Method for utilization of waste products from kraft pulping production as an alkali source in industrial wastewater purification

Abstract

The invention pertains to a method for treating acidic wastewater from a non-pulp and paper process, comprising the steps of neutralizing the acidic wastewater from a non-pulp and paper process to a pH>5.5 by addition of an alkaline waste product from kraft pulping, resulting in a neutralized wastewater, wherein the acidic wastewater having pH<3 and comprising at least one transition metal contaminant, and the alkaline waste product comprises at least one transition metal contaminant.

Description

[0001] METHOD FOR UTILIZATION OF WASTE PRODUCTS FROM KRAFT PULPING PRODUCTION AS AN ALKALI SOURCE IN INDUSTRIAL WASTEWATER PURIFICATION

[0002] Field of the Invention

[0003] This invention aims to enable the use of a residual product with a negative economic value as an alkali source in the purification of industrial wastewater. More specifically, the invention relates to the use of residual waste products from kraft pulping as an alkali source in a multi-stage purification process of acidic industrial wastewater comprising transition metal ions.

[0004] Background

[0005] Treatment of industrial wastewater by its very nature contains a variety of possible challenges. Quite often, transition metal ions are among the substances that the purification has as its main focus to remove. This applies, for example, to sewage from mines, smelters, foundries, metalworking workshops, surface treatment industries, pickling operations and also in relation to stormwater from places with hard surfaces where metal objects are stored and handled, e.g. in connection with port operations.

[0006] In some cases, the pH of the wastewaters in question is acidic, and then the wastewater, in addition requires not only a purification but also a neutralization in order to be subject for release into a recipient. This particularly applies to wastewaters from mines, pickling operations and smelters. For this purpose, an alkali source is required, capable of carrying out the neutralization. This alkali must also, perhaps especially in relation to mines, smelters and pickling operations where the flows are particularly large, be non-expensive in order to keep the production as such economical.

[0007] One commonly used alkali source is sodium hydroxide. This is suitable for small flows. When used for the neutralization of an acidic flow containing transition metal ions, some of these will ultimately precipitate as hydroxides or oxides allowing them efficiently to be removed from the water, whereas the sodium ions will stay in the solution. When larger acidic transition metal ion containing flows are to be neutralized sodium hydroxide is usually considered much too expensive.

[0008] Instead calcium hydroxide (slaked lime) is then the option near at hand. When slaked lime is added to an acidic sewage flow containing transition metal ions andwhere sulfate, carbonate and / or fluoride ion is also present, many transition metal ions will precipitate as oxides or hydroxides at the same time as the calcium will precipitate as calcium sulfate (gypsum), calcium carbonate and calcium fluoride. This will result in a dirty calcium rich sludge. It is a cheap and well-proven technique, e.g. in relation to mines. Slaked lime is cheap compared to sodium hydroxide, but it requires the quarrying of limestone and also gives rise to large climate-affecting carbon dioxide emissions in connection with the burning of the limestone that precedes the slaking. The standard bulk neutralization method using slaked lime thus still requires the purchase of virgin slaked lime, which not only presents a financial cost, it also gives rise to significant carbon dioxide emissions from its manufacture contributing to climate change. When used for neutralization of industrial waste flows it also gives rise to large amounts of contaminated sludge calling for further handling in addition to being a potential environmental hazard if just used in a landfill.

[0009] Thus, there is a need for methods to treat acidic industrial wastewater comprising transition metal ions in a more cost-effective as well as climate-effective manner.

[0010] Summary of the Invention

[0011] 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 method for treating acidic wastewater from a non-pulp and paper process, comprising the steps of: neutralizing the acidic wastewater from a non-pulp and paper process to pH>5.5 by addition of an alkaline waste product from kraft pulping, resulting in a neutralized wastewater; wherein the acidic wastewater having pH<3 and comprising at least one transition metal contaminant, and the alkaline waste product comprises at least one transition metal contaminant.

[0012] In one aspect, the acidic wastewater from a non-pulp and paper process comes from a non- forest industry process.

[0013] In one aspect, the acidic wastewater from a non-pulp and paper process comes from a process related to mining and / or metal processing.

[0014] In one aspect, the acidic wastewater from a non-pulp and paper process comes from mines, smelters, foundries, metalworking workshops, surface treatment industries,pickling operations, or stormwater from where metal objects are stored and handled, such as or port operations.

[0015] In one aspect, the acidic wastewater from a non-pulp and paper process has a pH<2.5, such as pH<2.

