Method for preparing alkaline solutions containing aromatic polymers

The invention proposes a process which enables separation of aromatic polymers which can be precipitated by acidification, as can be found especially in waste water from cellulose production, from an alkaline solution in a reliable manner and in pure form. To achieve good separation capacity, especially by filtration, it is proposed that the polymers be precipitated by reducing the pH at a relatively low temperature, typically between 15 and 60.degree. C., and the dispersion be subsequently reheated until it passes into filterable form, typically between 50 and 95.degree. C. The product separated in this way for example by filtration can be washed and can be dried at a high temperature, especially up to 110.degree. C., without becoming black.

The invention relates to a process for treatment of aromatic
 polymer-containing alkaline solutions which can be precipitated by
 acidification and which allows separation of these polymers in solid form
 and drying in air at normal pressure and using temperatures between 40 and
 110.degree. C. without becoming black.
 Most aromatic polymers are easily soluble under alkaline conditions. In
 certain cases, as in lignins obtained by alkaline leaching of wood,
 acidification enables precipitation and separation thereof. The problem
 which arises in doing so consists in finding out whether the precipitated
 product can be filtered and dried. It often precipitates specifically in
 gelatinous form and then cannot be centrifuged and is even less
 filterable. Even in the cases in which separation is possible by filtering
 or centrifuging, the resulting product tends to become black during
 drying. This often applies in lignins which have been subjected to
 chemical changes, in the course of which groups are introduced which
 increase the polarity of the molecules, i.e. improve solubility. It is
 thus difficult to find a process with which the separation of products of
 good quality is possible and which works for any type of precipitatable
 aromatic polymers. But there are still problems in conjunction with the
 low concentration (less than 15% by weight polymers). It is often
 impossible to separate polymers in these cases, even if the known
 processes described below are effective for the same polymers at higher
 concentrations. Based on the increased investment costs and the increased
 power consumption in industrial plants concentration by evaporation is not
 a solution to this problem either. It is rather a matter of finding an
 alternative method which is reliable and economical.
 The literature contains a large number of proposals for separation of this
 type of polymer, especially lignins, and for improving filterability.
 These processes include use of a mineral or organic acid for reducing the
 pH at a relatively high optimized temperature and subsequent filtration of
 the resulting precipitate with reheating (Wienhaus et al., Papier 1990
 (11), pp. 563-569; Binglin, Water Treatment 1988 (3), pp. 445-454; Alen at
 al., Tappi 1979 (11), pp. 108-110).
 A process with which it would be possible to make the separated product
 filterable and dryable and which is effective however for any type of
 aromatic polymers for which there is a precipitation pH in an aqueous
 medium has not been known to date.
 The object of the invention is to avoid the above described defects of the
 known processes. To achieve this object it is proposed as claimed in the
 invention that the solution be acidified and afterwards heated and the
 resulting precipitate separated as a solid. This process may seen
 nonsensical because in cold precipitation a mixture is produced which is
 extremely viscous and which has a very gelatinous appearance. In addition
 the precipitated polymers contained in this viscous suspension cannot be
 separated either by centrifuging or by filtration or by other methods. The
 suspension first becomes more liquid upon reheating and at a certain time
 exceeds a threshold (depending on the polymer type around 40-80.degree.
 C.) at which it surprisingly becomes filterable and gives no occasion for
 undesirable coloration during drying. The exact temperature at which good
 filtration becomes possible depends largely on the respective polymer and
 must be optimally selected for each case.
 The surprising element consists in that it is not a process which can be
 reversed in terms of heat engineering. It is absolutely essential to
 precipitate at a relatively cool temperature and afterwards to heat. If
 precipitation is done at an optimum temperature without reheating after
 precipitation, in certain cases a reasonable filtration capacity can be
 achieved; the result however is far from that effect which is achieved
 with proposed processes. Filtration takes place less promptly, and in most
 cases, for example in polymers with a relatively low precipitation pH of
 less than 5, the polymer becomes completely black during drying.
