A process for expanded use of a kraft pulp mill, for phosphorus refining by means of extended recovery

Abstract

The invention pertains to a process for the removal of a phosphorus-rich lime product from a kraft pulping process, wherein at least one, or at least part of one, phosphorus-containing waste stream from the from the kraft pulping process is purified resulting in a purified water fraction and a phosphorus-containing floc, the obtained dried phosphorus-containing floc is added to the spent liquor recovery process, resulting in a phosphorus-containing lime mud, part of which is removed as a phosphorus-rich lime product with a phosphorus content of at least 2 g / ton and a cadmium content in relation to phosphorus of not more than 100 g Cd / ton P. Further is provided a fertilizer product, comprising the phosphorus-rich lime product as a phosphorus component.

Description

[0001] A PROCESS FOR EXPANDED USE OF A KRAFT PULP MILL, FOR PHOSPHORUS REFINING BY MEANS OF EXTENDED RECOVERY

[0002] Field of the Invention

[0003] This invention aims for expanded use of a kraft pulp mill, for phosphorus refining by means of extended recovery, resulting in a phosphorus-rich limewith a low cadmium content, suitable e.g. for use as a fertilizer.

[0004] Background

[0005] In kraft pulp production, wood chips are cooked at elevated temperature and pressure together with white liquor, roughly made up of sodium hydroxide and sodium hydrogen sulfide. After cooking, the pressure is released and the chips are defibrated. The wood substance released during cooking is found in the liquid phase, which is now called black liquor. The pulp is washed and screened and could then also be subjected to oxygen delignification and bleaching before it is either dried and sold as a semi-finished product or directly goes on to a paper mill for the production of e.g. cardboard or paper.

[0006] Water is extensively used during pulping and bleaching and the resulting contaminated water, if released directly to a recipient, would negatively affect the environment there. To measure the degree of pollution in the effluent, analytical methods such as biological oxygen demand (BOD), chemical oxygen demand (COD), total organic carbon (TOC), the total content of phosphorus, suspended solids are used. In addition to this, the wastewater also contains e.g. transition metal ions. This wastewater cannot or should not be discharged untreated to the recipient. Commonly available wastewater cleaning technologies produce a sludge in which both organics, phosphorus and transition metal ions are found. Although the phosphorus is of interest e.g. as a fertilizer, the presence of potentially toxic organics and more so the presence of toxic transition metal ions, notably cadmium, effectively prevents the sludge from this kind of use thus making the sludge become a waste product to be safely disposed of instead of as a valuable resource.

[0007] Evidently, there is therefore, a need for novel processes capable of handling this kind of phosphorus containing wastewater from kraft pulp production in such a way that the phosphorus could be recovered as a non-contaminated phosphorus product thus unlocking its value. Summary of the Invention

[0008] Accordingly, the present invention preferably seeks to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination and solves at least the above mentioned problems by providing a process for expanded use of a kraft pulp mill, for phosphorus refining by means of extended recovery, wherein the kraft pulp mill comprises cooking, washing, potentially bleaching, optionally drying, and a recovery cycle, the recovery cycle comprising an evaporation step, wherein black liquor from cooking, screening, washing and postoxygen washing is evaporated, resulting in evaporated spent liquor which is incinerated in a recovery boiler and a white liquor preparation plant including a lime cycle, characterized in that at least one additional phosphorus-containing waste stream is added to the recovery process, the additional phosphorus-containing waste stream being one of several of: a waste flow from the washing, the bleaching and / or the drying of the kraft pulping process, or wherein the kraft pulping process further comprises a debarking step and / or a chipping / chip silo, and waste stream, or part of waste streams from the debarking step and / or chipping / chip silo, or an external waste flow comprising phosphorus and mainly organic matter, wherein the phosphorus-containing waste stream is concentrated and / or dried to a weight-based solids content of at least 1%, such as at least 3%, 5%, 10%, or at least 20%; and added to a black liquor flow or preferably to a medium-thick liquor flow in the evaporation process, such that the evaporation step results in a phosphorus-rich thick black liquor; the obtained phosphorus-rich thick black liquor is incinerated in a recovery boiler, resulting in a phosphorus-rich melt in the bottom of the recovery boiler, this melt is dissolved in a melt solvent to obtain phosphorus-rich green liquor; the obtained phosphorus-rich green liquor is purified by means of green liquor filtration or clarification, and causticized using reburnt lime from the lime kiln, resulting in a white liquor mixed with phosphorus-rich lime mud; the obtained white liquor mixed with phosphorus-rich lime mud is separated e.g. on a lime mud filter, and alternatively washed, resulting in a white liquor fraction and a phosphorus-rich lime mud fraction; accepting a higher phosphorus level in the lime kiln, such that the phosphorus-rich lime mud has a phosphorus content of at least 2 g / ton, such as at least 5 g / ton, 10 g / ton, 20 g / ton, or at least 50 g / ton making it suitable for use as a fertilizer, and removing part of the phosphorus-rich lime mud as a product and burning the remaining part together with low cadmium containing make-up lime in the lime kiln in order to obtain burnt lime mud, such that the phosphorus-rich lime mud after causticizing and separation gets a cadmium content in relation to phosphorus in the product not exceeding 100 g Cd / ton P, such as not exceeding 50 g Cd / ton P, not exceeding 25 g Cd / ton P, not exceeding 10 g Cd / ton P, not exceeding 5 g Cd / ton P, or not exceeding 2 g Cd / ton P.