[0016] In one aspect, the waste product from kraft pulping is selected from the group consisting of green liquor dregs, boiler fly ash, slaker grits and lime mud, or any mixture thereof.

[0017] In one aspect, the alkaline waste product from kraft pulping comprises at least 50 wt.-% green liquor dregs, such as at least 70 wt-%, and one or several of boiler fly ash, slaker grits and / or lime mud.

[0018] In one aspect, the alkaline waste product from kraft pulping is green liquor dregs.

[0019] In one aspect, the acidic wastewater is neutralized to pH>6, such as >6.5, such as >7.

[0020] In one aspect, the method further comprises a step of separating the neutralized wastewater into an aqueous fraction and a solids fraction.

[0021] In one aspect, the acidic wastewater comprises suspended solids, oil, phosphorus, and / or hydrocarbons.

[0022] In one aspect, the method further comprises a treatment step, wherein the neutralized wastewater or aqueous fraction is treated, resulting in a purified water fraction and an enriched contaminant fraction comprising the transition metal contaminants from both the acidic wastewater and the alkaline waste product.

[0023] In one further aspect, the treatment step uses osmosis techniques, such as reverse osmosis, to treat the neutralized wastewater or aqueous fraction.

[0024] In one aspect, the neutralized wastewater or aqueous fraction is treated using electrocoagulation (EC), resulting in a floc fraction and a purified water fraction, wherein the floc fraction comprises the transition metal contaminants from both the acidic wastewater and the alkaline waste product.

[0025] In one aspect, the enriched contaminant fraction of floc fraction comprising transition metal ions may be reused in a smelter, electric arc furnace or similar metal refining process.

[0026] In one aspect, the acidic wastewater comprises suspended solids, oil, phosphorus, and / or hydrocarbons.

[0027] In one aspect, the method further comprises a step of separating the neutralized waste water into an aqueous fraction and a precipitate.In one further aspect, the separation is performed using sedimentation, decantation and / or screening techniques.

[0028] In one aspect, the method further comprises a treatment step, wherein the neutralized wastewater or aqueous fraction is further treated by means of chemical coagulation / flocculation and / or separation using a filter or filtration techniques, resulting in a further treated aqueous fraction and a filter cake.

[0029] In one aspect, the neutralized wastewater, aqueous fraction or further treated aqueous fraction is treated using electrocoagulation (EC), resulting in a floc fraction and a purified water fraction, wherein the floc fraction comprises the transition metal contaminants from both the acidic wastewater and the alkaline waste product.

[0030] In one aspect, the precipitate, filter cake, and / or floc fraction comprises transition metal ions from both the acidic wastewater and the alkaline waste product and is suitable for recovery and / or reuse in metallurgical processes, including but not limited to smelting, electric arc furnace operations, or other metal refining or recycling processes.

[0031] Brief description of the drawings

[0032] 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;

[0033] Fig. 1 shows a schematic of one embodiment of the method of the invention, wherein an acidic wastewater comprising transition metal ions is neutralized using an alkaline waste product from kraft pulping process, wherein the kraft pulping process is shown schematically to illustrate suitable alkaline waste product streams;

[0034] Fig. 2 shows a schematic of one embodiment of the method of the invention, wherein an acidic wastewater comprising transition metal ions is neutralized using an alkaline waste product from kraft pulping, and where optional separation, treatment / filtration and electrocoagulation steps are shown, with optional recovery of transition metals from the precipitate / f liter cake and / or floc;

[0035] Fig. 3 shows a titration curve for acidic wash water neutralized with calcium hydroxide;

[0036] Fig. 4 shows a titration curve for acidic wash water neutralized with green liquor dregs;Fig. 5 shows the visual result of the purification trials with electrocoagulation with calcium hydroxide as neutralization agent. From the left rinse water after neutralization to pH 7.6 and on the right the same water after purification with electrocoagulation;

[0037] Fig. 6 shows the visual result of the purification trials with electrocoagulation with green liquor dregs as neutralization agent. From the left rinse water after neutralization to pH 7.7 and on the right the same water after purification with electrocoagulation; and

[0038] Fig. 7 shows the visual result of the purification trials with electrocoagulation with green liquor dregs as a neutralizing agent to pH 8.5. From the left, incoming rinse water, then the clear phase of the rinse water after neutralization with green liquor dregs (where sedimentation has therefore taken place after neutralization), then the clear phase after electrocoagulation purification with the floc settled at the bottom and then finally the purified water with the floc separated.

[0039] Detailed embodiments

[0040] The current invention relates to a purification method of acidic wastewater comprising transition metal ions using an alkaline waste product from kraft pulping and a multi-stage purification process for the wastewater.