 Especially good and reproducible results were achieved here when the
 polymers to be precipitated have the criteria that the peak on the
 viscosity curve is below pH of 8 or a lower inflection point is located on
 the titration curve below a pH of 6.5.
 The main advantage of the invention is that it enables production of
 products with very low contents of water-soluble minerals, especially
 sodium salts and hemicelluloses, at very low cost. When using products for
 example as copolymers in plastic materials (duroplastics or
 thermoplastics) the purity is an important factor for technical data
 (mechanical properties, absence of electrolytes, such as sodium, for
 electrical insulation capacity, etc.) of the end material. Since the
 product can easily be filtered using for example a Buechner filter or band
 filter, specifically the salts can be removed by washing. In particular in
 the case of highly diluted solutions, for example waste water from certain
 cellulose plants with increased consumption of washing water, the
 described process allows separation of a quality product at a competitive
 price, since the energy necessary for evaporation is saved.
 Filtration can be further improved when the product is allowed to mature
 after reheating for a few minutes with moderate to vigorous and very
 uniform stirring. This maturation time must however be limited. If it is
 extended over a very long time, for example by allowing the liquid to
 stand warm for several hours or by allowing it to cool, the filtration
 time increases again. In addition, cooling of the liquid following the
 primary stage of reheating or optionally also after an additional maturing
 time leads to an increase of the filtration rate.
 As another optional step the invention allows sedimentation of the polymer
 before filtration. In this way a reduction of the amount of water to be
 filtered by a factor over 5 which can extend to more than 20 is possible.
 In following the entire process as claimed in the invention the decisive
 factor is the pH value to enable sedimentation. If it is not low enough,
 the product does not settle even if it is filterable. This pH is typically
 at least one point below the peak of the viscosity curve. If conversely
 the pH value is too low, the filtration rate can become slower again.
 Other additional treatments can be done before or after precipitation and
 before or after heating: addition of flocculants (FeCl.sub.3, Al.sub.2
 (SO.sub.4).sub.3) or of polyelectrolytes (for example of the acrylic type)
 and electrolytic flocculation treatment. These measures can further
 improve separability and can also increase the amount of solids which can
 be separated, since they also allow precipitation of additional polymer
 fractions. The effectiveness of the flocculation aids is especially great
 in most cases when they are added before acidification and before heating.
 On the other hand, this process makes it possible to form a great diversity
 of new technical approaches to obtaining polymers with special properties
 adapted to the requirements. Due to the improved separation of polymers it
 becomes possible specifically to undertake chemical changes in the aqueous
 alkaline phase without the product thus becoming more difficult to
 separate and purify. The reactions which can take place in these changes
 include all those reactions which introduce chemical groups which change
 the charge distribution and thus the precipitation behavior:
 etherification (methylation, ethylation, carboxymethylation, alkoxylation
 with epoxy, etc.) and esterification (sulfonation, nitration, reactions
 with organic acids or diacids, etc.). The chemical reactions which form
 new functional groups by decomposition of the polymer are also named:
 oxidations, for example with oxygen, hydrogen peroxide, ozone, oxidation
 salts, such as periodates or permanganates, oxoammonolysis, oxidative or
 reductive electrochemical reactions or enzymatic reactions. These
 reactions can even influence the distribution of molecular size and
 especially cause a reduction of the average molecular size, but also
 condensations.
 These reactions can be accomplished on solutions which were produced from
 the polymers already separated beforehand, or they can be carried out
 directly on waste waters without the need to separate the polymer
 beforehand and to redissolve them. Some of these modifications, for
 example oxidation, can even promote filtration, when the process as
 claimed in the invention is used; this is not the case for known
 processes.
 This can be explained by the fact that the proposed process works better,
 the lower the maximum of the viscosity as a function of pH, or the lower
 the lower inflection point on the titration curve. For good operation
 thereof a certain polarity of the polymer (carboxyl or other polar groups
 in sufficient number) is essential. For example, certain weakly polar
 polymers which precipitate at pH values between 8 and 9 cannot be easily
 brought into a filterable form by this process. The nature of the acid
 used can be important even at the same pH. More polar acids exhibit a
 better effect than apolar ones and are preferably used at higher pH of the
 inflection point optionally jointly with CO.sub.2.