[0009] In one variant at least one, or at least part of one, phosphorus-containing waste stream from the kraft pulping process is purified using electrocoagulation (EC).

[0010] In one further variant, the electrocoagulation uses iron electrodes.

[0011] In one further variant, a voltage between the electrodes in the electrocoagulation is maximum 100 V, such as maximum 50 V, maximum 25 V, or maximum 10 V, such as between 5 and 50 V, or between 5 and 25 V.

[0012] In one variant, the phosphorus content in the phosphorus-rich lime mud product is primarily the result of the phosphorus in the black liquor, the additional phosphorus-containing waste stream and the re-burnt lime.

[0013] In one variant, the most part of the cadmium in the black liquor or phosphorus-containing waste stream will be removed during the extended recovery process of the kraft pulp mill, whereby cadmium in the phosphorus-rich lime mud product originates primarily from the make-up lime.

[0014] In one variant, the make-up lime has a cadmium content of max 0.5 g / ton, such as max 0.2 g / ton, max 0.1 g / ton.

[0015] In one variant, the concentrated phosphorus-rich waste stream contains phosphorus and precipitated transition metal ions and potentially organic matter.

[0016] In one variant, the concentrated phosphorus-rich waste stream comprises phosphorus and at least 1 mg / 1, such as at least 5 mg / 1, such as at least 50 mg / 1 precipitated transition metal ions.

[0017] In one further variant, the precipitated transition metal ions are selected from one or several of Cadmium (Cd), Arsenic (As), Lead (Pb), Chromium (Cr), Nickel (Ni), Copper (Cu), Zinc (Zn), Manganese (Mn), or Molybdenum (Mo) precipitated transition metals ions.

[0018] In one variant, the concentrated phosphorus-containing waste stream has a chloride concentration of less than 100 ppm, such as less than 50 ppm, such as less than 10 ppm.

[0019] In one variant, wherein the phosphorus-containing product is used as a fertilizer or phosphorus component in fertilizer or for soil improvement.

[0020] In one aspect is provided a fertilizer product or a soil improvement product, comprising the phosphorus-rich lime fraction as a phosphorus component. Brief Description of the Drawings

[0021] These and other aspects, features and advantages of which the invention is capable of will be apparent and elucidated from the following description of embodiments of the present invention, reference being made to the accompanying drawings, in which;

[0022] Figure 1 shows a schematic of one embodiment of the method of the invention, wherein a phosphorus-rich lime product is ejected from the kraft pulping process, wherein the kraft pulping process comprising a cooking step, a fiberline and a recovery cycle, the recovery cycle comprising a black liquor evaporation step, incineration in the recovery boiler and a lime cycle. The extended use of the kraft pulp mill for phosphorus refining by means of extended recovery is shown by the bold arrow, illustrating how a phosphorus-rich lime product with low cadmium content is removed;

[0023] Figure 2 shows a schematic of one embodiment of the method of the invention, wherein the bold arrow illustrates how a concentrated waste stream containing phosphorus is added to the black liquor flow (dotted arrow) or preferably to a mediumthick liquor flow in the evaporation process, resulting in a phosphorus-rich thick black liquor;

[0024] Figure 3 shows a schematic of one embodiment of the method of the invention, wherein a phosphorus-containing strong black liquor flow is burned in a recovery boiler, resulting in a phosphorus-containing bottom fraction, the obtained phosphorus-containing bottom fraction is dissolved in a melt solvent to obtain phosphorus-containing green liquor; and

[0025] Figure 4 shows a schematic of one embodiment of the method of the invention, wherein the recovery boiler loop (sodium loop) and the lime cycle (calcium loop) is illustrated, wherein phosphorus-containing green liquor is causticized, whereby a white liquor mixed with phosphorus-containing lime mud is obtained, which is separated resulting in phosphorus-containing lime mud, part of which is removed as a product. The other part is mixed with make-up lime having a low cadmium content and burnt in the lime kiln to produce a low cadmium burnt lime used in the causticizing process.

[0026] Detailed embodiments The current invention relates to a method for expanded use of a kraft pulp mill, for phosphorus refining by means of extended recovery, where a phosphorus-rich lime with low cadmium content may be recovered as a useful product, for instance as a fertilizer product.

