A METHOD FOR PRODUCING WATER-PURIFYING CHEMICALS

Abstract: The present invention relates to a method for producing water-purifying chemicals containing trivalent iron by oxidis­ ing a compound containing divalent iron, or iron via divalent iron in the presence of hydrochloric acid, the oxidation being ef­ fected by means of chlorate and/or hypochlorite ions to obtain formed ferri-ions in solution, the hydrochloric acid present in an amount beneath the stoichiometric amount.

Description: A METHOD FOR PRODUCING WATER-PURIFYING CHEMICALS DECRIPTION Technical Field The present invention relates to a method for producing from divalent iron a water-purifying chemical that contains trivalent iron. The object of the invention is to provide a simple and rational process for producing from a divalent iron starting material a water -purifying chemical that contains trivalent iron. More specifically, the object of the invention is to oxidise divalent iron to trivalent iron in a simple and rational manner. Background Art Salts containing trivalent iron are presently used as precipitating chemicals in water-purifying processes, the ferri-ions having the ability to form flocs containing colloidal substances present in the water, together with the co-precipitation of the phosphates present. The trivalent iron is often added in chloride form, or in the form of a double salt comprising chloride and sulphate anions. Such water-purifying chemicals are delivered in the form of solutions containing 12% iron and are used in large purifying plants with short through-flow times. With certain purifying plants it is possible to use ferrosulphate heptahydrate, which is less expensive, particularly in the case of long through-flow times. In this case, the ferro-ions are oxidised to ferri-ions by the biological substances present in the water, these ferri-ions serving as the coagulating (flocculating) ions in the system. Ferrosulphate-heptahydrate is a fatal product obtained in large quantities, inter alia, when pickling iron objects in sulphuric acid. Ferro-sulphate is used at present as a starting material in the manufacture of water-purifying chemicals containing trivalent iron, particularly ferri-chloride, the ferro-sulphate, dissolved in hydrochloric acid, being oxidised in counter-flow with chlorine gas in a reaction tower. For environmental reasons and reasons of working hygiene, the equipment and apparatus used with this manufacturing process must be well sealed and the surroundings well ventilated, which renders the process expensive. Consequently, there is a desire for simple manufacturing methods which can be carried out in simpler kinds of apparatus. In this respect, it would be of great advantage if oxidation of the ferro-sulphate could be effected at the local water-purification plant and that the need for large central production plants could be avoided, thereby eliminating the high costs incurred by the transportation of the chemicals required. Also commercially available is cheap scrap iron which could be oxidised to trivalent iron, particularly in conjunction with chlorine, since ferri-chloride is a water-soluble compound. When dissolving metallic iron in hydrochloric acid, however, only divalent iron is obtained, it being possible to obtain only some few percent Fe2+ in solution. It is also desirable to be able to oxidise metallic iron to trivalent iron to obtain iron chloride, and particularly iron chloride having a high Fe-content, since it is the Fe3+-ions which produce flocculation when used as a water-purifying chemical. Brief Description of the Invention Surprisingly, it has now been found possible to oxidise ferro-ions to ferri-ions in accordance with the invention, in simple apparatus and at a cost which competes well with previously known and used oxidation methods, despite the fact that these known methods use extremely cheap chlorine gas. Accordingly, the present invention is characterised by oxidising divalent iron in hydrochloric acid by means of a chlorate iron, to obtain ferri-ions in solution. Further characterising features of the invention are set forth in the accompanying claims. The present invention is well suited for oxidising ferrosulphateheptahydrate to ferrisulphate-chloride. The reaction follow the formula 6 FeS04. 