Carbonizing of black liquor The most important components of wood are essentially a-cellulose, consisting of hexose chains, hemicellulose, consisting of pentose chains, as well as lignin, also consisting of vanillin-like units in a variety of forms. The purpose in pulping is to recover as much as possible both a-and hemi- cellulose by dissolving the lignin component bonding these together. Lignin dissolves in sulphate cooking liquor with the aid of white liquor NaOH and Na2S as sodium lignosulphonate. After separating pulp from alkaline cooking liquor by filtering, the remainder will be black liquor. Cooking chemicals are recovered therefrom by burning black liquor concentrated as highly as possible in a soda recovery boiler. The chemistry of combustion is also quite complicated, but briefly it results in decomposition of sodium lignosul- phonate, and the sulphate sulphur contained therein reduces to sulphide and the sodium is present in molten ash as Na2CO3. This molten ash is quenched and dissolved, and the result is green liquor as it is yellowish green in colour.

The green liquor is causticized, i. e. a carbonate ion is replaced with a hydroxyl ion obtained from Ca (OH) 2 by using CaO.

In brief, a currently used method involves operations as follows 1) Pulping through the use of white liquor 2) Separation of pulp from black liquor for further processing 3) Concentration of black liquor 4) Burning of concentrated black liquor in a soda recovery boiler, including a secondary condensate which contains the methylmercaptan and other foul- smelling components. Energy is produced.

5) Dissolution of molten ash in water (green liquor) 6) Causticization of green liquor to white liquor (contains NaOH and Na2S) 7) Calcination of spent lime (CaC03, lime sludge) in a lime sludge reburning kiln for CaO.

In the currently used method, black liquor is fed into a soda recovery boiler at 70% and burned. Cooking chemicals melt and form Na-carbonate and first Na-sulphate, which then reduces at 1100°C to Na-sulphide. If there is too much combustion air, reduction will be incomplete, and if there is too little thereof, too much carbon will remain in the chemicals. The employed soda recovery boiler must be repaired from time to time because of oxidations caused by corrosion. This results in down-times at the factory. One reason for corrosion is chloride-containing waste liquor from pulp bleaching, which should be treated separately, if possible.

In a method of the invention, operations 4-7 of the above-described prior known method are replaced by applying operations as follows : 8) Carbonization of concentrated black liquor to coal dust, which contains sodium as soda (Na2CO3). In the carbonization process, coal dust and soda are separated while dry, and sulphur separates as hydrogen sulphide (H2S) and as COS breaking down to hydrogen sulphide, which are absorbed to an NaOH-solution and resulting in white liquor required for cooking.

9) Causticization of soda to NaOH. This particular operation can be carried out at almost 100%, as there is no Na2S present. (In a currently used method, with sulphide present, the sulphide present drops the degree of causticization to about 78%).

As long as coal dust and soda are absorbed along with sulphur compounds as an alkaline suspension, and carbon being separated therefrom after the dissolution of soda, it is feasible to use a conventional causticizing process for the preparation of white liquor. In that case, operations 6) and 7) must be included in this process with the above-explained downsides.

This method of the invention is so economical that it enables the utilization of the entire tree, including its stump, branches, bark, and sprigs, as the regeneration costs for chemicals are substantially lower. In addition, the need for equipment, operating costs and losses are reduced respectively.

Chemical principles for the novel method Since no thermodynamic values have been found for lignosulphonate as a function of temperature, the most unfavourable option is used: sodium sulphate (Na2SO4) in the reduction of sulphur. This refers to pyrolytic decomposition in a reducing atmosphere.

The lignin of black liquor decomposes in pyrolysis as temperature is rising to form H20, CO2, CO, and C. What ultimately remains is pulverized coal.

Cooking chemicals react spontaneously with pyrolysis products to ultimately form Na2CO3, COS, and H2S, with C and CO functioning as reducing agents.

In addition, solid intermediates may comprise for example Na2S, Na2S2, Na2S203. Gaseous intermediates may comprise COS, S2, H2S2 or CS2, as well as mer captans (CH3) S (CH3), (CH3) S2 (CH3), (CH3) SH and (CH3) S2H.

Black liquor carbonization process Drying is effected on black liquor as highly concentrated as possible. Drying must always involve coal dust for bringing it as evenly distributed as possible to a reduction process. The best way to do this is to admix in black liquor some of the coal produced thus far. As temperature is rising, C and CO activate, together with water vapour resulting from drying, a series of decomposing and reducing reactions, the ultimate result being C and Na2CO3, as well as gases H20, CO, COS and H2S.

