A METHOD AND AN AGENT FOR CHEMICAL PURIFICATION OF WATER BY MEANS OF CHEMICAL PRECIPITATION AND MAGNETIC SLUDGE SEPARATION

Abstract: A method and an agent for chemical purification of water is provided, especially waste water with subsequent magnetic sludge separation. A precipitation agent comprising as component aluminium sulphate and/or iron sulphate is used together with a magnetic material for the purification. In order to provide uniform distribution of the magnetic material in the agent composition it is added to the aluminium and/or iron sulphate component of the composition when said component is in the form of a melt or solution prior to crystallization of said composition.

Description: A METHOD AND AN AGENT FOR CHEMICAL PURIFICATION OF WATER BY MEANS OF CHEMICAL PRECIPITATION AND MAGNETIC SLUDGE SEPARATION The present invention relates to a method-and an agent for chemical pu¬ rification of water, especially wastewater with subsequent magnetic sludge separation. A precipitation agent or precipitant based on alumi¬ nium sulphate or iron sulphate is used together with a magnetic material for the purification. In chemical purification of wastewater, a chemical is added to the water which is precipitated in the wastewater in the form of flocculated che¬ mical compounds. The primary task of the added chemical is to precipi- tate the phosphates dissolved in the wastewater, which would otherwise result in fertilization of the receiving body of water. Furthermore, metals present in the water are precipitated to a large extent. The floes have the ability of thereby effectively binding any substances suspended in the wastewater. By this treatment there is thus obtained a clear water, to a large extent liberated from suspended substances, bac¬ teria, viruses, metals and phosphates. The content of oxygen-consuming organic substance falls considerably in this treatment. The sludge formed in the process sketched above, and containing the i - purities, is separated in conventional wastewater treatment plants by sedimentation or flotation. These operations are relatively voluminous since they cannot be carried out at high surface load. Sedimentation is 3 2 generally carried out at a surface load as low as about 1 m /m x h and 3 2 flotation at a maximum surface load of about 5-10 m /m x h. As an alternative to this conventional sedimentation or flotation, mag¬ netic separation has been brought in, which enables the removal of par¬ ticles down to colloidal size from many types of wastewater. The basic technique was originally developed at the end of the sixties in con- nection with the solution to the problem of removing ~ ry fine paramag¬ netic discolouring particles from kaolin clays used in the production of paper. It was quickly appreciated that the new technique was not limited solely to the treatment of kaolin clays, and could also be used for treating wastewater. It was then found that with the technique of ag- OMPI -A linn--- netic sludge separation the surface load could be increased substant- 3 2 ially also, right up to about 50-200 /m x h. This has the advantage that magnetic sludge separation requires less space than conventional sludge separation. To obtain an approximate idea of the differences, a plant for magnetic sludge separation can be compared to a plant hav¬ ing the same capacity for sedimentation, the former requiring at most 1/10 as great a space as the latter. Attempts have been made to utilize conventional flocculation agents in combination with magnetic material for purifying wastewater in a more effective and less voluminous way. Mechanical blending of, for example, aluminium sulphate and magnetic material has been used for this purpose, but it has been found difficult to provide even distribution of the mag¬ netic material in the floes formed. Such even distribution of the magne- tic material is necessary for the sludge separation to be carried out in an effective manner. According to the invention it has now been surprisingly found that if a composition is used as precipitant, this composition being obtained by adding the magnetic material to the flocculation chemicals of the preci¬ pitant when it is in the form of a melt or solution* very uniform dist¬ ribution of the magnetic material in conjunction with flocculation can be obtained, and thereby a more effective separation of the floes formed as well. The precipitants of interest in conjunction with the invention are aluminium sulphate and/or iron(II)sulphate. A preferred flocculant is aluminium sulphate which, when it is present as a melt, easily allows itself to be blended with the magnetic material, whereby the crystalli¬ sation of the melt under stirring leads to the formation of a composi¬ tion based on aluminium sulphate