[0027] Over the years, the pulping industry has gradually developed and taken its emissions to water more seriously. This has in effect been done for two different reasons: since it became apparent that the production had a negative impact on the environment and both public pressure and legislation forces mills to take action - and since taking care of waste streams also meant a potential to increase e.g. the pulp yield and therefore also economically benefitting from the action. Removing fibers in sedimentation ponds and the introduction of oxygen delignification are perhaps the most obvious examples of such win-win actions. However, when the idea was brought further to the bleach plant using all wash liquors in a strict counter-current mode eventually including them in the black liquor going to evaporation and incineration in the recovery boiler, the continuous operation of the mill soon turned out to be at stake. The idea of the 1970’s was that the organic content in the liquors instead of becoming a challenge to the environment in the sewer could be burnt and generate energy in the recovery boiler. The fact that the bleaching included chlorine-containing chemicals meant that chloride ion was present in the liquors causing severe corrosion in the mill. Accordingly, this development line was cancelled and bleach plant effluents continued to be poured out to the sewer. Instead, secondary treatment of the wastewaters was installed, beneficial to the environment but coming at an operational cost to the mill. Such aerated ponds / lagoons are by today very common for wastewater treatment. The pond makes use of microbes for a degradation of the organic components and the process is speeded up by the energy-consuming addition of air into the water. During production stops, the ponds could be emptied and the sludge excavated with dumpers. However, it is obvious that the contaminants in the wastewater is a resource awaiting to be used. It contains organics - potentially being a source for energy production, phosphorus - potentially being a component in fertilizer, transition metal ions - limiting the potential use of the phosphorus as a fertilizer and chloride ion - making the use of the liquor as such in evaporation impossible for reasons related to corrosion.

[0028] Phosphorus recovery

[0029] In recent years, interest in phosphorus recovery in particular has increased significantly. The phosphorus that is mined today is fossil and finite with an end date on known resources that is no more than a few decades into the future. Alternative phosphorus sources are of low, or very low, concentration and thus more expensive to extract, they occur e.g. out of the world ocean. Phosphorus in the pulp mill comes primarily from the wood being the main raw material in the pulping process. It mainly ends up in the acidic bleaching effluent and from there continues into the sewage system where it finally either ends up in the sludge that is precipitated in the sewage treatment together with larger organic substances / particles and transition metal ions or is released into the recipient. There, it accumulates over time and contributes to increasing eutrophication since phosphorus, unlike nitrogen compounds, does not form any gaseous degradation products enabling them to evaporate into the air and redistribute over land.

[0030] It has been discussed that phosphorus could be recovered from pulping, however, not from the pulping process, but from the sludge storage ponds. Johan Dahlbom and Rikard Wadsbom in Varmeforskrapport 939 (Effekter av PFG for integrerade pappersbruk vid indunstning och forbranning av bioslam i sodapannan) showed that burning sludge from (for example) aerated ponds is at least possible. The report discusses getting rid of a waste by incineration and, does not focus on recovery of phosphorus. None-the-less, it is mentioned that the stored sludge contains phosphorus, and phosphorus recovery could hypothetically be processed batch-wise when the aerated pond is dug out in connection with bigger shutdowns, perhaps once a year or even less often. Even more likely, it would be considered using the stored sludge directly, since the flock consists of both phosphorus and carbon and thus could hypothetically be used as a soil improver, rather than being incinerated first. However, if so, it would likely be realized that the levels of transition metal ions in the sludge were problematically high, this could lead to considerations of finding alternative methods for cleaning the bleaching effluents.

[0031] There might also be external waste flows containing phosphorus, that for similar reasons cannot be used directly. For instance, industries that use phosphorus- containing chemicals (such as those used in detergents) might discharge wastewater containing both phosphorus and trace amounts of transition metal ions. Agricultural runoff, including fertilizers and animal waste, is a significant source of phosphorus contamination. However, since these waste streams typically contains further contaminants, such as transition metal ions, they cannot be used directly as a phosphorus source. Another problem would be that if the waste stream is not first stored in a storage pond, the amount of phosphorus per liter of wastewater might make it impossible to evaporate the waste stream enough for it to be economically viable to recover the phosphorus.

[0032] In the invention, it was realized that several of these problems could be overcome using existing kraft pulp infrastructure together with an additional concentrating / drying step.

[0033] Process for expanded use of a kraft pulp mill for phosphorus refining

[0034] It was realized that a concentrated waste stream containing phosphorus, along with contaminants such as transition metals making it unusable for soil improvement, may be further process using the kraft pulp mill infrastructure (see figure 1).

