sub.2 CH.sub.2 OH, and R" is selected from the group consisting of R, --CH.sub.2 CH.sub.2 OH, and ##STR2## where each M is independently selected from the group consisting of H, NH.sub.4 -- and alkali metal, "n" is a whole number from 1 to 3, and subjecting the thus treated waste water to distillation to remove substantially all of the ammonia and acid gases from the waste water.

Preferably, this process is achieved by two separate and successive distillations and comprises separating acid gases and ammonia from a dilute aqueous solution thereof, the solution also containing free and fixed ammonia salts. This solution is first subjected to a countercurrent multi-stage continuous distillation, the distillation being conducted by having a gradient of ammonia concentration decreasing towards the region of bottom liquid removal which results in the bottom liquid having a pH of less than 8 and preferably between about 5 and about 8 when measured at 50.degree. C. An overhead vapor stream from this distillation contains a major proportion of stripping vapor, and a major proportion of the acid gases in the solution, and a major proportion of the ammonia from the free ammonia salts. An aqueous bottom stream from this first distillation contains a major proportion of water and substantially all of the fixed ammonia salts. Lime is then added to the bottom stream in an amount sufficient to form an insoluble precipitate and evolve ammonia contained in the fixed ammonia salts during a second distillation of the mixture. A threshold amount of at least one scale inhibitor compound of the above mentioned general formula is added to the diluted aqueous solution to thereby inhibit the deposition of scale on the trays of the distillation towers and especially the tower in which the second distillation is conducted. This bottom stream is then subjected to a second countercurrent multistage continuous distillation whereby an overhead vapor stream is withdrawn containing a major proportion of stripping vapor and a major proportion of the ammonia from the fixed ammonia salts, and an aqueous bottom stream is drawn, such stream having a substantially reduced concentration of acid gases and ammonia as compared to the original aqueous solution, and preferably being substantially free of acid gases and ammonia.

The first distillation or other waste water pretreatment step may be eliminated so long as the composition of the waste water immediately prior to lime addition comprises:

(a) free ammonia in a concentration between of up to about 1000 ppm (parts per million based on the total weight of the waste water) and preferably between about 35 and about 1000 ppm and more preferably between about 35 and about 300 ppm,

(b) fixed ammonia in a concentration of between about 50 and about 50,000 ppm and preferably between about 500 and about 4000 ppm and more preferably between about 700 and about 2000 ppm,

(c) acid gases in a concentration of between about 5 and about 200 ppm, and

(d) sulfate ion in a concentration of less than about 1500 ppm and preferably less than about 800 ppm, the process comprising (a) adding lime to said waste water, said lime and fixed ammonia salts being present in amounts sufficient to form an insoluble precipitate and evolve ammonia contained in the fixed ammonia salts during a distillation of said mixture; (b) adding a threshold amount of at least one scale inhibitor compound of the above described general formula to said dilute aqueous solutions to thereby inhibit the deposition of scale on the trays of the distillation tower in which the distillation is conducted; (c) subjecting the thus treated waste water to a countercurrent multi-stage continuous distillation, (i) withdrawing an overhead vapor stream, said vapor stream containing a major proportion of stripping vapor and a portion of the ammonia from the fixed ammonia salts, (ii) withdrawing an aqueous bottom stream, said stream having a substantially reduced concentration of acid gases and ammonia as compared to said waste water. Carbon dioxide is a common acid gas that may be present in the waste water and immediately prior to lime addition in a concentration between about 5 and about 75 ppm.

The invention described herein also includes the apparatus for carrying out the described processes.

FIG. 1 is a flow diagram of the waste water purification process of this invention.

FIG. 2 is a flow diagram illustrating the basic principles of the preferred two distillation column process of this invention.

By conducting the process as described, it has been found that there is an improved yield of both acid gases and ammonia, the concentrations of acid gases and ammonia in the liquid effluent can reach and be maintained at desirable low levels, preferably provides a "fixed still" containing more than 12 trays and thereby the energy required per gallon of aqueous feed is reduced, the recovery of the gases is simplified, and fouling or scale deposition on the trays of the distillation columns and particularly of the "fixed still" is substantially reduced.

