The present invention relates to a process and apparatus for treating and detoxifying nitrite- and sulfite-containing caustic scrubber waste to a chemical form suitable for land application such as fertilizer or direct disposal. In particular, the process and apparatus of the present invention oxidizes nitrites and sulfites to nitrates and sulfates, respectively, and neutralizes the resulting solution to neutral pH's near 7. The process as described can be carried out in situ with existing NO.sub.x and SO.sub.x scrubber systems, or on stored scrubber waste.
Dinitrogen tetroxide (N.sub.2 O.sub.4) is used at East and West Coast space launch facilities as hypergolic fuel, but the liquid waste generated by the NO.sub.x scrubbers associated with those facilities poses a serious environmental problem. Industrial processes such as fired process heaters, fossil fuel fired boilers, production of nitric acid, and IC engine exhausts also have the potential of producing environmentally dangerous NO.sub.x and SO.sub.x emissions. Scrubber waste can also produce NO.sub.x emissions which makes the waste extremely hazardous since the OSHA STEL is 1 ppm.
Previous processes for treating and detoxifying wastes used strong acids such as nitric acid which made the final pH of the waste difficult to control and rendered the final product unsuitable for direct disposal unless additional treatment steps were taken for pH adjustment.
A process described in U.S. Pat. No. 5,206,002, for example, removes SO.sub.x and NO.sub.x gas by oxidation in the gas-phase, absorbs the oxidized species into solution, and then neutralizes the solution. This process does not, however, treat existing waste. The scrubbing of SO.sub.x and NO.sub.x streams is also known from U.S. Pat. Nos. 3,473,298; 4,784,835; 4,799,941; and 4,999,167.
Processes for the treatment of gas streams containing nitrogen oxides have also been described in U.S. Pat. Nos. 4,603,036 and 5,017,348 which use absorption of nitrogen oxides into an aqueous solution of hydrogen peroxide with the subsequent conversion of the nitrogen oxides to acids. The absorption solution in these known processes is acidic, and the final solution is not neutralized for disposal.
Other processes for the production of liquid fertilizer are described in U.S. Pat. Nos. 3,711,269; 4,013,443; and 5,211,735 which utilize reactions which include phosphoric acid, sulfuric acid, monobasic potassium phosphate (MKP), and ammonia. These conventional processes do not use MKP for the neutralization of salts-hydroxide strictly for the production of neutralized solution to be applied as liquid fertilizer.
It is an object of the present invention to provide a method and apparatus which produce a final product with high potassium and phosphate content directly suitable as liquid fertilizer or a base blend substrate for liquid or solid fertilizer.
It is another object of the present invention to produce a final product of neutral pH and low nitrite or sulfite content which is suitable to meet local, state, and federal EPA regulations for direct sewage disposal.
The foregoing objects have been achieved by the use of processes for detoxifying nitrite (NO.sub.2.sup.-) and sulfite (SO.sub.3.sup.2-) containing salts-hydroxides (e.g., NaOH, KOH, Ca(OH).sub.2) first by oxidation of the nitrites and sulfites to nitrates (NO.sub.3.sup.-) and sulfates (SO.sub.4.sup.2-), respectively, and second by the subsequent neutralization of the salts-hydroxide to a final neutral pH.
An advantage of the method of the present invention is that it is also applicable to most sulfite, nitrite, NO.sub.x, and SO.sub.x by-product streams which are prevalent in commercial processes. Consequently, the detoxification processes described herein, along with a scrubber operation such as that used in the launch scrubbers, can eliminate or at least greatly reduce environmental NO.sub.x /SO.sub.x emissions.
More specifically, processes which we have found to oxidize and neutralize a caustic waste solution containing NaNO.sub.2, NaNO.sub.3, and NaOH comprise a first process (a) on which there is a continuous flow oxidation of the waste by ozone (O.sub.3) followed by a neutralization step with KH.sub.2 PO.sub.4, or monobasic potassium phosphate (MKP); and, alternatively, a second process (b) comprising a one-step oxidation and neutralization with the addition of H.sub.2 O.sub.2 and MKP.
The first-mentioned process is amenable to batch treatment or continuous flow gas-liquid contacting towers, and to direct integration into scrubber system operations with the addition of ozone in the NO.sub.x scrubber stream. The details of these processes also include configurations which provide a "secondary" scrubber safeguard against NO.sub.x emissions (OSHA STEL of 1 ppm) utilizing additional scrubber waste vessels connected in series with the primary treatment vessel.