[0035] In the invention was proposed a process for expanded use of a kraft pulp mill, for phosphorus refining by means of extended recovery, where the kraft pulp mill comprises cooking, washing and potentially (optionally) bleaching, optionally drying and a recovery cycle. The recovery cycle comprises an evaporation step, wherein black liquor from cooking, screening, washing and post-oxygen washing is evaporated, resulting in evaporated spent liquor which is incinerated in a recovery boiler and a white liquor preparation plant including a lime cycle.

[0036] The process is characterized in that at least one additional phosphorus- containing waste stream is added to the recovery process. The additional phosphorus- containing waste stream being one of several of: a waste flow from the washing, (optional) bleaching and / or (optional) drying of the kraft pulping process (in the kraft pulp mill), or (if the kraft pulp mill further comprises a debarking step and / or a chipping silo) a waste flow from the debarking step and / or chipping silo, or an external waste flow comprising phosphorus, transition metal ions and organic matter.

[0037] In the process, the phosphorus-containing waste stream is concentrated and / or dried to a weight-based solids content of at least 1%, such as at least 3%, 5%, 10%, or at least 20%.

[0038] The concentrated phosphorus-containing waste stream is added to a black liquor flow or preferably to a medium-thick liquor flow in the evaporation process, such that the evaporation step results in a phosphorus-rich thick black liquor.

[0039] The obtained phosphorus-rich thick black liquor is incinerated in a recovery boiler, resulting in a phosphorus-rich melt in the bottom of the recovery boiler, this melt is dissolved in a melt dissolver to obtain phosphorus-rich green liquor. The obtained phosphorus-rich green liquor is purified by means of green liquor filtration or clarification, and causticized using rebumt lime from the lime kiln, resulting in a white liquor mixed with phosphorus-rich lime mud and the obtained white liquor mixed with phosphorus-rich lime mud is separated e.g. on a lime mud filter, and alternatively washed, resulting in a white liquor fraction and a phosphorus-rich lime mud.

[0040] By accepting a higher phosphorus concentration in the lime kiln (resulting in a higher dead-load), such that the phosphorus-rich lime mud has a phosphorus content of at least 2 g / ton, such as at least 5 g / ton, 10 g / ton, 20 g / ton, or at least 50 g / ton (thus making it suitable for use as a fertilizer), part of the phosphorus-rich lime mud can be removed as a product.

[0041] The remaining part is burned together with low cadmium containing make-up lime in the lime kiln in order to obtain burnt lime mud, such that the phosphorus-rich lime mud after causticizing and separation gets a cadmium content in relation to phosphorus in the product not exceeding 100 g Cd / ton P, such as not exceeding 50 g Cd / ton P, not exceeding 25 g Cd / ton P, not exceeding 10 g Cd / ton P, not exceeding 5 g Cd / ton P, or not exceeding 2 g Cd / ton P.

[0042] Thus, the process of the invention makes use of the extended recovery process of the kraft pulp mill to make a contaminated phosphorus source, having negative or zero value, refining it into a phosphorus-rich lime usable for soil improvement or as a fertilizer product.

[0043] Concentrating the waste stream for addition to the evaporation process Preferably, the concentrated phosphorus-containing waste stream should be added to a liquor flow in the evaporation process of similar solids content. This avoids wasting energy on concentrating the phosphorus-containing waste stream, making it more viable to purify the phosphorus from the phosphorus-containing waste stream.

[0044] A waste stream having a weight-based solids content of between 1 to 20%, preferably at least 5%, may be fed to the recovery cycle of the kraft pulp mill without much need for further energy required for evaporation. In fact, it is suitable for feeding into the medium-thick liquor, before the final evaporation steps, on its way to the recovery boiler, where any organic fraction will contribute to the energy production in the boiler while the ecotoxic organic components are safely destroyed.

[0045] If concentrating the phosphorus-containing waste stream to a (weight-based) solids content of 1 to 10 %, it may preferably be added to the medium -thick liquor flow in the evaporation process. For higher solids content (such as 10 to 20 %) it may be preferably to add it to the black liquor flow in the evaporation process.

[0046] Another consideration is if the solids content is under 20%, it is often still possible to pump the concentrated phosphorus-containing waste stream, which also may provide cost-saving benefits.

[0047] Simultaneous removal of contaminants

[0048] Feeding the phosphorus containing waste stream to the recovery cycle of the kraft pulp seems counter-productive, since the medium-thick liquor is in itself contaminated, however, by co-processing the medium-thick liquor waste stream containing phosphorus, toxic transition metals can be removed from the enriched phosphorus at the same time as the phosphorus is made particularly bioavailable.

[0049] Thus, in the process, the concentrated phosphorus-rich waste stream may comprise phosphorus and at least 1 mg / 1, such as at least 5 mg / 1, such as at least 50 mg / 1 precipitated transition metal ions. The precipitated transition metal ions may be selected from one or several of Cadmium (Cd), Arsenic (As), Lead (Pb), Chromium (Cr), Nickel (Ni), Copper (Cu), Zinc (Zn), Manganese (Mn), or Molybdenum (Mo) precipitated transition metals ions. However, it may also contain other precipitated transition metal ions.