The flow diagram of FIG. 1 shows waste water containing fixed ammonia and ions which upon reaction with lime produce water-insoluble compounds entering through inlet 101 into a vessel 102 where lime is added through inlet 103 preferably in the form of a lime-water slurry. The lime is added in at least sufficient amount to assure conversion of the fixed ammonia present to free ammonia upon subsequent distillation. The waste water is then passed from vessel 102 through line 104 to vessel 105 where a threshold amount of a scale inhibitor compound of the above described general formula is added through inlet 106. The waste water is then passed from vessel 105 through line 107 to the top of distillation apparatus 108 where steam is added to the bottom through inlet 110 and ammonia, steam and other gases are removed overhead through outlet 109. The treated waste water is removed through line 111. Substantially all contaminates except thiocyanate and organic materials such as phenol have then been removed from the waste water without any significant solids built up on the trays of distillation apparatus 108. Preferably at this point the water is then passed through a biochemical oxidation treatment plant and then discharged into streams, sewers or the like or, if desired, recycled for use such as for coke-quenching.

In FIG. 2, a flow diagram illustrates the principles of a preferred embodiment of this invention. The aqueous feed composition is a dilute solution of acid gases, ammonia, fixed ammonia salts, and free ammonia salts. This may be waste water from a coke-oven plant or coal-conversion plant or similar installation. This solution flows by line 101 into a first distillation column 102. In this column, the solution is heated and its pH values are in a range to decrease the solubility of acid gases. In the distillation of a multi-component liquid, the upflowing vapor which becomes enriched with the more volatile components of the liquid is described as having a stripping action on the liquid, hence, this vapor is termed a stripping vapor. The stripping vapor may be a condensable gas or a non-condensable gas or a combination of these; e.g., steam, air, hydrogen, nitrogen, and methane. The stripping vapor may be generated by vaporization of the liquid or by injecting vapor into the distillation or by a combination of these. Because this invention concerns distillation of dilute aqueous streams, the stripping vapor will generally comprise a predominant proportion of steam. The overhead vapor leaving the column by line 103 will have a major proportion of stripping vapor such as steam, substantially all of the acid gases and free ammonia. Heat for the first column can be provided by direct injection of steam or other stripping medium or indirect heat exchange or a combination of these; as shown, steam is directly injected into the column by line 105. From the first column, a bottom liquid is withdrawn by line 104 and mixed with lime and a threshold amount of the scale inhibitor compound of this invention in the chamber 108; also, grit and other readily separable solids such as precipitates can be removed in the chamber. This bottom liquid stream flows by line 110 to a second distillation column 111. In this column, the liquid is heated and an overhead vapor of ammonia and water is removed by line 112. The conditions in this second column are selected for maximum recovery of ammonia. This column can be heated by direct injection of stripping vapor such as steam or by other art recognized methods or by combinations of these; as shown, steam is withdrawn by line 113; it has low concentrations of the acid gases and ammonia. It is now suitable for further processing, an example of which is further treatment to provide a quality such that it can be discharged into rivers or reused as a process water. By utilizing the multiple distillation of this invention, low levels of toxic cyanides can be achieved in the effluent so that after biological oxidation, the treated water has such low levels of toxic substances that additional cyanide removal by way of chlorination or the like may be avoided.

The scale inhibitor compound of this invention is that of the above-mentioned general formula which is a well known class of scale inhibitor compounds as shown, for example, by U.S. Pat. No. 3,336,221, incorporated herein by reference. Preferably, the scale inhibitor compound is of the formula ##STR3## where each M is independently selected from the group consisting of H, NH.sub.4, and alkali metal, the amount of said compound added being no more than a threshold amount up to about 200 ppm and in a weight ratio of said compound to the cation component of said scale-forming salts not in excess of 0.5 to 1.

Preferably, the concentration of scale inhibitor compound does not exceed 100 ppm, more preferably does not exceed 80 ppm and most preferably does not exceed about 20 ppm.

A preferred scale inhibitor compound is a water soluble alkali metal salt of an aminotri(lower alkylidene phosphonic acid) and more preferably the scale inhibitor compound is penta sodium aminotri(methylphosphonate).