in which the magnetic material is very evenly distributed. Iron(II)sulphate is also amenable to be transferred into a melt or solution, but in this case some water must be added for maintaining the right water content, to compensate for the water which . is driven off by vaporization in connection with melting the iron sul¬ phate. By "magnetic materials" are meant here and in the following such mate¬ rials which are attracted by magnets, i.e. materials that can be sepa- -xZVREAΪ O PI rated with the help of magnetic fields. Magnetic materials thus com¬ prise ferromagnetic materials, which are attracted already by relati¬ vely weak magnets, and paramagnetic materials which can be attracted by stronger magnets, e.g. of the high-gradient type which can achieve a magnetic flux density of about 2 tesla (20000 gauss) and above, or such superconductive types with a maximum magnetic flux density of 10-15 tesla. Included in the ferromagnetic materials are iron, nickel and cobalt, as well as certain alloys and oxides of these substances. Such magnetic materials based on iron are, of course, to be preferred due to their easy availability and low price. The use of iron oxides is especially preferred. The magnetic material used in the present inven¬ tion thus expediently consists of magnetite or other magnetic iron ox¬ ide, such as the iron oxide obtained in magnetite - yielding roasting of pyrites or other materials containing iron sulphide. Seen in percent by weight, the magnetic material preferably constitu¬ tes the minor portion of the composition, e.g. 1-50 percent by weight, suitably 2-30 percent by weight and especially 3-10 percent by weight of the composition. The invention thus relates to a process for chemical purification of water with subsequent magnetic sludge separation, and to an agent for carrying out this method. Remaining characterizing features of the in¬ vention are apparent from the patent claims. The invention will now be described in detail by means of non-restricting examples in conjunction with the appended drawing, which diagrammatically shows a system for magnetic separation used in the development of the technique according to the present invention. The apparatus shown diagrammatically on the attached drawing is based on a Sala-HGMS magnetic separator (from Sala International, Sala, Sweden). The central portion of the plant consists of a magnet generally denoted by the numeral 1, provided with a matrix-containing through- flow tube 2 having a diameter of about 10 cm and a length of about 15 cm. In the present experiments, a matrix 3 of expanded metal was used, i.e. a metal given a wire-netting-like appearance, which was packed complet- ely randomly. The magnet was otherwise conventionally provided with an iron core 4 and a magnet winding 5. Vertically above the matrix 3 there is arranged a flocculation vessel 6 provided with a propeller st rrer 7 and interior baffles, not shown. This flocculation vessel 6 is connected to the matrix 3 by means of a pipe 8. In the embodiment in question, the flocculation vessel 6 has a volume of about 20 liters while the pipe 8 has a bore of about 10 mm. Associated with the magnetic separator there is a current supply unit 9, a heat exchanger 10 for cooling water, and a control panel 11 for regu¬ lating valves etc. The current supply is adjustable in eight steps, and the magnetic flux density achieved is in the range 0-2.0 T (tesla). The water flow through the apparatus is regulated by a throttle valve 13 in the outlet pipe 12 downstream of the magnet. The valve 13 is controlled by the control panel 11 through regulator 14. Purified water leaves the apparatus as indicated by arrow 15. The load is given in meters per hour (m/h) calculated on the cross sectional area of the through-flow tube. Side pipes 16, 17 with associated shut-off valves 18, 19 controlled by the control panel 11 through regulators 20, 21 are arranged for counter flushing, to clean the matrix. Water is lead to pipe 16 as indi¬ cated by arrow and out from pipe 17 as indicated by arrow. When per¬ forming counter flushing the pipes 8, 12 are shut off by shut-off valves 13, 22 associated with regulators 14, 23. The regulators 14, 20, 21 and • 23 are controlled via conduits from control panel 11 as indicated at 24. The current supply to magnet 1 and control panel 11 is taken from current supply unit 9 as indicated by conduits 25. The heat exchanger 10 cools outgoing cooling water from the magnet 1 as indicated by conduit 26 and may also influence the power supply as indicated by connection 27. So-called "beaker experiments" were carried out using 1-liter glass beak¬ ers provided with gate sti rers with the object of finding out whether addition of magnetic material has any disturbing effect on