[0050] When the waste stream containing phosphorus is added to the black liquor flow or preferably to a medium-thick liquor flow in the evaporation process, it results in a phosphorus-rich thick black liquor (see figure 2).

[0051] This is in turn incinerated in a recovery boiler, resulting in a phosphorus-rich melt in the bottom of the recovery boiler. The melt is then dissolved in a melt solvent to obtain phosphorus-rich green liquor (see figure 3).

[0052] In the process, the particularly toxic transition metal ions (e.g. Cd, As, Hg, Pb, Zn) mainly end up in the boiler fly ash. After the melt dissolver, elements foreign to the process are ejected as green liquor dregs, and there now mainly ends up the remainder of the transition metal ions that the floc carried with it (e.g. Fe, Cu, Mn, Al, Mg).

[0053] New phosphorus-rich lime by a modified Lime cycle

[0054] The resulting phosphorus-rich green liquor is purified by means of green liquor filtration or clarification, and causticized using rebumt lime from the lime kiln, resulting in a white liquor mixed with phosphorus-rich lime mud. The obtained white liquor mixed with phosphorus-rich lime mud is separated e.g. on a lime mud filter, and alternatively washed, resulting in a white liquor fraction and a phosphorus-rich lime mud - in fact, phosphorus, being a non-process element in relation to the kraft process, builds up over time an inert fraction of calcium phosphate in the lime mud. Part of this phosphorus-rich lime mud can then be removed from the process as a product suitable for use as a fertilizer or phosphorus component in fertilizer or for soil improvement (see figure 4).

[0055] By accepting a higher phosphorus level in the lime kiln (which also means a higher dead-load), the resulting phosphorus-rich lime mud can be made to have a phosphorus content of at least 2 g / ton or even higher, such as at least 10 g / ton, at least 20 g / ton, or at least 50 g / ton.

[0056] In the invention, it was found that there is economically viable to accepting a higher dead-load of phosphorus in the lime kiln in order to get a phosphorus content sufficiently high to make the lime suitable for use as a fertilizer, thus expanding the use of a kraft pulp mill.

[0057] Cd / P ratio

[0058] The lime manufacturer Nordkalk reports a typical composition of its quicklime as follows (https: / / nordkalk.se / wp-content / uploads / 2022 / 03 / Nordkalk_rapport_kce.pdf (Bilaga A)): Cd 0.5; Co 5; Cr 25; Cu 167; Hg <0.05; Ni 15; Pb 22; V 11; Zn 195 mg / kg in CaO.

[0059] In SOU 2017: 102 Tax on cadmium in certain products and chemical plant protection products https: / / data.riksdagen.se / fil / F9E104Fl-E65B-4CB3-8D2F- 7A648D8420F4 (page 90) it is stated that ”In product information that the investigation has seen varies the cadmium content in various lime products for spreading on arable land from various production facilities between <0.1 and 1.8 grams per ton.” where Nordkalk's commercial product thus ends up in the middle of the specified range. In the same way, it is important to note the cadmium content of the mineral calcium phosphate which forms part of commercial fertilizers. According to the same SOU (page 94), the level of cadmium ’’during the period 2003 / 2004-2015 / 2016 [...] was around 5 grams per ton of phosphorus. The lowest reading was reached in 2012 / 13 (4.4 g per ton of phosphorus) and in 2015 / 2016 the level was 6.1 g per ton of phosphorus (figure 5.4).” These figures should be compared to the 175 g Cd / ton P that the floc exhibited.

[0060] It was realized that if the phosphorus in the waste stream containing phosphorus is to be able to be used in a fertilizer context, in addition to being purified by incineration, it also needs to be either mixed with another very pure phosphorus fraction or refined in some way so that the Cd / P ratio decreases. A combination of both measures is of course also always a possibility.

[0061] In a comparison between different pulp mills made at Chalmers University of Technology (https : / / publications. lib . chalmers. se / records / fulltext / 219202 / 219202.pdf (page 18, Table 4)) a phosphorus content in lime mud of 3.5-3.8 g / kg is reported and a corresponding cadmium content of 0.6-1.6 mg / kg, which corresponds to a cadmium content in the same order of magnitude as in the floc reported above (170-420 g Cd / ton P). During normal operation of the pulp mill, the ejected lime mud should therefore not be suitable for use as a phosphorus component in fertilizer due to excessively high cadmium content.