The scale inhibitor compound of this invention may be added at a number of different points in the process such as prior to or during the first or second distillation. Usually it is preferred to add the scale inhibitor compound subsequent to the addition of the lime. The second distillation column of this invention, commonly referred to as the "fixed still" preferably contains more than 12 trays and more preferably contains between about 15 and about 60 trays. Between about 18 and about 44 trays are more preferred. Preferably, these trays are dualflow trays, which are perforated trays without a downcomer (a separate channel for liquid). Therefore, in a dualflow tray the liquid and gas are forced to go countercurrent alternately through the same opening. These dualflow trays generally contain substantially circular holes having an average diameter of at least one-half inch and preferably between about one inch and about two inches. The trays generally have an average diameter of between ahout 3 and about 20 feet.

The scale inhibiting compound of this invention is that described by the previously mentioned general formula.

As is shown the vapor stream from the second distillation is kept separate from the liquid in the first distillation. It has been found that this separation of second distillation vapors from the first distillation provides optimum equilibrium conditions for removal of acid gases in the first distillation and avoids unnecessary vaporization of second distillation ammonia vapor with subsequent recondensation in the presence of the acid gases. In addition this method of operation permits optimum conditions to be used for recovery of ammonia in the second distillation.

Generally, the dilute aqueous solutions which will be treated by this invention are those having acid gases and ammonia in the solution together with fixed and free ammonia salts. By dilute is meant solutions having water as its major component, where the total dissolved acid gases and fixed and free ammonia are up to 10 percent by weight. The acid gases include CO.sub.2, HCN, and H.sub.2 S or mixtures thereof. Any one of these may be present by itself or in combination with one or more of the others. The simultaneous low concentration of acid gas and ammonia in the treated solution is an important aspect of this invention. Where the acid gases include CO.sub.2, the simultaneous reduction of CO.sub.2 and NH.sub.3 concentrations in the bottom liquid from the first distillation tends to reduce the amount of sludge produced after lime addition. Where the acid gases include HCN, the reduced concentrations of ammonia and cyanide in the exit water are very useful when activated sludge plants are subsequently used to remove biodegradable materials from the water.

The most common dilute aqueous solutions will contain CO.sub.2, H.sub.2 S, and HCN with NH.sub.3 as well as the fixed and free ammonia salts; Van Krevelan et al, Recueil 68 (1949) pp. 191-216 describes the vapor pressures of such solutions as well as the ionic species of acid gas salts and ammonium compounds in such solutions, which would be representative of the aqueous solutions upon which the invention may be practiced. Commonly the acid gases and free and fixed ammonia comprise up to about 0.6% by weight of the aqueous solutions.

Where the solution is a waste water of streams collected from coke plants and coal conversion plants, other components may include tars, phenols, fluorides, chlorides, sulfates, thiosulfates, and thiocyanates. In these circumstances, the tars would be removed by decanting and then the ammonia and acid gases would be removed according to the subject invention.

The collected waste waters from coke or other coal conversion plants are often referred to as ammoniacal liquors. The principal free and fixed salts present in the liquors are as follows:

In addition to ammonia and ammonium salts, the waste waters contain low concentrations of suspended and dissolved tarry compounds. The most important of these compounds are the phenols or "tar acids", the concentration of which usually ranges from about 0.3 to about 15 grams per liter of liquor. Pyridine bases, neutral oils, and carboxylic acids are also present but in much lower concentrations.

The insoluble precipitates formed on the addition of lime to the waste water generally include calcium sulfate, calcium sulfite, calcium carbonate, calcium fluoride, calcium phosphate or mixtures of the same.

Typical compositions of liquors from various sections of the coke oven operation are:

A fuller discussion of the recovery of ammonia from coke oven gases and the origin of the various ammonia salt species in the various sections of coke oven plants is given in the book, COAL, COKE and COAL CHEMICALS, P. J. Wilson and J. H. Wells, McGraw-Hill Book Company, Inc., N.Y., 1950, particularly Chapter 10, pp. 304-325.