the floccula¬ tion. The flocculation chemicals were added as a 10 percent by weight so- lution during a quick admixture of 5-10 seconds, whereafter stirring was reduced to a peripheral speed of the stirrer of about 0.1 m/sec, for a ρUREΛ ? s J)MPI_ flocculation period of 10 minutes. After 10 minutes sedimentation, 100ml samples were decanted for analysis with respect to turbidity (JTU; Jackson Turbidity Units) and total phosphorus (P tot ) in mg/1. The experiments in the magnetic separation apparatus described above were carried out in batches in such a way that the whole system was filled with drinking water up to the bottom of the flocculation vessel 6. All valves were subsequently closed and 5 liters of wastewater was transferred to the flocculation vessel. Dosing of the chemicals was car¬ ried out with a 10 percent by weight solution during a rapid admixture for 10 seconds. Stirring was thereafter reduced to the lowest possible, i.e. about 30 revolutions per minute for 5 minutes for flocculation. The water is thereafter allowed to pass through the magnetic separator under gravity, and samples for analysis were taken out after 3 liters of water had passed through the magnetic separator. The experiments were carried out at varying loads and with varying magnetic flux density. In the per- culation tube, the load was thus varied between 62 and 470 meters per hour and the magnetic flux density between 0.07 and 1.60 T. It was found that the purification effect deteriorated with increased load or falling flux density. The values selected for the experiments represent suitable experimental conditions in the apparatus used. After each experiment, the current supply was cut off and the matrix was flushed clean by means of the counter flushing arrangement. Samples taken were analyzed for turbidity and total phosphorus. Some of the experimental results are given in the tables 1-3 below. The flocculation chemicals used in the experiments are wastewater grade alu¬ minium sulphate (AVR), to which, while in the form of a melt had been added 5 percent by weight of magnetite or 1-20 percent by weight of iron oxide (JOX). AVR is a special chemical for wastewater purification, con¬ taining active aluminium, iron and silicon compounds. Its chemical com¬ position is as follows: Al appr. 7 % Fe " 3 % Sulphuric acid deficiency " 1 % Waterinsoluble " 2.5 % Active substance 3.2 moles/kg Aluminium is present as Al 2 (S0 4 ) 3 • 16-17 H 2 0 Iron is present as Fe 2 (S0.) 3 • 9 H 2 0 The water-insoluble part consists mainly of silicate mineral. The AVR with the addition of magnetic material has been produced in the laboratory especially for these experiments. A mechanical mixture of AVR and magnetic material has been prepared, as well as a mixture con¬ sisting of magnetic material added to the final solution in connection with the normal AVR-manufacture. The AVR solution or melt, and the mag¬ netic material are blended by stirring for 5 minutes, after which the melt is allowed to solidify while forming the solid crystalline product with the magnetic material uniformly distributed therein. The product is crushed and dissolved in water to a content of about 10 percent by weight for use as a flocculation chemical. The magnetite used in the experiments is of laboratory quality, puriss (Kebo). The iron oxide (JOX) comes from a sulphuric acid factory and 2+ contains 65.8 % ^ e +0 » 0-5 % being Fe . For a magnetic flux density of 0.15 T the iron oxide contains 24.5 % magnetic material. Wastewater used for the experiments was collected daily from Sala Mu¬ nicipality Wastewater Treatment Works, which is a conventional plant 3 working with postprecipitation and an AVR dosage of 75 g/m . Table 1 Experiments with biologically purified wastewater Experiment Chemical Pt.ot. mιg/1 Turbidity JTU 1 Untreated water 0.59 9.8 2 75 mg/1 AVR 0.15 12 3 75 mg/1 AVR + 5 % magnetite 0.18 12 4 0.08 4.5 5 0.20 8.5 Experiment 1 relates to assessment of the unpurified wastewater without any treatment. Experiments 2 and 3 relate to beaker experiments, experi- ent 2 solely using conventional flocculating agent (AVR) and experi¬ ment 3 being performed in beakers with AVR containing 5 percent by weight magnetite in a fused-in homogeneous form. Sludge separation takes place here by sedimentation. Experiment 4 relates to the use of the same flocculation agent as in Experiment 3, but carried out with magnetic se¬ paration. Finally, Experiment 5 relates to magnetic separation carried out with a solely mechanically blended composition of AVR and magne¬ tite. During the magnetic separation a load of 62 m/h and a magnetic flux density of 0.47 T were used. The effect of applying the technique according to the present invention is directly apparent from a comparison between