FIELD OF THE INVENTION

The present invention relates to the combustion of cellulose bleach plant waste liquors, particularly waste liquors from bleaching processes comprising chlorine dioxide bleaching steps. The invention also relates to the environmentally safe disposal of such liquors and to their reuse.

BACKGROUND OF THE INVENTION

The discharge of cellulose bleach plant waste liquors from bleaching processes comprising chlorine dioxide bleaching steps is considered to be one of the most important remaining environmental issues for kraft pulp mills (Ken L. Patrick et al. , Closing the loop - The effluent-free pulp and paper mill, Pulp and Paper International. April 1994, page 1 - 27. Such waste liquors have a low solids content and, i.a., contain a variety of chlorinated organic compounds resulting from the action chlorine on wood components. These chlorinated organic compounds are toxic and highly resistant to biodegradation. At present their traditional discharge to the biosphere is severely restricted and may be entirely prohibited by law in the near future. The same also holds true for waste liquors generated during THF (totally chlorine free) bleaching of pulps. Although the content of chlorinated compounds in TCF effluent streams is low they may contain other compounds hazardous to life which should be destroyed prior to discharge.

The main thrust in the search for elimination of this problem has been directed to the recirculation of bleach

plant waste liquors. A number of processes including a concentration (evaporation) step, a dechlorination step and a dry incineration step have been proposed. The description of these tentative processes in the cited publication indicates a number of major obstacles that have not yet been solved.

The need for a method for destruction of environmentally harmful cellulose bleach plant liquors thus is evident.

Occasionally inorganic matter contained in cellulose bleach plant liquors might be considered for reuse in the mill. In such case the organic matter contained therein, including environmentally or otherwise harmful organic compounds, would have to be destroyed prior to reuse.

OBJECTS OF THE INVENTION

It is a primary object of the present invention to provide a method for treatment of cellulose bleach plant waste liquors, particularly waste liquors from bleaching processes comprising chlorine dioxide bleaching steps, to make their contents environmentally acceptable or to make them fit for reuse in the mill.

It is another object of the invention to design such method in an energy-efficient way.

A further object of the invention is to provide a method for environmentally safe disposal of such bleach plant liquors.

An additional object of the invention is to provide a method for reuse of such bleach plant liquors.

SUMMARY OF THE INVENTION

The present invention is a method for complete combustion of cellulose bleach plant waste liquors, particularly waste liquors from bleaching processes comprising chlorine dioxide bleaching steps, having a dry substance net calorific value of 3.0 - 9.0 GJ/metric ton, in a reactor to obtain inorganic products suitable for discharge into the environment or reuse in the mill, particularly in form of their aqueous solutions.

According to the invention, the method for complete combustion of cellulose bleach plant waste liquors, particularly waste liquors from bleaching processes comprising chlorine dioxide bleaching steps, having upon concentration a dry substance net calorific value of 3.0 - 9.0 GJ/metric ton, preferably 5.0 - 9.0 GJ/metric ton, comprises:

- providing a bleach plant waste liquor having a pH of at least 5.0, preferably of at least 8.0, and being concentrated to a solids content of at least 35 % by weight, preferably from 40 % to 60 % by weight,

- feeding, separately of each other, said concentrated bleach plant waste liquor and an oxygen containing gas to a burner in a reactor, said burner including a liquor lance and an atomizing nozzle having a downstream front side,

atomizing the concentrated bleach plant waste liquor by contacting it with said oxygen con-taining gas inside the burner or immediately in front of the burner, for forming a spray of atomized concentrated bleach plant waste liquor emanating in a generally axial direction in a divergent manner from the

downstream front side of the nozzle to sustain a stable flame in the reactor,

wherein said oxygen containing gas comprises more than 50%, preferably more than 80 %, of the oxygen required for complete combustion (stoichiometric ratio of l.o) supplied to the reactor, and wherein the total amount of oxygen containing gas being supplied to the reactor corresponds to a stoichiometric ratio of from 1.0 to 1.5, preferably from 1.0 - 1.2.

An important advantage of the method according to the invention is that, in spite of the very low calorific value of the fuel, the flame in the reactor is self- igniting at operating conditions and that no extra fuel or, in some extreme cases, only a minor quantity of extra fuel is needed besides the concentrated bleach plant waste liquor to sustain the flame. The calorific value of said minor quantity of extra fuel preferably amounts to less than 20 % of the calorific value of the fuel.