The following Table 3 illustrates the range of compositions in coke plant waste water that comprise aqueous solutions especially suitable for the practice of this invention:

The practice of our invention on materials such as described above can be achieved by having two successive and separate continuous distillations each operating under counter-current multistage separation conditions. In the first distillation, the process conditions are selected so that essentially all of the acid gases and essentially all of the ammonia from the free ammonia salts are vaporized and removed from the solution. The general conditions to achieve this result are to have an inlet feed temperature such that there is substantial rectification in the first distillation, an overhead vapor in a condition for maximum removal of acid gases and ammonia and a bottom stream that is essentially depleted of the free ammonia and said gases.

Because of its desirable physical properties and ready availability, the preferred stripping vapor consists essentially of steams in which minor amounts of non-condensables may be present. The hereinafter specific description of the preferred practice of this invention is made with reference to the stripping vapor being steam. It being understood that due allowance be made for variations in operating conditions where the stripping vapor contains a significant proportion of a non-steam component. The inlet feed temperatures may be in the range of about 60.degree.-212.degree. F., the overhead temperatures in the range of about 140.degree.-265.degree. F., and the bottom temperatures in the range of about 160.degree.-275.degree. F. The first distillation will be conducted at a pressure in the range of about 0.3 to 3.0 atmos. abs. The low part of the pressure range allows for efficient use of low pressure steam while the higher pressures give more efficient removal of cyanides. In this first distillation, the gradient of ammonia concentration is controlled to be decreasing towards the bottom of the column. Thus, substantially all of the free ammonia salts, e.g., sulfide, carbonate and cyanide, are decomposed into ammonia and acid gases that are removed by the stripping vapor. The decreasing ammonia concentration results in decreasing the pH which enhances the stripability of the acid gases in the lower section of the distillation tower. The results in the solution at the bottom of the column becoming midly acid, ca. pH of about 5 to 6, the acid gases can thus be removed substantially completely from the solution with the result that the still bottoms is also very low in acid gas content. The preferred concentration of ammonia in the first distillation bottom stream is about 40 to 200 ppm. As another aspect of these conditions, if direct injection of steam is used to supply part or all of the heat required for vaporization, then this steam should be substantially free of ammonia so that the pH range in the first distillation can be maintained of less than 8, and preferably in the mildly acid range.

The ratio of internal liquid to internal vapor flow should be as high as possible to achieve good economy of operation; the L/V values may be in the range of about 10/1 to 2/1 for this first distillation.

The bottom stream from this first distillation is treated by addition of lime to increase its pH such that the bottoms stream from the second distillation column has a pH of 9.5-12 when measured at 50.degree. C.

The lime reacts with both fixed ammonia salts and any residual acid gases. With respect to the ammonia salts, the principal reaction is with ammonium chloride although ammonium thiocyanide and sulfate also react according to the following equations:

the reactions between lime and any residual acid gases are according to the following equations:

the major portion of any residual acid gases is CO.sub.2 and the calcium carbonate thus formed tends to consume lime and forms additional lime sludge and thus causes scaling or fouling in distillation equipment.

The treated stream is subjected to a second distillation. As noted above, the high pH and heat cause the "fixed" ammonia salts to decompose with liberation of the ammonia. The resulting overhead vapors are essentially ammonia and water. The feed temperature may be in the range of about 155.degree.-270.degree. F., the overhead vapors may be at a temperature in the range of about 140.degree.-290.degree. F., and the bottom stream may be at a temperature in the range of about 160.degree.-295.degree. F. The pressure may be up to about 60 psia. The bottoms stream from the second distillation will have a low concentration of total ammonia and of cyanides. The pH will be in the range of 9.5-12. The total ammonia may be as low as 25 ppm. The total cyanides, including complexed cyanides and free cyanides expressed in terms of equivalent HCN concentration [HCN (total)*], may be as low as 22 ppm; the free cyanides, cyanides amendable to chlorination, in some cases may even be less than 1 ppm. This bottom stream can be clarified and then treated to remove other organic materials, such as phenols.