the Experiments 4 and 5, where Experiment 4, according to the table, gives substantially better purification, both with regard to the figure for P. . and for turbidity. Table 2. Further experiments carried out with varying compositions. Load: 62 m/h Magnetii : flux density: 0.47 T Experiment Chemicals 1 P tot m /] Turbidity JTU 6 Untreated water 0.72 11 7 75 mg/1 AVR, plus 5 % fused-in magnetite 0.07 4.2 8 75 mg/1 AVR plus 5 % fused-in ground magnetite 0.17 4.0 9 75 mg/1 AVR plus y 1 % fused-in JOX 0.32 18 10 75 mg/1 AVR plus 1 % fused-in ground JOX 0.38 17 11 75 mg/1 AVR plus 1 % JOX, mechanical blend 0.43 18 12 75 mg/1 AVR plus 5 % fused-in JOX 0.12 6.1 13 75 mg/1 AVR plus 5 % fused-in ground JOX 0.13 6.7 14 75 mg/1 AVR plus 5 % JOX, mechanical blend 0.35 20 15 75 mg/1 AVR plus 10 % fused-in JOX 0.20 8.2 16 75 mg/1 AVR 0.29 23 x) Iron oxide The technical effect on applying the technique according to the present invention will be seen from this table, i.e. in comparing between the examples 13 and 14, for example with fused-in and mechanically blended iron oxide. It is also apparent from the table that the use of 5 percent by weight of ferromagnetic material gives the best effect. In Experi¬ ments 8, 10 and 13 magnetic material was used which was ground for 8 minutes in a shatter box mill. This grinding operation did not appear to have any substantial effect. Table 3. The effect of the time for adding magnetite in experiments with biologically purified wastewater. Load: 62 m/h Magnetic flux density: 0.47 T Chemical dosage: 75 mg/1 AVR + 3.75 mg/1 magnetite Magnetite addition tot mg/1 Turbidity JTU 1 minute before adding AVR 0.15 12 Fused-in into AVR 0.07 " 6.5 1 minute after adding AVR 0.16 13 The advantage with fusing-in the magnetite into the aluminium sulphate will" be seen immediately from these experimental results, compared with separate addition, independent of whether the latter takes place before or after the addition of aluminium sulphate. Tentative experiments have shown that the use of paramagnetic substances, such as oxides and hydroxides of manganese, chromium and tri-valent iron (hematite) give similar results to the above for ferromagnetic substan¬ ces. However, a higher magnetic flux density and/or lower water flow rate is required when using paramagnetic substances, than for the use of ferromagnetic additives to achieve the same amount of purification. Further experiments have shown that the grain size of the magnetic ma- UREAZ OMPI terial used has an influence of great importance on the purifying re¬ sult. Thus, small grain sizes have shown to provide that no substantial sedi- mentation of the magnetic material occurs and that all floes which are formed at the chemical precipitation will contain magnetic material. The importance of the grain size is illustrated of the following results, ■ which have been obtained when performing a chemical precipitation of biologically purified waste water and a subsequent magnetic sludge sepa¬ ration. The waste water had previous to the precipitation a content of total phosphorus (P t t ) of 2.3 mg/1 in test run I and 1.1 mg/1 in test run II, respectively. The precipitation was carried out by means of 150 mg/1 AVR containing 5% by weight fused-in magnetite. The magnetic flux density of the used magnet was 0.1 T and the surface load was approxi¬ mately 100 m/h. The grain size is given as the grain size median, i.e. 50% by weight of the material has smaller grain sizes than the median value. Magnetic material Phosphorus content grain size separated u mg/1 I II 2.5 1.30 0.50 8 0.70 0.35 16 0.60 0.25 30 0.50 0.25 75 0.40 0.20 The obtained results unequivocally exhibit that the purifying effect in¬ creases with decreasing grain size and with increasing P. . previous to precipitation and that the grains should be at least below 8 Aim, prefer¬ ably below 3 i-m in size. It has also been found possible to incorporate activated carbon into the precipitant composition while in a molten state, the carbon also being evenly distributed in the crystallized product. The advantage of having activated carbon in the product is that it has the ability of absorbing dissolved organic compounds out of the wastewater, these compounds not being usually removed in chemical purification. Since such compounds are often of a toxic nature, it will be seen that treatment of wastewater with active carbon is of great value. It has been found that a composi¬ tion of the kind described hereinbefore, containing about 10 percent active carbon, apart from about 5 percent magnetite, for example, and at a dosage of 75 mg/1 has the ability of removing up to 3 mg per liter dis¬ solved organic substance, in addition to the amount removed by a composi- tion not containing activated carbon.