[0041] Especially, the present invention is about finding an inexpensive alkali, which could lower the costs for purification of acidic transition metal-containing wastewater flows without compromising purification efficiency. Further, in the invention, it was found that this can be achieved at the same time as solving an environmental problem in the kraft pulping industry.

[0042] In the search for inexpensive alkali sources for treatment of acidic industrial wastewaters comprising transition metals, alkalis contaminated with e.g. different transition metal ions resulting from industrial processes could theoretically be considered. However, for obvious reasons it would be counter-intuitive to use such contaminated sources in a wastewater purification context. After all, the alkali source would contribute to increasing the content of transition metal ions that the wastewater purification actually aims to remove.

[0043] However, in the invention, it was realized that under certain circumstances, there might be a synergy effect in co-treating different waste products, and that one suchsynergy can be found for acidic transition metal-containing wastewater and alkaline waste products from kraft pulping.

[0044] Acidic wastewaters

[0045] The acidic wastewaters in the invention may come from a process related to mining and or metal process, such as metal refining or metal working processes (a nonpulp and paper process). More specifically, this may be from mines, smelters, foundries, metalworking workshops, pickling operations, surface treatment industries or stormwater from metal industry (from places with hard surfaces where metal objects are stored and handled) or port operations.

[0046] When the wastewater is acidic, it needs to be neutralized (the pH needs to be increased) in addition to being cleaned of impurities, before it could be released to the recipient. This particularly applies to wastewater from smelters and mines.

[0047] The acidic wastewater may have pH<3, such as <2.5 or pH<2.

[0048] Some of the transition metal contaminants found in industrial or mining wastewater are especially problematic due to their toxicity and potential to bioaccumulate in the food chain, such as Chromium (Cr). Thus, it is of high importance to remove such transition metals from the wastewater flow before it can be safely released. However, other transition metals may also have a commercial value, such as Copper (Cu), Nickel (Ni), Cobalt (Co), Zinc (Zn), and Molybdenum (Mo), which are widely used in the metal and electronics industries and often end up in wastewater. The recovery of these metals not only reduces environmental pollution but also provides economic benefits by allowing their reuse in industrial processes

[0049] Kraft pulping

[0050] In kraft pulping, wood chips are cooked at elevated temperature and pressure together with white liquor, where the active species are hydroxide ion and hydrogen sulfide ions. After cooking, the pressure is released, and the cooked chips are defibrated. The wood substance dissolved during cooking is found in the spent liquor, which is now called black liquor.

[0051] The black liquor is the raw material in the recovery cycle of the (kraft pulp) mill, which aims to regenerate the active cooking chemicals from the black liquor at the same time as the dissolved wood substance from the cook is burned and the energy benefitted from. In the recovery cycle, the black liquor is evaporated into thick liquor and incinerated in a recovery boiler. The organic components in the thick liquor are thenburned whereby heat energy is released in the boiler. A melt forms in the bottom of the recovery boiler and from there flows down into a melt dissolver forming green liquor.

[0052] Small amounts of transition metal ions and phosphorus are present in the wood and thus also in the black liquor. The particularly toxic transition metal ions (e.g. Cd, As, Hg, Pb, Zn) are vaporized in the recovery boiler and mainly end up in the top of the recovery boiler as boiler fly ash.

[0053] After the smelt dissolver, other elements foreign to the process (so called nonprocesses element, NPEs) are ejected as green liquor dregs in conjunction to the green liquor filtration and / or clarification, and now mainly the remainder of transition metal ions (e.g. Fe, Cu, Mn, Al, Mg) end up there.

[0054] In the lime slaker, where burnt lime mud and make-up lime are added to the filtered and / or clarified green liquor, a solid by-product is formed in the form of slaker grits.

[0055] The phosphorus that came with the wood and continues into the recovery cycle builds over time up an inert fraction of calcium phosphate in the lime mud. To ensure that the concentration of calcium phosphate is maintained at a suitable level (being a deadload in the lime kiln), a small part of the lime mud is constantly ejected from the lime cycle. Four residual products are thus formed in the recovery cycle of the kraft pulp mill:

[0056] 1. Green liquor dregs (GLD), an alkaline solid waste product containing transition metal ions and requiring safe handling, today usually used as cover material on closed mines.

[0057] 2. Slaker grits (SG) is a byproduct of the slaking. SG is a solid waste that contains calcium carbonate and other impurities. It is typically used as a filler in asphalt and other construction materials, or possibly as a soil conditioner if not too toxic.