Research work was prepared for by building a highly insulated reactor drum (D = 320 mm, L = 1000 mm). Heating was by liquid gas. In order to avoid caking, the drum was provided with 30 mm steel balls, as well as with a long (D = 50 mm) steel rod. In order to commence another run, some coal dust was left in the drum from the preceding run, and hence the necessary coal was already there. The feed was effected with 30% black liquor, which is highly pumpable, even when cold. The black liquor was metered with a gear pump, operating at the intervals of a few seconds, minutes. The feed solution was metered in the amount of 27 liters over the period of 7 hours.

It was noted that the reaction progressed most conveniently at a tempera- ture of between 420° and 440°C. However, information was not available regarding the precision of temperature measurement. This developed hyd- rogen sulphide, which was absorbed with a wet scrubber into a 10% lye solution. The water vapour produced by drying expelled the resulting coal dust out of the reactor. Whenever necessary, it was possible to run super- heated water vapour (whose temperature had been adjusted to the same temperature range). In order to prevent the condensation of water vapour, the apparatus had been insulated as thoroughly as possible.

The developed coal dust must be separated in dry state from a product stream. By using an enhanced feed, the dried black liquor has no time to pulverize to a dust form, the coal being thus more readily separable as powder by means of dry separators. Consequently, the reducing process must be provided with a sufficient delay time.

Since the available dry separator requires major gas flows, only a water scrubber was used upstream of a lye scrubber. As slightly alkaline tap water was used first, the scrubber's exhaust gas had a strong odour of hydrogen sulphide. All coal along with Na2CO3 remain in said water scrubber.

When tap water was replaced with feed water used in a soda recovery boiler, the resulting COS did not fully absorb therein. The water was colored dark green and the gas evaporating from the water had a very strong foul odour.

The smell of hydrogen sulphide was not detected in this water, thus not retained in the discussed water scrubber. Alkaline water decomposes COS spontaneously into hydrogen sulphide (H2S) and carbon dioxide (CO2), even at room temperature. By treating COS separately after a dry separation process, the hydrogen sulphide can be recovered from other gases.

The pyrolysis product obtained from the dry matter of black liquor comprises carbon and soda roughly to less than a half. When using a dry separator, the coal dust, and Na2CO3 contained therein, can be recovered separately and the gases COS and H2S are recovered by means of an alkaline scrubber.

Since the reaction of CO2 with NaOH is very slow, Na2CO3 develops in quite a small amount. Some of the coal appeared in the form of such a fine dust that it was air-borne as the reactor lid was opened.

If coal dust is not allowed to become very finely pulverized, it can be separated in dry state from an exhaust gas flow. This facilitates greatly the treatment thereof, the sulphide sulphur having been recovered separately.

After the recovery of chemicals, the obtained raw coal can be calcinated further for separating volatile components. Hence, it can be pulverized to very fine dust, which requires no further elutriation. It can also be pelletized and activated.

An advantage gained by very fine coal dust is that its reactions with a gas phase are very rapid. Furthermore, the extraction and regeneration of chemicals can be effected by using methods which cannot be otherwise applied to the sludge of conventional coal powder.

Very fine coal dust and coarser coal powder in refined form have their own important applications.

The access of atmospheric oxygen into the reactor must be blocked as effectively as possible. At one point some air was accidentally allowed into the reactor, which brought the reactor instantly to white heat. Nothing indicative of explosion was detected.

Thus, in practice, the simple test indicated as follows : Black liquor can be pyrolyzed in a reducing atmosphere to coal dust.

Chemicals used in pulping are all recovered in the pyrolysis of black liquor.

Na2CO3 migrates along with coal in a solid phase, and sulphide sulphur in a gas phase.

Na2CO3 was dissolved with water from coal. It should be appreciated that Na2CO3 has a negative solubility, i. e. more of it dissolves in cold water than in hot water. Sulphide sulphur is recovered from a gas phase by absorbing it in an alkaline solution.

An advantage gained here is that there is a possibility of effecting the causticization of Na2CO3 to NaOH by almost 100%. When sulphide sulphur is present, the causticization reaction progresses not higher than by about 78%.

The causticization is followed by blending together the obtained Na2S-and NaOH-solutions for the preparation of alkaline cooking liquor.

The finest coal dust and CO contained in exhaust gases will be exploited as a heat source in a carbonization process.