In a preferred embodiment of the method according to the invention the combustion is carried out by use of a burner of the type disclosed in the co-pending U.S. Patent Application Ser. No. 07/929,875, the contents of which are incorporated into this specification by reference. The nozzle of this burner is of a "twin-fluid type" making use of relatively high gas flow rates for the supply of energy needed for atomization. Ato ization can be further enhanced by flashing the concentrated cellulose bleach plant waste liquor into the reactor; in this case the liquor is pre-heated to a temperature above its boiling point at the operating pressure of the reactor.

Particularly preferred is the use of a burner head including the nozzle arrangement of the embodiment shown in Figs. 3 and 4 of US Patent Appln. Ser. No. 07/020,875. The predominant direction of atomized liquor spray emanating from the nozzle to form the flame is an axial direction of the terminal liquor lance/nozzle portion of generally cylindrical shape. A major difference between conventional oil burners and the burner disclosed in said U.S. Patent Application is that the latter provides for particularly efficient atomization; a stable flame can be formed by use of a considerably lower amount of oxygen containing gas. A reactor suitable for use in the process according to the invention is disclosed in U.S. Patent No. 4,808,264.

Due to the very low calorific values of some of the waste liquors considered as feedstock in the process according to the invention support f el, such as gas, diesel oil or heavy oil, may be added to an extent sufficient to raise the net dry substance calorific value of the combined concentrated waste liquors and the support fuel to at least 3.0.

The rheological properties of the concentrated cellulose bleach plant waste liquors affect their atomization in the burner; this atomization (physical disintegration to small droplets or particles) is of major importance for successful combustion. For this reason it is preferred for the viscosity of the concentrated bleach plant waste liquor to be lower than 200 cSt; particularly preferred is a viscosity at 100 ^* C of about 100 cSt or lower. The gas flow rate in the burner nozzle is preferably from 40 to 350 m/sec.

Pre-heating of the oxygen containing gas to at least about 100° C, preferably to about 250 - 350° C, allows to increase the gas velocity at constant mass feed rate.

The concentrated cellulose bleach plant liquors that can advantageously be incinerated by the process of invention have an ash content of from about 20% to about 60 % by weight on dry solids and a chloride content of from about 3 to about 40 % on dry solids. At stoichiometric combustion the preferred air/fuel weight ratio for bleach plant waste liquor concentrates at is from about 2:1 to about 4:1. The preferred operation temperature of the reactor is from 700° to 1,100°; most preferred is a temperature from about 850° to about 1,000° C.

The method of environmentally safe disposal of cellulose bleach plant waste liquors, particularly waste liquors from bleaching processes comprising chlorine dioxide bleaching steps, according to the present invention comprises the steps of:

providing a cellulose bleach plant waste liquor,

concentrating said bleach plant waste liquor to a solids concentration of from 45 % to 70 % by weight, more preferred of 45 - 55 % by weight,

feeding, separately of each other, said concentrated bleach plant waste liquor and an oxygen containing gas to a burner in the reactor, said burner including a liquor lance and an atomizing nozzle having a downstream front side, the terminal front end portion of said burner having a generally cylindrical configuration,

atomizing the concentrated bleach plant waste liquor by contacting it with said oxygen containing gas inside the burner or immediately in front of the burner, for formation of a spray of atomized concentrated bleach plant waste liquor emanating in a generally axial direction in a divergent manner from the downstream front side of the nozzle to sustain a stable flame in the reactor, wherein said oxygen containing gas comprises more than 50 % of the oxygen required for complete combustion (stoichiometric ratio of 1.0) supplied to the reactor, the total amount of oxygen containing gas supplied to the reactor corresponting to a stoichiometric ratio of from 1.0 to 1.5, preferably from 1.0 - 1.2.

discharging the inorganic salts formed in the reactor chamber in molten form,

quenching said inorganic salts with water or an aqueous medium to form an aqueous salt solution preferably having a solids content of from 0.1 to 25 %,

optionally adjusting said aqueous salt solution to a pH of 10 or lower by addition of a neutralizing agent, and

disposing of said optionally pH-adjusted aqueous salt solution to the environment.