Claims: CLAIMS : - 1. A method of chemically purifying water with subsequent magnetic sludge separation, the purification being carried out while using a crystalline precipitant based on A!,,(SO.), and/or FeSO., together with a magnetic material, characterized by using as a precipitant a compo- sition which, for attaining uniform distribution of the magnetic mate¬ rial in the composition, is obtained by adding the magnetic material to the A1 2 (S0.) 3 or FeSO. component of the precipitant when it is in the form of a melt or solution prior to crystallization. 2. A method as claimed in claim 1, characterized in that the magnetic material consists of magnetite or other magnetic iron oxide, such as is obtained in magnetite yielding roasting of pyrites. 3. A method as claimed in claim 1, characterized in that the magnetic material constitutes 1-50, preferably 2-30, and especially 3-10 percent by weight of the composition. 4. A method as claimed in any of the preceding claims, characterized in that the magnetic material has a grain size median substantially below 8 ^ιm, preferably below 3 yum. 5. A method as claimed in any of the preceding claims, characterized in that there is used as precipitant a composition also containing activa¬ ted carbon. 6. A precipitant for carrying out the method according to claim 1, for chemical purification of water with subsequent magnetic sludge separation, purification being carried out while using a crystalline precipitant based on A1 2 (S0.) 3 and/or FeSO,, together with a magnetic material, characterized in that it consists of a composition which, for attaining uniform distribution of the magnetic material in the com¬ position, has been obtained by the addition of the magnetic material to the A1 2 (S0 4 ) 3 and/or FeSO^ component of the precipitant when this is in the form of a melt or solution before crystallization. 7. A precipitant as claimed in claim 6, characterized in that the mag- JUREATΓ OMPI netic material consists of magnetite or other magnetic iron oxide, e.g. obtained in magnetite yielding roasting of pyrites. 8. A precipitant as claimed in claim 6 or 7, characterized in that the magnetic material constitutes 1-50, preferably 2-30, and especially 3-10 percent by weight of the composition. 9. A precipitant as claimed in any of the preceding claims, character¬ ized in that the magnetic material has a grain size median substant- ially below 8 urn, preferably below 3 um. 10. A precipitant as claimed in any of claims 6-9, characterized in that the composition also contains activated carbon. AMEN L (received by the International Bureau on 2 March 1979 (02.03.79)) 1. A method of chemically purifying water with subsequent magnetic sludge separation, the purification being carried out while using a crystalline precipitant based on A1 2 (S0 4 ) 3 and/or FeSO-, together with a magnetic material, characterized by using as a precipitant a solid, crystalline composition which, for attaining uniform distribution of the magnetic material in the composition, is obtained by adding the magnetic material to the A1 2 (S0.) 3 or FeSO. component of the precipitant when it is in the form of a melt or solution prior to crystallization. 2. A method as claimed in claim 1, characterized in that the magnetic material consists of magnetite or other magnetic iron oxide, such as is obtained in magnetite yielding roasting of pyrites. 3. A method as claimed in claim 1, characterized in that the magnetic material constitutes 1-50, preferably 2-30, and especially 3-10 percent by weight of the composition. 4. A method as claimed in any of the preceding claims, characterized in that the magnetic material has a grain size median substantially below 8 μm, preferably below 3 μm. 5. A method as claimed in any of the preceding claims, characterized in that there is used as precipitant a composition also containing activa¬ ted carbon. 6. A precipitant for carrying out the method according to claim 1, for chemical purification of water with subsequent magnetic sludge separation, purification being carried out while using a crystalline precipitant based on A1 2 (S0.) 3 and/or FeSO., together with a magnetic material, characterized in that it consists of a solid, crystalline com¬ position which, for attaining uniform distribution of the magnetic mate¬ rial in the composition, has been obtained by the addition of the mag¬ netic material to the A1 2 (S0.) 3 and/or FeSO. component of the precipi¬ tant when this is in the form of a melt or solution before crystallization. f -BUREAU O PI y. STATEMENT UNDER ARΗCLE 19 This is in reply to the international search report of 1979-01-18. Considering the documents found in the inter¬ national search report we file a new page 11, where we have made the following amendments: The words "solid, crystalline" have been inserted in our claim 1 line 4 after "as a precipitant a - " and in our claim 6 line 30 after "it consists of a - ". These amendments are based upon what is said on page 6 line 12 in the description, and are made for the purpose of showing in a more clear way how the invention differs from the prior art. Further, the amendments have only a minute impact on the description since one of the essential objectives of the invention is the use of a solid agent as a precipitant. However, to uniform the description and the claims the words "solid, crystalline" could be inserted on page 2 line 28 after "the formation of a - ", even though it is said on page 2 l ne 26-28 that the melt is crystallized. lϊU EA^ OMPI

Inventors: SIGVARD H
Applicants: BOLIDEN AB, SIGVARD H
Application year: 1978
Main classification: C02C5
IPC codes: B03C1, C02F1