When using the above-described method, a soda recovery boiler becomes superfluous. The factory's own energy demand can be satisfied by using refined coal dust, either by a gas turbine or a residual heat boiler, as much as its own demand requires. The rest of the produced coal can be sold and used in place of heavy fuel oil, or else as a source of coal for various applications.

Example 1 Tests and apparatus were based on carbonization experiments of sugar cane waste (bagasse). Pyrolysis was checked over the range of 250°-710°C at the steps of 20°C. Half-way through the temperature range, the temperature step was 10°C. The conducted study shows that, when pyrolysis was performed between 410°-420°C, the yield of coal is at its optimum, and inorganic components entrapped within coal could all be dissolved most effectively.

Example 2 Above a liquid-gas grill a rotatable steel drum (L = 1 m, D = 0,3 m) was built. The drum is topped by a hood for cutting heat losses. Inside the drum 25 mm nuts for grinding bodies were installed. The ends had openings for feeding black liquor and discharging gases. The purpose was to clarify the behaviour of black liquor in pyrolysis.

The drum was supplied with 10 liters of 70% black liquor. By the time the last moistures started to escape from black liquor, the same began to expand or swell like foamed plastic, and nearly dry black liquor extruded out of the

ends like a sausage along with exhaust gases. This was collected and re-fed in small batches.

When the dry stock started to carbonize, plenty of coal dust emerged with pyrolysis gases. When the discharge of gas ceased almost completely, heating was stopped. Temperature could not be measured as yet, but in light of earlier experience it was probably 400°-500°C. The obtained coal was treated with water to extract cooking chemicals and then with 10% HCI to extract water-insoluble metals originating principally from the drum surface.

The amount of ash was quite insignificant.

This qualitative test series demonstrated that: - carbonization of black liquor is highly successful, - special attention in carbonization should be paid to expansion process, and grinding associated therewith, - when black liquor was added, a sudden increase of moisture caused a discharge of the already carbonized finest fraction along with developing steam. In other words, the feed must be added in small amounts and at a relatively high frequency, preferably continuously.

- the nuts used as grinding bodies were not good. These should be equal- sized steel balls. Smooth balls grind more uniformly, and metal from the walls ends up in coal as little as possible.

Pyrolysis gases in the carbonization process had a strong odour of hydrogen sulphide, which prompted a suspicion that sulphate had reduced to sulphide, which was expelled by carbon dioxide as H2S. A H2S-content of more than 50 ppm was measured near the drum.

Example 3 In the next run, the amount of black liquor fed in the beginning was only 5 liters and when pyrolysis gases began to appear, another 5 liters of black

liquor was added. This resulted in very fine coal dust which, upon opening the box lid, was air-borne for a long time. It was noted that pyrolysis gases had a strong odour of hydrogen sulphide. Unfortunately, the drum temperature could not be measured.

In the next run, black liquor was added in 2-liter batches up to 10 liters, whenever pyrolysis gases began to form. This time, care was taken that as little coal dust as possible would escape along with pyrolysis gases.

Concentrations of more than 52 ppm were measured with a portable hydrogen sulphide meter in the proximity of the drum's discharge opening for gases. Exhaust gases were sucked out by a flue gas extractor.

The obtained coal was studied and it was found that all black liquor had carbonized. By leaching with hot water, calcinated soda was extracted from coal. Coaling was followed by only a slight accumulation of ash, and even that was greenish as a result of metal alloys present in the drum wall and grinding bodies.

Based on the tests, it could be confirmed that the black liquor sulphur in lignosulphonate used for pulping in Finland can be reduced at a temperature considerably lower than that used today for the same purpose. Thus, 300°- 500°C will be sufficient instead of more than 1100°C used at present. This reduces decisively the costs of recovering pulping chemicals. Theoretical examination prompted a further study of whether sulphur can be recovered separately in a gas phase and soda in a solid phase.

The examination came up with a hypothesis that it was first necessary to dry the black liquor evaporated to a concentration as high as possible, but the resulting water vapour or steam is needed in the immediately following reduction process, wherein sodium sulphate (Na2SO4) developed from lignosulphonate is obtained as soda (Na2CO3) and sulphur compounds are

obtained in a gas phase (H2S and COS). The discussed reaction required the presence of carbon (C), carbon monoxide (CO), water vapour (H2O), and carbon dioxide (CO2). The reactions include a multitude of intermediate processes and intermediate products, yet the final result comprises soda (Na2CO3) and hydrogen sulphide (H2S).

Example 4 The preceding test results supported the theory. Based on said results, still some new calculations and modifications were made in the apparatus. The apparatus was subjected to modifications as follows : - the drum's bearings were replaced and provided with cooling.