[0058] 3. Lime mud (LM) is the name of the mud mainly consisting of calcium carbonate that is separated from the causticizing process (turning green liquor into white liquor) and then burnt to form burnt lime to be used again to treat green liquor. Since calcium phosphate builds up in the lime mud over time being an energy-consuming deadload in the lime kiln and a certain amount of lime mud needs to be ejected from the lime cycle constantly. LM is a solid waste that contains calcium carbonate, calcium hydroxide, and other impurities. It is usually used for coverage of old mines, but has a potential sometimes to be used also for other purposes.

[0059] 4. Boiler fly ash (BFA) is a byproduct of the combustion process in the recovery boiler, which is used to recover chemicals and produce energy in the kraftpulping process. BFA is a fine powder that contains mainly ash, and other impurities, notably toxic transition metal ions. It is usually put into landfill or released into the sea.

[0060] Thus, boiler fly ash, green liquor dregs and slaker grits are normally deposited. The ejected excess lime mud may be partially used in the green liquor dregs filtration process, but otherwise it also goes to landfill or possibly to external rebuming for use in other applications. As such, there are both environmental and financial benefits in finding uses for these waste products from the kraft pulping process.

[0061] The chemical composition of the four residues identified above can be found in more detail, e.g. in the publication Quina et al (2020) - see table 1.

[0062] Table 1. Chemical composition: Green liquor dregs (GLD); Slaker grits (SG); Lime mud (LM); Boiler fly ash (BFA).

[0063] GLD SG LM BFA

[0064] H i- i] [- '] n [- 1] i-d m i>] [ ] [- ■] i-N i-'i CaO 34.3 330 34.9 4945 55.8 444-52.0 57.12 54.1 167 34.9 0.8-104 16.5 MgO 109 4.65 594 045 0.47 0.6-3.4 091 0.86 3.44 4.4 0.7-1.9 3.07 SiO20 23 235 ni 0.47 1.31 34-11 0 358 0.34 385 11 6 33.9-597 34 0 AI2O31.94 0.69 0.47 029 0.42 0.5-1.4 0.07 0.07 14.8 4.4 16.5-35.4 13.5 FejOj 0.61 0.65 0.59 005 <0.1 0.2-1.2 0.20 0.15 5.94 2.6 1.5-19.7 4.95 Na2O 2.10 11 7 949 452 0.60 ni 232 0.91 1.53 1.4 ni 1.52 K2O <01 1.03 0.37 027 <0.1 ni 0.26 0.06 5.97 6.5 ni 5.49 P2O5ni 0.33 0.37 0.38 0.65 ni 0.03 0.96 1.12 1.6 ni 1.11 TIOT ni <01 ni ni <0 1 ni ni ni 076 025 ni 065 MnO 4.21 0.37 0.06 ni <0 1 ni ni 0.09 0.50 1.4 ni 0.45 SO33.6 2.82 ni 186 0.11 ni 0.4 ni 2.66 11.4 ni 2.77 1.01 ni 4210 ’ ni 41 1 4010 31.5^135 ni ni 638 158 1 2-33.6 143

[0065]

[0066] LOl-Loss on ignition; * Determined at 1200 °C; ni-not indicated.

[0067] As seen in table 1 , the alkali content - here expressed mainly in terms of CaO, MgO, Na20 and K2O is high in all four residual products. Given a content of transition metal ions of various kinds (which is not further specified here) at least the green liquor dregs and the boiler fly ash are problematic in nature and clearly have negative economic value, while the value of the slaker grits and the ejected lime mud is somewhat less negative.

[0068] The largest by-product by mass is the green liquor dregs, with the filter ash and the excess lime mud in a shared second place and the slaker grits constituting the smallest fraction. The proportions are roughly 1 part slaker grits, 12 parts boiler fly ash, 12 parts excess lime mud and 25 parts green liquor dregs - all expressed in dry matter.In green liquor dregs and boiler fly ash, various types of elements that interfere with the process from the perspective of the kraft pulping process, so called non-process element (NPEs), are concentrated, as summarized by Johan Dahlbom and Rikard Wadsbom in Varmeforsk report 939. In this regard, the particularly toxic transition metal ions (i.e. Cd, As, Hg, Pb, Zn...) end up mainly in the boiler fly ash, while the green liquor dregs mainly concentrates non-toxic or less toxic transition metal ions and other metal ions (i.e. Fe, Cu, Mn, Al, Mg...).