[0062] When waste stream containing phosphorus is added to the recovery cycle, more phosphorus and more cadmium will be present in the green liquor after the recovery boiler. Given that the green liquor consists of a high concentration of sulfide ions, the transition metal ions including cadmium that are present in the melt are precipitated very efficiently and end up in the green liquor dregs that is separated from the green liquor during the green liquor filtration and / or clarification. In Varmeforsk's report (https: / / energiforskmedia.blob.core.windows.net / media / 18057 / traepulverfoerbraenning- i-sodapanna-vaermeforskrapport-1248.pdf (page 23)) the behavior of phosphorus in the recovery is explained in more detail: ’’Since the solubility of phosphorus is much higher in green liquor than in white liquor, negligible amounts of P are removed in the green liquor dregs and this PFG instead tends to accumulate in the recovery cycle, probably as calcium phosphate, Cas(PO4)2, or calcium hydroxylapatite, CasOH PChjs”. Only a very small part of the cadmium in the melt ends up in the lime mud.

[0063] The small amount of cadmium that is still found in the mesa comes mainly from the make-up lime. This means that the more waste stream containing phosphorus that is added to the black liquor, the higher the phosphorus content is found in the lime mud and the more well filtered / clarified the green liquor is and, above all, the cleaner the make-up lime is, the lower the cadmium content in the lime mud.

[0064] Thus, the pulp mill could, through the choice of operating mode, influence the cadmium / phosphorus ratio in the lime that is removed from the mill such that it becomes suitable for use as a fertilizer / soil improvement.

[0065] Most part of the cadmium in the black liquor or phosphorus-containing waste stream will be removed during the extended recovery process of the kraft pulp mill, whereby cadmium in the phosphorus-rich lime product originates primarily from the make-up lime.

[0066] The presence of phosphorus in the lime cycle is normally problematic for the pulp mill as it is primarily this that contributes to the dead-load of the lime kiln. During normal operation, the pulp mill thus makes a trade-off between, on the one hand, increased use of make-up lime (by means of a bigger amount of lime mud being ejected) and, on the other hand, a poorer heat economy in the lime kiln. With this invention at hand, a third parameter is added to the balance, namely what phosphorus concentration could be tolerated in the lime mud in order to get a sufficiently low cadmium / phosphorus ratio in the ejected lime product that makes this product commercially interesting for use as a phosphorus component e.g. in fertilizers.

[0067] Thus, by using make-up lime with low cadmium content, it is ensured that the resulting phosphorus rich lime mud product will have a cadmium content in relation to phosphorus making it suitable for use as a fertilizer.

[0068] In the process, the make-up lime may have a cadmium content of max 0.5 g / ton, such as max 0.2 g / ton, max 0.1 g / ton.

[0069] Thus, the phosphorus-rich lime may be used as a phosphorus fertilizer or for soil improvement.

[0070] One example of the invention is a soil improvement product or a fertilizer product comprising the phosphorus-rich lime.

[0071] Converging phosphorus from the wood raw material to the lime cycle

[0072] During normal operation in a kraft pulp process, a large part of the phosphorus found in the wood raw material does not end up in the lime cycle. Instead, phosphorus will flow uncontrolled through different flows of the pulp mill and partly ending up in the lime mud, partly in the wastewater treatment sludge and partly in the recipient.

[0073] In the invention, waste streams containing phosphorus are cleaned and the sludge / floc is sent into the medium thick liquor, ensuring that most of the phosphorus will end up in the lime kiln. As described in the invention, by accepting external contaminated phosphorus rich waste streams, even more phosphorus could be concentrated in the lime mud.

[0074] By converging the phosphorus to the lime kiln, it becomes advantageous to run the lime kiln in the way of the invention (for example by accepting a higher deadload and using make up-lime low in cadmium), since phosphorus that would normally not be utilized, and in part end up in contaminated waste of negative value, will instead be concentrated in a phosphorus-rich lime of positive economic value.

[0075] Benefitting from phosphorus-rich wastestreams without putting the mill at risk As described above the addition of chloride containing liquors to the evaporation would cause severe corrosion to the mill - and the same would go for solid material including substantial amounts of chloride. Should the organics and phosphorus from the wastewater from the bleaching thus be recovered the cleaning technology must not recover the chloride ions. Many technologies do so in theory, but the amount of water following the separated solids is crucial for the actual outcome. It is thus also important to obtain a high dry content of the solids in order to minimize the amount of chlorine atoms in the fraction containing the separated contaminants. Furthermore, the cleaning technology should discriminate between the phosphorus and organics on the one hand and the transition metal ions on the other. In practice it proves difficult to find such a technology since the transition metal ions and the phosphorus tend to follow eachother closely.

[0076] Electrocoagulation (EC)

[0077] Electrocoagulation is a well-established purification method for a wide range of contaminants in water. Suspended solids, organic matter, phosphorus and transition metal ions belong to the things that are usually removed effectively. However, passing e.g. alkali metal ions, chloride and many small and polar organic substances normally go right through the process and thus accompany the water instead of being collected in the flock that electrocoagulation also gives rise to. This floc is also well-known to be obtained with a high solids content and to be possible further to dewater, unlike a traditional sludge.