 These changes allow the properties of the polymers to be influenced and
 controlled; they then can be separated using the process as claimed in the
 invention and can be recovered in a commercially usable form. Based on
 their special properties these polymers therefore represent high
 performance products for use in plastic materials as copolymers, fillers
 or additives (antioxidants, production aids, dispersants for pigments,
 emulsifiers, flame-resistant substances). They can also be used as
 surfactants (dispersants, emulsifiers, surfactants), as complex-forming
 agents (sequestration agents, chelates, fixing agents for heavy metals)
 and in the area of nutrition and medicine as a high purity product with
 emulsifying, dispersing, antioxidizing, antibacterial, antiviral and
 digestion-regulating (antidiarrheal) properties for improving the energy
 yield of foods.
 In the industrial sector, especially in cellulose plants, after separation
 of polymers the filtrate can be treated by means of conventional water
 treatment systems such as biological anaerobic and/or aerobic treatments
 and oxidation in the wet phase. Biological treatment is facilitated
 especially as a result of the reduction of loading with aromatic polymers
 which act as inhibitors. In the case of wet oxidation the lignin
 contributes to economic profitability of the entire treatment process due
 to the profit as a result of its sale. Thus this invention allows
 formation of a profitable system for treatment of industrial waste water.
 Another important advantage of the process as claimed in the invention
 consists in a much lower water content of the polymer after separation,
 especially after filtration. In the cases in which separation of the
 polymers with existing methods is possible, the water contents are often
 in the range from 75 to 85%. At the same concentrations the new method
 allows water reduction down to 50% by weight. This corresponds to a factor
 between 3 and 5.6 for the amount of energy needed for drying.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Example 1
 Black liquor from straw with a CSB of 115 g/l and a total alkali content
 expressed in sodium hydroxide of 17.4 g/l, an inflection point on the
 titration curve at pH 2.9, as is shown in FIG. 1, and a maximum on the
 viscosity curve at pH 2.1, as is shown in FIG. 2, at a temperature of
 85.degree. C. is acidified with sulfuric acid to a pH 1.0. The resulting
 precipitate passes through any type of filter paper.
 The same liquor is acidified at a temperature of 35.degree. C. with
 sulfuric acid to pH 1.0. The resulting precipitate almost immediately
 clogs any type of filter paper and can be separated neither by
 sedimentation nor by centrifuging.
 The lignin separated from the same liquor by flocculation using
 polyelectrolytes and by centrifuging has a coal-like appearance when it is
 dried in air and it has an ash content of 27.2%.
 200 ml of the same liquor are acidified at a temperature of 35.degree. C.
 with sulfuric acid to pH 1.0, heated to 85.degree. C. and kept at this
 temperature for 10 minutes with uniform stirring during the entire
 process. The suspension is then cooled to 30.degree. C. This liquid can be
 sedimented and can be filtered via a Buechner filter 7 cm in diameter in a
 period of 7 minutes. Washing with 50 ml water lasts 5 minutes. The cake
 dried in a furnace at 80.degree. C. has a clear yellow color, an ash
 content of 1% by weight, a carboxyl group content of one milliequivalent
 per gram of dry substance and weighs 7.9 g. The filtrate has a CSB of 65.7
 g/l.
 If 200 ml are mixed at 85.degree. C. with 25 ppm of a cationic acrylic
 amide flocculent and then precipitated at a pH 2.0, the filtration time is
 30 minutes. If the flocculent is added at 25.degree. C. and precipitation
 is done at the same temperature and at pH 2.0, the filtration time after
 heating to 85.degree. C. is only 8 minutes.