[0069] Thus, the alkaline waste product is primarily an unprocessed waste product from kraft pulping, such as green liquor dregs, boiler fly ash, slaker grits, or lime mud. This waste product can be used either as a single waste stream or as a mixture of several such streams from the kraft recovery cycle. In a preferred embodiment, the waste product comprises at least 50 wt.-% green liquor dregs, such as at least 70 wt.-%. In another embodiment, the waste product is green liquor dregs.

[0070] It should be understood that where a 'mixture' of alkaline waste products is mentioned herein, this refers to a blend of raw, unprocessed waste streams originating from the kraft pulping recovery cycle.

[0071] In the invention, it was realized that if the wastewater contains transition metal ions, such as a wastewater from mining, pickling or smelting the additional amount of transition metal ions supplied with the dirty alkali from the pulping process is of the same nature. This way, the waste disposal problem / cost for the kraft pulp mill may be solved / diminished at the same time as the need for purchase of strongly carbon dioxideemitting virgin slaked lime for the wastewater treatment could be terminated.

[0072] This stands in stark contrast to prior art methods that rely on the addition of multiple 'clean' virgin chemicals and commercial by-products, such as slaked lime or purified lignin derivatives, which all have a positive economic value and represent a significant operational cost. Furthermore, by avoiding the need for virgin slaked lime, which is produced by burning limestone, the present method offers a substantial climate benefit by eliminating the associated carbon dioxide emissions from its manufacture.

[0073] Thus, in the method of the invention acidic wastewater from a non-pulp and paper process comprises the steps of: Neutralizing the acidic wastewater from a nonpulp and paper process to a pH>5.5 by addition of an alkaline waste product from kraft pulping, resulting in a neutralized wastewater. The acidic wastewater has pH<3 and comprising at least one transition metal contaminant, and the alkaline waste product comprises at least one transition metal contaminant.It was found that not only do the profiles of contaminants match, the alkaline waste product did indeed neutralize the acidic wastewater. The alkaline compounds in these alkaline waste products, such as carbonates and hydroxides, react with hydrogen ions and thereby increase the pH of the solution. This can be seen in the titration curves in Figures 3 and 4, where slaked lime is compared to green liquor dregs for acidic wastewater neutralization. As can be seen in the titration curves, the pH can easily be adjusted to >5.5, in fact the pH can be adjusted even higher. Thus, the acidic wastewater may be neutralized to pH>6, such as >6.5, such as >7.

[0074] In fact, by monitoring the pH in a neutralization stage where alkaline waste products from kraft pulping are added to the acidic wastewater, the neutralization process can be followed in real time. Preferably, the water is agitated as the alkali is added, such as through stirring, whereby the neutralization is speeded up.

[0075] It also follows that if an efficient wastewater purification process is applied after the neutralization there should be reason to assume a similarly pure purified wastewater should be obtained with the contaminated alkaline byproduct from kraft pulping as alkali source in the neutralization as would be the case when fresh slaked virgin lime is used for this purpose in the standard way for neutralization.

[0076] By neutralizing an acidic wastewater flow by means of an alkali charge, transition metal ions could be precipitated. Thus, the method may comprise a step of separating the neutralized wastewater into an aqueous fraction and a precipitate, where precipitated transition metal ions may end up in the precipitate.

[0077] Such a separation may be performed using sedimentation, decantation and / or screening techniques.

[0078] However, the majority of organic substances will remain in the wastewater. In addition, although many transition metal ions precipitate in solid form at the higher pH (and thus could be removed in a subsequent separation step), the purification is rarely so effective that low pg / 1 levels and thus environmental standards put up by the authorities to comply with are reached by neutralization alone.

[0079] This means that although many transition metal ions in acidic wastewater will indeed precipitate during a neutralization step. Practically it is seldom feasible to clean the wastewater to such an extent that the water is deemed clean just through a neutralization step. For wastewater flows with transition metal ion concentrations in the mg / 1 scale or even 10-100 pg / 1 scale, or for wastewater flows also comprising other (organic) pollutants, a multistep process would be required to achieve a good purification result. Although several transition metal ions precipitate during theneutralization, it is challenging to reach limits put up in environmental standards for all transition metals ions, such as nickel, where pH>9 being required for effective precipitation. However, such high pH levels will result in that other transition metal species will re-dissolve. Further, such high pH wastewater would have to be down-adjusted again, in order to comply with discharge regulations. When it comes to the removal of chromium(VI) a reduction to chromium(III) is also required in order to enable a precipitation meaning a neutralization alone will not be sufficient.

[0080] A mere neutralization evidently is a first and necessary step in the purification of an acidic transition metal-containing wastewater. However, in order to make such a water truly clean, the transition metal ions also need to be efficiently removed, not just merely reduced.