[0078] The floc separated by electrocoagulation of bleaching filtrate mainly consists of organic material (preferably larger and relatively non-polar structures), phosphorus and transition metal ions, while the effluent after purification mainly contains alkali metal ions, alkaline earth metal ions, chloride and small polar organic substances.

[0079] The advantages of using electrocoagulation for phosphorus removal include its effectiveness in treating water with low phosphorus concentrations, its ability to remove both dissolved and suspended forms of phosphorus, and its relatively low operating costs compared to some other treatment methods. Importantly, while phosphorus will be concentrated in the floc, chloride ions will remain in the water fraction, thus being separated from the phosphorus fraction, thus not putting the mill in danger of corrosion.

[0080] As such, it was found that it the concentrating and / or drying step of the phosphorus-containing waste stream may preferably be done using electrocoagulation (EC).

[0081] In the process, the concentrated phosphorus-containing waste stream may have a chloride concentration of less than 100 ppm, such as less than 50 ppm, such as less than 10 ppm.

[0082] That EC is well suited is shown in Table 1, which show the composition of the water in a bleaching effluent before and after electrocoagulation treatment.

[0083] Table 1.

[0084] The results clearly show that the purification of suspended solids and phosphorus is very effective. In the same way, it is noted that the purification of transition metal ions is almost complete in all cases. The Cd / P-ratio in the water thus is 640 g Cd / ton P. Should the phosphorus be used as for fertilizing reasons on a field, the long-term detrimental effect of the simultaneously added cadmium (and other transition metal ions) would thus be unavoidable. Simply burning the floc would indeed recover the energy from the organics, but would still not refine the phosphorus, in the sense that it would be purified from the transition metal ions and notably the cadmium.

[0085] Developing such a phosphorus refining process is therefore key not only to the realization of the phosphorus resource in the bleach plant effluent floc but also in any solid material containing phosphorus, transition metal ions and potentially organics.

[0086] All the substances that are separated from the wastewater end up in the solid phase, called floc, which the purification process gives rise to. In addition, the floc also contains some iron or aluminum released during the process as well as flocculant polymer. In the example above, the floc (not to be confused with the ratio in the wastewater) contained 570 mg P / kg and 0.1 mg Cd / kg, which corresponds to 175 g Cd / ton P. In addition, there are also other transition metal ions.

[0087] Depending on several factors, the separated floc may have a solids content of from 1% up to 20%. However, a final concentration of at least 5% is preferable, since in the process of the invention, the floc will be added to the evaporation step of the recovery cycle. As such, the EC coagulation step is preferably combined with suitable floc separation techniques.

[0088] The electrocoagulation preferably uses iron electrodes. However, other electrode materials, such as aluminum, or a combination of materials, such as iron (stainless steel and / or iron) and aluminum, could also be used.

[0089] The voltage between the electrodes in the electrocoagulation is preferably maximum 100 V, such as maximum 50 V, maximum 25 V, or maximum 10 V, such as between 5 and 50 V, or between 5 and 25 V.

[0090] In the case of electrocoagulation, suspended solids, phosphorus as well as transition metal ions and larger organic substances are obtained in the solid phase and can then proceed to processing.

[0091] Thus, although electrocoagulation can solve several of the phosphorus recycling problems presented above, it could not separate phosphorus and organic substances on the one hand from transition metal ions on the other. Thus, in case the phosphorus containing waste stream contains toxic transition metals, the resulting phosphorus containing floc would retain toxic transition metals and thus would likely not be used as a fertilizer without further purification. References

[0092] Page 23 of: Johan Dahlbom and Rikard Wadsbom in Varmeforskrapport 939 (Effects of PFG for integrated paper mills when evaporating and burning biosludge in the soda boiler): https: / / www.osti.gov / etdeweb / servlets / purl / 20674942 Bilaga A of: Lime manufacturer Nordkalk typical composition of quicklime: https: / / nordkalk.se / wp-content / uploads / 2022 / 03 / Nordkalk_rapport_kce.pdf

[0093] Page 90, 94 and figure 5.4 of: SOU 2017: 102 Tax on cadmium in certain products and chemical plant protection products: https: / / data.riksdagen.se / fil / F9E104Fl-E65B-4CB3-8D2F-7A648D8420F4 Page 18, Table 4 of: Comparison between different pulp mills made at

[0094] Chalmers University of Technology: https : / / publications. lib . chalmers. se / records / fulltext / 219202 / 219202.pdf