 Example 2
 200 ml of black liquor from paper hemp with a CSB of 165 g/l, a total
 alkali content in sodium hydroxide of 57.5 g/l, an inflection point on the
 titration curve at pH 6 and a peak on the viscosity curve at pH 5.5, at a
 temperature of 65.degree. C. are acidified with sulfuric acid to a pH 5.
 This liquid can be filtered in an interval of 30 min via a Buechner filter
 with a diameter of 7 cm. Washing with 10 ml lasts 30 min. The cake
 contains 80% by weight moisture. It is divided into two parts. The first
 is dried for several days at ambient temperature and then has a highly
 dark brown color and weighs 8.2 g. The second part is dried in a furnace
 at 80.degree. C. and becomes completely black and very hard and brittle.
 It weights 7.9 g. The ash content of the two parts is 1% by weight.
 Washing with more than 10 ml water is not possible, since this leads to
 blockage of the filter.
 200 ml of the same liquor at a temperature of 35.degree. C. are acidified
 with sulfuric acid to a pH 5 and afterwards heated to 65.degree. C. with
 simultaneous stirring during the entire process. This liquid can be
 filtered in an interval of 9 min via a Buechner filter with a diameter of
 7 cm. Washing with 50 ml water lasts 4 min. The cake contains 50% by
 weight moisture. It is dried in a furnace at 80.degree. C., it has a
 bright brown color and an ash content of 0.1% by weight.
 Example 3
 10 g of the product from example 2 are dissolved in 100 ml water with 1 g
 NaOH and 3 g ethylene oxide. The solution is allowed to react for 10
 hours. Afterwards at a temperature of 65.degree. C. it is acidified with
 sulfuric acid to pH 5. This liquid can be filtered in an interval of 2 h
 via a Buechner filter with a diameter of 7 cm. Washing with 50 ml water
 lasts 1 h. The resulting product becomes completely dark after drying at
 ambient temperature and very hard and brittle.
 The same solution after conversion with ethylene oxide is acidified at
 35.degree. C., heated to 65.degree. C., and kept at this temperature for
 10 minutes with simultaneous stirring during the entire process. The
 suspension is cooled afterwards to 30.degree. C. This liquid can be
 filtered via a Buechner filter with 7 cm diameter in an interval of 4 min.
 Washing with 50 ml water lasts 30 s. The cake contains 50% by weight
 moisture. It is dried in a furnace at 80.degree. C., it has a bright-brown
 color, and an ash content of 0.1% by weight.
 Example 4
 One liter of waste water from cellulose production from straw at the output
 of biological treatment with a CSB of 3 g/l are acidified at a temperature
 of 35.degree. C. with sulfuric acid to pH 0.5 and afterwards heated to
 40.degree. C. with uniform stirring during the entire process. The
 suspension is then allowed to settle for 15 min and the supernatant is
 separated. It has a CSB of 0.9 g/l. The resulting volume is 200 ml. This
 liquid is then heated to 65.degree. C. and allowed to settle again for 15
 minutes and decanted. The remaining volume is then 50 ml. It can then be
 filtered via a Buechner filter 7 cm in diameter within 1 minute. Washing
 with 50 ml water lasts 15 s. The cake dried in a furnace at 80.degree. C.
 has a bright brown color and an ash content of 1% by weight.
 Example 5
 200 ml of the black liquor as in example 1 are mixed with 3 g of calcium
 oxide and stirred for 10 hours at 60.degree. C. This liquid has two peaks
 for the viscosity, one at pH 11.1 and one at pH 5.3, as is shown in FIG.
 3. 50 ml of this sample are acidified at 60.degree. C. by blowing in
 CO.sub.2 up to a pH of 9.0. The resulting liquid cannot be filtered and is
 centrifuged for 5 minutes at 4500 rpm. After subsequent separation of the
 two phases 40 g are in the sedimented phase and only 10 g in the liquid
 phase.
 Another 50 ml are likewise brought to a pH of 9.0 at 20.degree. C. likewise
 with CO.sub.2 and then held at 60.degree. C. for 20 minutes. When
 centrifuged the liquid yields a ratio of sediment to liquid phase of 1 to
 3.