[0081] However, as hypothesized in the invention, an efficient wastewater purification process applied after the neutralization step should result in a similarly pure purified wastewater, regardless of whether a contaminated alkaline byproduct from kraft pulping is used as alkaline source or fresh slaked virgin lime, because their contaminant profile is similar.

[0082] Thus, the method may comprise a treatment step wherein the neutralized wastewater or aqueous fraction is further treated e.g. by means of chemical coagulation / flocculation and / or separation using a filter or filtration techniques, resulting in a further treated aqueous fraction and a filter cake.

[0083] Depending on water quality, filtration techniques may include from simple filters to osmosis techniques, such as reverse osmosis, to treat the neutralized wastewater or aqueous fraction.

[0084] However, the acidic wastewater streams may in some cases also comprise other contaminants, such as organic contaminants, such as oil / fat in smelter sewage water. Thus, the acidic wastewater may comprise suspended solids, oil, phosphorus, and / or hydrocarbons.

[0085] In this case, it was found that these contaminants (not precipitating as a result of neutralization) might instead interfere with the separation process after the naturalization, for instance by clogging up filters in the separation step. Further, the waste product from kraft pulping may not be homogeneous, and may comprise particles of as small size as a few micrometers. As such, it might be hard to separate out all of the added waste product from the neutralized water, for instance through sedimentation (and as explained above, fine filters may be clogged up by other contaminants).For such dirtier waste waters, electrocoagulation may be used to treat the neutralized wastewater or aqueous fraction. Electrocoagulation is a well-established purification method for a wide range of contaminants in water. Transition metal ions, suspended solids, oil, phosphorus, many hydrocarbons with low polarity, etc. are usually removed effectively using EC.

[0086] Thus, the neutralized wastewater, aqueous fraction or further treated aqueous fraction may be treated using electrocoagulation (EC), resulting in a floc fraction and a purified water fraction, wherein the floc fraction comprises the transition metal contaminants from both the acidic wastewater and the alkaline waste product.

[0087] Electrocoagulation efficiently separates contaminants e.g. transition metal ions, suspended solids, emulsified oil / fat and organic pollutants with low polarity from the purified wastewater and into a high density floc, resulting in a purified water with low p.g / 1 levels of transition metal ions. The floc thus contains all the separated contaminants.

[0088] Thus, by neutralizing the wastewater to a pH suitable for discharge (e.g., pH 6-9) and then employing a highly efficient purification step like electrocoagulation, the entire process is completed without the need for a final pH reduction step, thereby saving chemicals, time, and costs.

[0089] The full effect of this multi-step treatment can be seen in figures 4 to 6 and tables 2 and 3. Figure 5 shows the visual result of the purification trials with electrocoagulation with calcium hydroxide as neutralization agent. From the left rinse water after neutralization to pH 7.6 and on the right the same water after purification with electrocoagulation. As shown, there is a great difference between the water after neutralization and after EC. As described above, the water also has a low pg / 1 levels of transition metal ions.

[0090] More interestingly, figure 6 shows the visual result of the purification trials with electrocoagulation with green liquor dregs as neutralization agent. From the left rinse water after neutralization to pH 7.7 and on the right the same water after purification with electrocoagulation. As hypothesized in the invention, although a waste product (here green liquor dregs) is used for neutralization of the acidic wastewater, the purified water fraction is comparable to that of figure 5, having as low pg / 1 levels of transition metal ions.

[0091] The benefits of using an alkaline waste product from kraft pulping for neutralization are evident for each method step, which is illustrated in figure 7, which shows the visual result of the purification trials with electrocoagulation with greenliquor dregs as a neutralizing agent to pH 8.5. From the left, incoming rinse water, then the clear phase of the rinse water after neutralization with green liquor dregs (where sedimentation has therefore taken place after neutralization), then the clear phase after electrocoagulation purification with the floc settled at the bottom and then finally the purified water with the floc separated. The chemical analyses for each sample can be seen in table 2.

[0092] A further, unexpected advantage of the present method, particularly when followed by a purification step such as electrocoagulation, is the nature of the resulting solid residue. Because the neutralization is achieved using a primarily mineral-based waste product, the resulting precipitate, filter cake, or floc fraction is primarily inorganic. This provides superior properties for dewatering and handling compared to processes that rely on the mandatory addition of organic complexing agents, such as lignin derivatives. Such processes inherently generate a voluminous, organic-rich sludge that is difficult to dewater and costly to dispose of. The inorganic sludge of the present invention is, in contrast, more compact and significantly more suitable for potential recovery and reuse in metallurgical processes.