- the rotating through-holes at both ends of the drum were provided with packings.

- a thermometer was installed for measuring the drum's internal temperature, which measured the temperature at the accuracy of about a degree, but the real temperature is probably 10°-20°C lower due to thermal radiation from the drum surface to the sensor. The sensor's measuring point was in the middle of the drum in longitudinal direction, so the temperatures of the ends are probably within the same accuracy range.

- although the feed of 30% black liquor produced water vapour, it was desirable to make sure that steam required by reduction reactions would be available. Therefore, alongside the feed was fitted a duct for supplying the drum thereby with high-temperature superheated steam, which had been preheated while the drum was heated. For steam generation, the duct was supplied with a minor flow of water. The amount of steam is not of major importance, as long as it is sufficiently available.

- the drum's rotating speed could be adjusted by adjusting the motor's cycling speed.

the previously used 25 mm nuts were replaced with 25 mm hard chrome- plated balls, the number of which is 200. In addition, the drum was fitted with (at least) one 50 mm thick rod with a length almost equal to that of the drum for enhancing the grinding during the drying process. a small gear pump was installed for metering black liquor, which had a delivery of 10 seconds at the intervals of one minute. The metering and rest periods could be adjusted steplessly. The employed setup delivered 28 liters of 30% black liquor over 7 hours. at the discharge side a dry separator for coal dust was first built. This was carefully insulated thermally in order to prevent the condensation of water vapour. However, since the coal appeared in such fine dust that it was capable of penetrating through dry separators, a decision was made to replace it with a water scrubber. the entire support frame of the apparatus was reconstructed. In fact, the only components of the old apparatus included a drum and a liquid-gas heating mechanism and an insulated hood for reducing heat losses. the water scrubber was highly effective in separating coal dust. It was thought that hydrogen sulphide would all be absorbed in the downstream scrubber. Then, as the employed tap water was replaced with clean boiler feed water, it was wondered why its colour turned dark green. The water emanated pungent-smelling white steam, although its temperature was about 20°C. Theoretical investigation indicated that it was carbon oxisulphide (COS), which decomposed in slightly alkaline tap water to CO and H2S, but clean neutral water did not decompose this particular gas. this indicates that temperature was within a favourable range, as COS developed, i. e. the sulphur of black liquor can be separated in a gas phase (CO and H2S). for a second separator, in series with the preceding one, a venturi scrubber was built, wherein a 10% NaOH solution was sprayed at a 10 bar pressure for recovering hydrogen sulphide and fine coal dust residues. This scrubber was so powerful that it generated a sufficient

negative pressure for sucking gases from the reactor. No odour of hydrogen sulphide could be identified in exhaust gases. Since the reaction of CO2 with lye is very slow, it is possible to conclude that the recovery of sulphur compounds has been successful according to theory. The small amount of C02 absorbed does not substantially hamper causticization.

One week of test runs was enough to prove the theory truthful. However, the runs were continued for producing a sufficient amount of coal. Samples of the coal will be supplied to potential users.

Conclusions The theory for the method proved correct. The discussed method can be used for the recovery of pulping chemicals, sulphur in a gas phase separately from soda. At the same time, the organic matter of black liquor can be used for producing carbon which is much purer than what is obtained for example from pit coal, for specific applications. In addition, the discussed coal is one of the renewable natural resources. It is very difficult to obtain coal dust from wood, which would be equally fine and would contain equally few volatile components.

Dry separation is a primary way of separating coal dust. Therefore, coal dust must be sufficiently coarse-grained, not completely misty. However, the finest coal dust, capable of passing through dry separation, is recovered in wet separation.

A secondary method is a water scrubber upstream of an alkaline scrubber, which immediately picks up even fine coal dust. Thus, it is preferred that coal dust be separated from water (as quickly as possible) before the soda contained in carbon begins to dissolve. Sulphur containing gases (CO and H2S) are principally recovered in an alkaline wet scrubber.

It is nevertheless possible to take lime (carbon + soda + hydrogen sulphide) in an alkaline scrubber. In this case, coal dust must be separated at a stage as early as possible from sludge, for example by nanofiltering or by using a filter press and polyelectrolytes before its soda has time to dissolve too much. Soda is causticized from carbon after extraction.

It should be appreciated that the recovery apparatus will be much simpler, if there is no desire to recover coal separately but it is absorbed together with gases. However, a consequence of this is that the causticization process complies with the old prior known method.