The preferred neutralizing agent is carbon dioxide. It is also preferred to use the flue gases formed during the combustion of the concentrated bleach plant waste liquor for neutralization. According to an advantageous aspect of the invention the cellulose bleach plant waste liquor has a pH of at least 5.0, preferably of at least 8.0, or

it or its concentrate is brought to such pH by addition of a base, particularly by addition of sodium hydroxide or sodium carbonate. Particularly preferred is the integration of the quenching and neutralization steps into one single step. An suitable apparatus for such combined quenching and neutralization, by quenching the smelt in presence of the combustion gases formed in the incineration step, is described in the PCT Application WO 91/19042, the contents of which are hereby incorporated into this specification by reference.

The method for recovery of inorganic chemicals from cellulose bleach plant waste liquors, particularly waste liquors from bleaching processes including chlorine dioxide bleaching steps, according to the invention comprises:

providing a cellulose bleach plant waste liquor,

- concentrating the bleach plant waste liquor to a solids concentration of from about 35 % to about 70 % by weight, more preferred from about 45 to 60 % by weight,

- feeding, separately of each other, said concentrated bleach plant waste liquor and an oxygen containing gas to a burner in a reactor, said burner including a liquor lance and an atomizing nozzle having a downstream front side, the terminal front end portion of said burner having a generally cylindrical configuration,

atomizing the concentrated bleach plant waste liquor by contacting it with said oxygen containing gas inside the burner or immediately in front of the burner, for forming a spray of atomized concentrated

bleach plant waste liquor emanating.in a generally axial direction in a divergent manner from the downstream front side of the nozzle to sustain a stable flame in the reactor, wherein said oxygen containing gas comprises more than 50 % of the oxygen required for complete combustion (stoichiometric ratio of 1.0) supplied to the reactor, the total amount of oxygen containing gas being supplied to the reactor corresponding to a stoichiometric ratio of from 1.0 to 1.5, preferably from 1.0 - 1.2,

discharging the inorganic salts formed in the reactor chamber,

- quenching said inorganic salts with water or an aqueous medium to form an aqueous salt solution preferably having a solids content of from 0.1 to 25 %,

- transferring the aqueous inorganic salt solution thus obtained to a chemicals recovery or purification stage for reuse.

Preferably the pH of said aqueous inorganic salt solution is lowered by addition of carbon dioxide from flue gases prior to said transferral. It is also preferred for the bleach plant waste liquor or the concentrated bleach plant waste liquor to be brought to a pH of about 5, preferably of about 8 or higher. According to an advantageous aspect of the invention the aqueous inorganic salt solutions is depleted of chloride ion, for instance by use of ion exchange resins, prior to reuse.

Further particulars of the invention are evident from the appended claims and the following detailed description of a preferred embodiment. The invention will be more fully

understood by studying this description which is given by way of exemplification only and not intended to limit the scope of the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

Three experiments were carried out with chlorine dioxide bleach plant waste liquors of different composition (Examples 1, 2, and 3). Percentages are by weight if not indicated otherwise.

General procedure

By combination of the waste liquors from the various bleaching and alkaline washing (extraction) steps of a chlorine dioxide bleach plant a combined bleach plant waste liquor is obtained. This combined waste liquor has a pH in the order of 9.0. The alkaline pH of the combined waste liquor is a result of the neutralization of the acidic waste liquors from the chlorine dioxide stages by the alkali (sodium hydroxide) used in the extraction/washing stages. In case the waste liquors or the combined waste liquor have a pH lower than about 8, it is preferably brought to that or a higher level by the addition of alkali, for instance sodium hydroxide or sodium carbonate. Preferably any material precipitating during neutralization, such as calcium carbonate, is separated from the aqueous solution, for instance by filtration. The alkaline combined waste liquor is then concentrated in an evaporator. Solids content and other relevant data are given below. The concentrated combined waste liquor has a pH of about 9.5.