[0093] In fact, there may be even more advantages when combining waste products from kraft pulping with acidic wastewater from processes such as mining or metal processing. The separated contaminants (precipitate, filter cake and / or floc fraction) contain transition metal ions originating both from the acidic wastewater and from the alkali source. As such they could be utilized in a smelter, electric arc furnace steel plant or similar metal extracting plant as part of the feedstock, this mostly relating to the solids fraction and floc fraction. This way the transition metal content of the acidic wastewater is recovered increasing the overall yield in the process. Similarly, the transition metal content of the alkali source adds further to this yield - at the same time as environmental hazards concerning the safe disposal of this pulp and paper waste product are omitted. The transition metal ions initially withdrawn from the ground by the trees are sent into the circular flows of societies’ metals.

[0094] Experimental examples

[0095] In a series of experiments, the neutralization of acidic washwater from pickling of stainless steel using conventionally used slaked lime and, as an alternative to this, with transition metal ion-containing green liquor dregs from a kraft pulp mill. The slaked lime was a commercial product used in an existing neutralization process at aSwedish steelworks and provided by the same steelworks, which also provided the acidic wash water on which the experiments were performed. Green liquor dregs were provided from a Swedish kraft pulp mill.

[0096] Initially, the acidic wash water was titrated with respective slaked lime and green liquor dregs respectively, in order to establish titration curves. The amount of green liquor dregs was converted to dry matter after dry content determination. Titration curves for the respective slaked lime and green liquor dregs could be seen in Figures 3 and 4.

[0097] In a second series of experiments, neutralization of 201 of water with respective slaked lime and liquid green liquor dregs was carried out to a pH of around 7.5; for the calcium hydroxide a final pH of 7.6 was obtained and for the liquid green liquor dregs 7.7. A further neutralization was also carried out using green liquor dregs to a final pH of 8.5. The green liquor dregs, being the alkali source, were added under vigorous stirring to ensure homogeneity. The neutralized solutions were allowed to stand overnight and then settled. Prior to the purification tests, the samples were remixed vigorously in the first two cases, while the clear phase was sucked off in the third case for use in the further tests.

[0098] Purification of the water then took place with a bench-scale electrocoagulation system using an aluminum electrode and a current of 10 A and a flow of 1001 / h. The water neutralized with calcium hydroxide had a conductivity of 2280 mS / cm, while those neutralized with liquid green liquor dregs had conductivities of 5180 and 5830 mS / cm, respectively. This resulted in voltages of 25 V for the first sample and around 14 V for the two green liquor neutralized samples respectively

[0099] The visual results of the purification of the neutralized waters are presented in Figures 5 to 7. The results of the chemical analyses are reported in Tables 2 and 3.

[0100] Table 2. Results of chemical analyses of purification trials with AxoPur (EC). IN is the average of two independent measurements. All values given in pg / l.

[0101] EC=Electrocoagulation, GLD=Green liquor dregs

[0102] IN After After After After EC After EC GLD clear After EC Neutr Neutr Neutr Lime 7.6 GLD 77 phase 8.5 GLD 8.5 Lime 7.6 GLD 77 GLD 8.5

[0103] Arsenic 130 120 140 220 31 31 33 27 Barium 44 41 1600 2700 18 7.8 14 2.2 Lead 5.55 56 37 59 <0.5 <0.5 1.4 <0.5 Cadmium 1 0.5 57 78 0.12 0.38 1.5 0.48 Cobolt 1600 1500 1400 1500 6.4 3.9 21 7.8

[0104]

[0105] Copper 4900 4400 4500 5700 31 22 130 17 Chromium 120000 110000 110000 140000 390 180 1700 210 Nickel 95000 89000 85000 96000 380 230 1200 150 Vanadium 950 840 970 1800 3 3.2 28 6.4 Zinc 130 790 7400 10000 35 14 170 14

[0106]

[0107] Table 3. Percentage reduction of transition metal ions relative to acidic wash water included, where GLD refers to Green liquor dregs. The fourth column instead measures the reduction of transition metal ions relative to EC included water, i.e. after neutralization.