A summary of the reactions involved in the first-mentioned process in accordance with the present invention is as follows:
(A) Oxidation EQU NO.sub.2.sup.- +O.sub.3 NO.sub.3.sup.- +O.sub.2 [ 1] EQU SO.sub.3.sup.- +O.sub.3 SO.sub.4.sup.- +O.sub.2 [ 2] PA0 (B) Neutralization EQU NaOH(+NO.sub.3.sup.- +SO.sub.4.sup.-)+KH.sub.2 PO.sub.4 (K,Na,PO.sub.4,NO.sub.3,SO.sub.4).sub.salts +H.sub.2 O [3] PA0 (A) Oxidation EQU NO.sub.2.sup.- +H.sub.2 O.sub.2 NO.sub.3.sup.- +H.sub.2 O [4] EQU SO.sub.3.sup.- +H.sub.2 O.sub.2 SO.sub.4.sup.- +H.sub.2 O [5] PA0 (B) Neutralization EQU NaOH(+NO.sub.3.sup.- +SO.sub.4.sup.-)+KH.sub.2 PO.sub.4 (K,Na,PO.sub.4,NO.sub.3,SO.sub.4).sub.salts +H.sub.2 O [3] PA0 (a) evolution of NO from solution EQU 3NO.sub.2.sup.- +2H.sup.+ 2NO(g)+NO.sub.3.sup.- +H.sub.2 O [6] PA0 (b) production of "NO.sub.x " gases ##EQU1##
The second-mentioned detoxification process offers simplified treatment which does not require ozone. The emission of any NO.sub.x resulting from the waste acidification is trapped in a "secondary scrubber" vessel of untreated scrubber waste or redirected to an NO.sub.x scrubber tower.
The reactions summarizing the second-mentioned treatment process in accordance with the present invention are as follows:
The neutralization step of both processes produces a fertilizer with high potassium and phosphate content which can serve as a base blend or as a stand-alone liquid product for numerous agricultural applications. The acid utilized in the process, namely monobasic potassium phosphate (MKP), is a solid characterized by low toxicity, low cost, and pK.sub.a 's which provide for a controlled neutralization process resulting in fertilizer having a pH of approximately 7.
A further advantage is that the detoxification of scrubber waste can be accomplished at ambient temperatures and pressures, with commercially available equipment such as mixing tanks, mixers, ozone generators, and associated valves and fittings. The NO.sub.x emissions resulting from the oxidation and neutralization steps can be controlled below 1 ppm (by volume), the current OSHA STEL.
We have also discovered that the neutralization process of nitrite-containing waste must result in pH's slightly basic (greater than 7) in order to mitigate the evolution of toxic NO.sub.x. When the pH of nitrite containing solutions drops below a pH of 7, nitric oxide can be produced to react with oxygen in the air and produce NO.sub.2 and other nitrogen oxides (NO.sub.x) as follows:
The process of the present invention minimizes the above reactions (Equations 6 through 8) first by oxidizing the nitrites and then neutralizing the reaction product to a pH greater than 7 so as to minimize the NO release described in Equation 6 as the increased concentration of H.sup.+ shifts the reaction to the right.
The final product of the oxidation and neutralization process of the present invention is usable, as noted above, for direct land application or disposal. Liquid fertilizers have become a viable source of agricultural nutrient addition. Consequently, the conversion of the scrubber waste to a usable fertilizer form potentially provides for at least part of the cost recovery of the scrubber operation and detoxification processes.
There is a particularly high demand for fertilizer with the potassium content being similar to the phosphorous content as noted in U.S. Pat. No. 3,711,269. Fertilizer is typically rated as to its nitrogen, phosphorous, and potassium content, N-P-K. The numbers are weight per cent expressed for the composition of N as atomic nitrogen, the equivalent of P as P.sub.2 O.sub.5, and the equivalent of K as K.sub.2 O. The neutralized waste stock generated by the methods of the present invention produce liquid fertilizer with potassium (K.sub.2 O) to phosphorous (P.sub.2 O.sub.5) ratios of 1 to 1.5. Higher final concentrations of potassium are possible for scrubber waste solutions comprised of potassium hydroxide. Without departing from the principles of the present invention, low N content can be augmented by addition of urea and ammonium nitrate, as is commonly practiced in the fertilizer industry, to produce a "complete" fertilizer.
Definitions for certain terms used throughout this Specification are as follows:
Salts-Hydroxide or Caustic: Solutions comprised of hydroxides such as Ca(OH).sub.2, KOH, and NaOH, with pH's generally at 12 or higher. Caustic solutions are used in effluent scrubbing operations due to their reactive capabilities of removing SO.sub.x and NO.sub.x gas producing sulfites and nitrites in solution, respectively.
NO.sub.x : A collective term used to describe the family of nitrogen oxide gases. These gases are generally produced from nitric oxide NO and subsequent oxidation with oxygen in air. Nitrogen dioxide NO.sub.2 is one of the more toxic oxides with an OSHA STEL of 1 ppm.
Liquid Fertilizer: Aqueous solutions containing varying weight percents of nitrogen, potassium, and phosphorous for use in land application fertilization. Liquid or solid fertilizer is generally rated in weight percent as to its total nitrogen content N, total potassium content K as K.sub.2 O equivalent, and total phosphorous content P as P.sub.2 O.sub.5 equivalent, and expressed as N-P-K.
MKP: Monobasic potassium phosphate, or KH.sub.2 PO.sub.4 ; molecular weight 136.085.