Claims

CLAIMS1. A process for expanded use of a kraft pulp mill, for phosphorus refining by means of extended recovery, wherein the kraft pulp mill comprises cooking, washing, potentially bleaching, optionally drying, and a recovery cycle, the recovery cycle comprising an evaporation step, wherein black liquor from cooking, screening, washing and post-oxygen washing is evaporated, resulting in evaporated spent liquor which is incinerated in a recovery boiler and a white liquor preparation plant including a lime cycle, characterized in that at least one additional phosphorus-containing waste stream is added to the recovery process, the additional phosphorus-containing waste stream being one of several of: a waste flow from the washing, bleaching and / or drying of the kraft pulping process, or wherein the kraft pulp mill further comprises a debarking step and / or a chipping silo; a waste flow from the debarking step and / or chipping silo, or an external waste flow comprising phosphorus, transition metal ions and organic matter, wherein the phosphorus-containing waste stream is concentrated and / or dried to a weight-based solids content of at least 1%, such as at least 3%, 5%, 10%, or at least 20%; and added to a black liquor flow, or preferably to a medium-thick liquor flow, in the evaporation process, such that the evaporation step results in a phosphorus-rich thick black liquor; the obtained phosphorus-rich thick black liquor is incinerated in a recovery boiler, resulting in a phosphorus-rich melt in the bottom of the recovery boiler, this melt is dissolved in a smelt dissolver to obtain phosphorus-rich green liquor; the obtained phosphorus-rich green liquor is purified by means of green liquor filtration or clarification, and causticized using rebumt lime from the lime kiln, resulting in a white liquor mixed with phosphorus-rich lime mud;the obtained white liquor mixed with phosphorus-rich lime mud is separated e.g. on a lime mud filter, and alternatively washed, resulting in a white liquor and a phosphorus-rich lime mud; accepting a higher phosphorus level in the lime kiln, such that the phosphorus-rich lime mud has a phosphorus content of at least 2 g / ton, such as at least 5 g / ton, 10 g / ton, 20 g / ton, or at least 50 g / ton making it suitable for use as a fertilizer, and removing part of the phosphorus-rich lime mud as a product and burning the remaining part together with low cadmium containing make-up lime in the lime kiln in order to obtain burnt lime mud, such that the phosphorus-rich lime mud after causticizing and separation gets a cadmium content in relation to phosphorus in the product not exceeding 100 g Cd / ton P, such as not exceeding 50 g Cd / ton P, not exceeding 25 g Cd / ton P, not exceeding 10 g Cd / ton P, not exceeding 5 g Cd / ton P, or not exceeding 2 g Cd / ton P.

2. The process according to claim 1, wherein the at least one phosphorus-containing waste stream is concentrated using electro-coagulation (EC).

3. The process according to claim 2, wherein the electro-coagulation uses iron electrodes.

4. The process according to claim 2 or 3, wherein a voltage between the electrodes in the electrocoagulation is maximum 100 V, such as maximum 50 V, maximum 25 V, or maximum 10 V, such as between 5 and 50 V, or between 5 and 25 V.

5. The process according to any one of claims 1 to 4, wherein the phosphorus content in the phosphorus-rich lime mud product is primarily the result of the phosphorus in the black liquor, the additional phosphorus-containing waste stream and the reburnt lime.

6. The process according to any one of claims 1 to 5, wherein the most part of the cadmium in the black liquor or phosphorus-containing waste stream will be removed during the extended recovery process of the kraft pulp mill, whereby cadmium in the phosphorus-rich lime mud product originates primarily from the make-up lime.

7. The process according to any one of claims 1 to 6, wherein the make-up lime has a cadmium content of max 0.5 g / ton, such as max 0.2 g / ton, max 0.1 g / ton.

8. The process according to any one of claims 1 to 7, wherein the concentrated phosphorus-rich waste stream contains phosphorus and precipitated transition metal ions and potentially organic matter.

9. The process according to any one of claims 1 to 8, wherein the concentrated phosphorus-rich waste stream comprises phosphorus and at least 1 mg / 1, such as at least 5 mg / 1, such as at least 50 mg / 1 precipitated transition metal ions.

10. The process according to any one of claims 9, wherein the precipitated transition metal ions are selected from one or several of Cadmium (Cd), Arsenic (As), Lead (Pb), Chromium (Cr), Nickel (Ni), Copper (Cu), Zinc (Zn), Manganese (Mn), or Molybdenum (Mo) precipitated transition metals ions.

11. The process according to any one of claims 1 to 10, wherein the concentrated phosphorus-containing waste stream has a chloride concentration of less than 100 ppm, such as less than 50 ppm, such as less than 10 ppm.

12. The method according to one of claims 1 to 11, wherein the phosphorus-containing lime product is used as a fertilizer or phosphorus component in fertilizer or for soil improvement.

13. A fertilizer product or a soil improvement product, comprising the phosphorus-rich lime obtained according to any one of claims 1 to 12 as a phosphorus component.

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