By having the acid gases and free ammonia removed in the first distillation, the second distillation can be conducted for the optimum removal of ammonia from the alkaline fixed ammonia salt solution. Also, the fixed ammonia salt solution will have minimum deposits of salts that arise from the presence of the acid gases. Also, the low concentration of ammonia in the bottoms of the first distillation allows substantially complete removal of acid gases, such as hydrogen cyanide, carbon dioxide, and hydrogen sulfide, from the bottom stream so that the tendency to deposit salts in the second distillation is reduced. It is another benefit of this invention that the various streams may be utilized as heat sources in the distillations to minimize the consumption of energy for achieving effluents with low concentrations of ammonia and cyanide. As well, the overheads of the first distillation may be treated to separate the ammonia and acid gases; the overheads from the second distillation are essentially water and ammonia from which the ammonia may be recovered.

The following examples illustrate the practice of the invention.

An apparatus essentially as described in FIG. 2 contained a fixed distillation column of a height of 108 ft., a diameter of 121/2 ft., having 44 dualflow trays each containing one inch diameter circular holes to provide a hole area of about 23% for each tray. In this apparatus a waste water collected from a coke plant was treated according to this invention.

The waste water was heated to a temperature of 160.degree. F. and then admitted near the top of a first distillation column. The liquid to vapor flow rates were about 10:1. The overhead vapors were at a temperature of 204.degree. F. and 13 psia. The bottom streams were at a temperature of 219.degree. F. and 16.5 psia.

The concentrations of acid gases and ammonia in the first column were as follows:

The bottom stream from the first distillation column was withdrawn and then treated with a slurry of lime in an aqueous vehicle so that its pH was increased such that a pH of 9.5-12 was maintained in the bottoms of the second distillation column. A threshold amount of penta sodium aminotri(methylphosphonate) was added as a scale inhibitor compound to achieve a concentration of such compound of less than about 80 ppm. Thereafter, the treated stream was fed to near the top of the second distillation column. The overhead vapors from this column were at 268.degree. F. and 42.4 psia, the bottom stream was at 275.degree. F. and 45.8 psia. Their compositions were as follows:

The bottom fraction from the second column was flashed to a pressure of about 22 psia. The vapor was essentially pure steam and a part of it was used with high pressure steam for direct injection into the second distillation column. The remainder, approximately 85 percent of the flashed vapor, was directly injected into the first column to supply both heat and ammonia-free stripping steam therein.

The bottom stream from the flasher was cooled to about 130.degree. F. and clarified with flocculating agents. The clarified bottom stream was then sent to treatment where aerobic bacteria under the influence of oxygen cause biological degradation of the reactive organic matter remaining in the water.

The effects upon purification of the waste water are illustrated by the following compositions for the initial feed water and for the effluent water sent to the biological treatment system.

This example represents the achievement of a reduction in ammonia content of more than 98 percent and in cyanide content of about 94 percent. Only about one pound of steam is required per gallon of feed in the process of this invention as compared to conventional ammonia stills, which require about 2 to 3 pounds of steam per gallon of feed.

The process of this example was run for 7 months with essentially no scale deposition on the trays of the fixed still. For a conventional prior art free and fixed still process operated under substantially identical conditions except that no anti-scaling agent is utilized the trays are generally completely plugged up with scale at about 6 months of operation of the process with scale building up gradually from the start of operation of the process.

The process of Example 1 is repeated except that the sulfate ion is allowed to increase in the initial waste water feed. When the concentration of sulfate ion in the waste water feed reaches above about 1500 ppm, the trays in the fixed still very rapidly plug with scale.

When an effluent of the type set forth in Example 1 is subjected to biological oxidation in the form of an activated sludge plant, the concentrations of toxic substances may be reduced as follows: phenol, > 1 ppm; thiocyanate, > 1 ppm; cyanides amenable to chlorination, > 1 ppm.

It is within the practice of this invention to make use of such apparatus, to use additional materials and processes as would be familiar to those skilled in this art. For example, the first and second distillations may be performed in one or more vessels. Also, reboilers and heat exchangers may be used at intermediate locations in both the first and the second distillations. Such variations are intended to be within the scope of the invention set forth in the claims.