The concentrated alkaline combined waste liquor is fed to a prototype burner incorporating the design principles

disclosed in U.S. Patent Application Ser. No. 07/929,875 (embodiment shown in Figs. 3 and 4) and mounted in an incinerator (reactor) of the type disclosed in U.S. Patent No. 4,808,264, the contents of which are hereby incorporated into this specification. The front portion of the nozzle/liquor lance har a generally cylindrical configuration with a central axis. The fuel and the air emerge at the front side of the nozzle in a generally axial direction and form an atomized spray in the immediate vicinity of said front side; the atomized spray diverges radially while maintaining its generally axial direction under formation of a stable flame. The fuel can also be atomized within the nozzle (U.S. Patent No. 4,808,264, embodiment shown in Figs. 1 and 2). In the latter case the atomized fuel in combination with air leaves the nozzle at its front side in a generally axial direction while diverging radially. The result in terms of flame configuration and flame location in respect to the burner nozzle is similar in both cases, i.e. a flame having a major extension in the downstream axial direction and being set off from the front side of the nozzle.

The refractory lined reactor can be operated at pressures ranging from ambient up to 2,5 MPa. By operating at higher pressures the heat generated in the process can be transformed into low pressure steam in accordance with the principles disclosed in U.S. Patent No. 4,808,264. Air was used as oxidant in all experiments. The main incineration products are a smelt containing non-volatile inorganic matter, including inorganic salts, and flue gas. The smelt was quenched in a quench water chamber positioned beneath the reactor (combustion) chamber. Compositions of concentrated alkaline waste liquor and incineration products as well as reaction parameter values are given in Tables I - VI below.

TABLE I

Concentrated alkaline combined waste liquor (fuell

Example Fuel feed rate Feed Solids Calorific # (solids, kg/h) temp. content value (MJ/kg

(°C) (%) solids)

1 1,025 90 50.4 8,100

2 8,040 95 50.6 8,975

3 4,550 98 41.0 3,450*

*) includes added support fuel (diesel oil; calorific value 41 MJ/kg; addition rate 200 kg/h)

TABLE II

Oxidant fairl

Example Air factor Air flow Air temperature

# (m ³ N/h) (preheated to °C)

1 1.2 3,012 250

2 1.1 18,448 250

3 1.3 17,050 250

TABLE III

Concentrated alkaline combined waste liquor, solids composition, in % by weight

Example Na*) S Cl C H 0**)

#

1 18,0 0.2 7.9 31.7 3.1 32.4

2 21.5 0.22 5.2 23.3 5.1 44.5

3 20.7 0.3 5.7 20.7 3.4 balance

*) Includes other cations, such as potassium. **) Includes trace anions, such as phosphate.

TABLE IV

Reactor operating data

Example Temp. Pressure # (°C) (bar)

1 1085 1.35

2 880 20.0

3 975 1.35

TABLE V

Flue gas composition (% by volume) and total gas flow

Example C0 ₂ 0 ₂ N ₂ H ₂ 0 total gas flow # (m ³ N/h)

1 11.8 2.3 51.8 34.1 4,599

2 8.8 1.1 45.4 44.7 32,120

3 10.7 2.2 47.5 39.6 7.650

TABLE VI

Smelt, composition*! (% by weight fmol/ko solids1.

Example Na ₂ C0 ₃ Na ₂ S0 ₄ NaCl NaOH Total # (kg/h)

I 61.3 2.1 31.2 5.3 422

[2.44] [0.06] [2.26] [0.56]

II 70.3 1.8 16.9 10.8 3,907

[3.25] [0.06] [1.41] [1.32]

III 67.3 1.7 23.7 6.3 1,750 [3.11] [0.06] [1.97] [0.77]

*) at reactor exit

The flue gas exiting the reactor (incinerator) contains only small amounts of particulate matter which can be removed in a scrubber or an electrostatic precipitator. The quenched combustion products solution (solution of inorgnic salts) is discharged at the bottom of the reactor through a cooled outlet lead (passing a steam generator) into a storage tank for monitoring its composition, storage and/or further dilution with water. Alternatively the smelt can be drafted into a separate tank for solidification and subsequent dissolution in water or an aqueous medium.

The resulting alkaline aqueous solution can be advantageously neutralized or brought to a pH of lower than about 10 before recirculation for chemicals recovery or discharge into a suitable water recipient. Acids or, preferably, carbon dioxide can be used for such pH- reduction or neutralization.

After neutralization with carbon dioxide (from pulping chemicals recovery flue gases; the flue gas from the incineration step containing carbon dioxide can also be used for this purpose) the solution is ready for discharge to a suitable water recipient.

The quenching liquid is water or an aqueous solution, preferably a condensate from the cooling of flue gas in a scrubber.