[0108] Slaked lime GLD 7.7 GLD 8.5 GLD 7.7*

[0109] 7.6

[0110] Arsenic 76% 76% 79% 78%

[0111] Barium 59% 82% 95% 100%

[0112] Lead <100% <100% <100% <100%

[0113] Cadmium 88% 62% 52% 99%

[0114] Cobalt 100% 100% 100% 100%

[0115] Copper 99% 100% 100% 100%

[0116] Chromium 100% 100% 100% 100%

[0117] Nickel 100% 100% 100% 100%

[0118] Vanadium 100% 100% 99% 100%

[0119] Zinc 73% 89% 89% 100%

[0120]

[0121] Table 4. Chemical composition of the green liquor dregs with respect to transition metal ions relative to dry substance (DS).

[0122] Lime act as CaO 42 % DS

[0123] Lead 9.8 mg / kg DS

[0124] Phosphorus 240 mg / kg DS

[0125] Cadmium 12 mg / kg DS

[0126] Potassium 6800 mg / kg DS

[0127] Copper 140 mg / kg DS

[0128] Chromium 99 mg / kg DS

[0129] Mercury <0.047 mg / kg DS

[0130] Nickel 50 mg / kg DS

[0131] Silver 3.8 mg / kg DS

[0132] Tin <0.24 mg / kg DS

[0133] Zinc 1600 mg / kg DS

[0134]

[0135] Sources

[0136] 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 / serylets / purl / 20674942

[0137] Quina, Margarida & Pinheiro, Carolina. (2020). Inorganic Waste Generated in Kraft Pulp Mills: The Transition from Landfill to Industrial Applications. Applied Sciences. 10.2317. 10.3390 / app 10072317: https: / / www.researchgate.net / figure / Chemical-composition-of-green-liquor-dregs-GLD-slaker-grits-SG-lime-mud-LM-and_tbl2_340320203

Claims

1. CLAIMS1. A method for treating acidic wastewater from a non-pulp and paper process, comprising the steps of:neutralizing the acidic wastewater from a non-pulp and paper process to a pH>5.5 by addition of an alkaline waste product from kraft pulping, resulting in a neutralized wastewater;whereinthe acidic wastewater having pH<3 and comprising at least one transition metal contaminant, andthe alkaline waste product comprises at least one transition metal contaminant.

2. The method according to claim 1, wherein the acidic wastewater from a non-pulp and paper process comes from a non-forest industry process.

3. The method according to claim 1, wherein the acidic wastewater from a non-pulp and paper process comes from a process related to mining and / or metal processing.

4. The method according to claim 1, wherein the acidic wastewater from a non-pulp and paper process comes from mines, smelters, foundries, metalworking workshops, surface treatment industries, pickling operations, or stormwater from where metal objects are stored and handled, such as or port operations.

5. The method according to any one of claims 1 to 4, wherein the acidic wastewater from a non-pulp and paper process has pH<2.5, such as pH<2.

6. The method according to any one of claims 1 to 5, wherein the waste product from kraft pulping is selected from the group consisting of green liquor dregs, boiler fly ash, slaker grits and lime mud, or any mixture thereof.

7. The method according to any one of claims 1 to 6, wherein the alkaline waste product from kraft pulping comprises at least 50 wt.-% green liquor dregs, such as at least 70 wt-%, and one or several of boiler fly ash, slaker grits and / or lime mud.

8. The method according to any one of claims 1 to 6, wherein the alkaline waste product from kraft pulping is green liquor dregs.

9. The method according to any one of claims 1 to 8, wherein the acidic wastewater is neutralized to pH> 6, such as >6.5, such as >7.

10. The method according to any one of claims 1 to 11, wherein the acidic wastewater comprises suspended solids, oil, phosphorus, and / or hydrocarbons.

11. The method according to any one of claims 1 to 10, further comprising a step of separating the neutralized wastewater into an aqueous fraction and a precipitate.

12. The method according to claim 11, wherein the separation is performed using sedimentation, decantation and / or screening techniques.

13. The method according to any one of claims 1 to 12, further comprising a treatment step, wherein the neutralized wastewater or aqueous fraction is further treated by means of chemical coagulation / flocculation and / or separation using a filter or filtration techniques, resulting in a further treated aqueous fraction and a filter cake.

14. The method according to any one of claims 1 to 13, wherein the neutralized wastewater, aqueous fraction or further treated aqueous fraction is treated using electrocoagulation (EC), resulting in a floc fraction and a purified water fraction, wherein the floc fraction comprises the transition metal contaminants from both the acidic wastewater and the alkaline waste product.

15. The method according to any one of claims 13 to 14, wherein the precipitate, filter cake and / or floc fraction, comprises transition metal ions from both the acidic wastewater and the alkaline waste product and is suitable for recovery and / or reuse in metallurgical processes, including but not limited to smelting, electric arc furnace operations, or other metal refining or recycling processes.

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