7H20 + NaC103 + 6HC1 6 FeSO4Cl + lOH2O + NaCl Stoichiometrically, 1th of a mole C10 3 should be consumed for each mole mole of Fe2+. Surprisingly, it is found that when practising the method according to the present invention only 60-96% of the stoichiometric amount of chlorine is required for oxidation. If only 60% chlorate ions are used, a minor quantity ( < 5% Fe) will remain as Fe2+. The resultant solutions, however, contain no residues that are liable to settle, or in any event, no disturbing residues. In the odd case, it is possible that a bottom sediment of one or two tenths of one percent is obtained immediately after oxidation. If hydrochloric acid is not present in the amount stated, oxidation will be incomplete and an insoluble ferro-sulphate precipitate is obtained, which results in a poor end product. In accordance with the invention, it is also possible to dissolve metallic iron in concentrated hydrochloric acid, the hydrochloric acid obtaining a highest concentration of 3-4% Fe. By continually adding chlorate ions to such a hydrochloric solution, dissolved Fe2+ is oxidised to Fe3+, and more iron can be dissolved out and 3+ oxidised to Fe These reactions follow the formuli 6 Fe + 12 HCl --- > 6 FeCl2 + 6H2 6 FeCl2 + NaClO3 + 6 HCl --- > 6 FeCl3 + 3H2O + NaCl. Through this aspect of the invention it is also possible to obtain a ferrichloride-solution containing up to 15% Fe. In accordance with this method, scrap iron is dissolved in concentrated hydrochloric acid, whereafter chlorate is added continously in an amount corresponding to the amount of iron dissolved and oxidised, i.e. the amount of Fe2+ obtained. A minor quantity of water should be added, in order to increase the solubility of FeC12. FeC12 is more soluble in water than in hydrochloric acid, and provided that the solution contains large quantities of hydrochloric acid, crystallisation of FeC12 can take place. It has been found, however, that the amount of hydrochloric acid required in the reaction is less than the stoichiometric amount, while the amount of chlorate required is the stoichiometric amount, or an amount slightly thereabove. If the chlorate ions are added at a faster rate than Fe2+ is formed, chlorine dioxide is liable to fume off, as a result of the reaction temperatures generated. The reaction Fe2+ ? foe3+ is exothermic. The invention will now be described in more detail with reference to the following working examples, of which some have been included for comparison purposes. Example 1 1000 grams of ferrosulphate-heptahydrate were suspended in 340 grams of 32-HC1 and 110 grams of H20, whereafter 50 grams of NaC103 were added. During the process of oxidation, oxidised iron passes into solution, whereupon an end product is obtained in the form of a clear solution containing less than Otl residual Fe2+. After being left to stand for one week, the vessel was found to have a bottom sediment of 0.1%, while the solution was quite clear. EXAMPLES Ex FeSO4 . 7H2O HCl NaClO3 H2O Cl2 NaCl Comments 32% nr g (18%Fe) 9 9 g 9 g 2 100 34 3.5 12.5 - - Good; 3% Fe2+, clear 3 100 30 5 15 - - Good; < #O.1% Fe2+,clear < 0.2% sed. 4 100 34 4 12 - - Good; 1% Fe2+ ,clear 5 100 25 3.5 21.5 - - Good; 3% Fe2+ ,clear < 0.1% sed. 6 100 - 35 15 - - Poor;total cryst prec. 7 100 37 - 13 - - Poor; 30% undissolved FeSO4 8 100 35.6 1.75 12.6 - - Poor; 20% undissolved FeSO4 9 100 19 5 18.5 - 7.5 Poor; high cryst. prec. 10 100X 12 4.5 38 Poor; 5% bottom sediment (1 W) turbid 11 100 28 8.8 lb - - Poor; 3% bottom sediment 12 100 32 5.3 13 - - Unsuitable, 0.5% bottom sediment (1 W) 13 100 12 3.5 34.5 - - Poor; 2% bottom sediment (1 W) turbid 14 100 45 5.0 - - - Poor; C102 formation 15 100 22.5 5.0 22.5 - - Poor; precipitation 16 100 12 - 38 5.5 - Poor; precipitation x) 11.6 Fe total Example 17 64 grams of scrap iron were dissolved in 320 grams of 32%-HC1 and 40 grams of water. After eight hours, the hydrochloric acid contained 3.5% Fe2+. A total of 21.5 grams sodium chlorate was then added continuously for 16 hours. As an end product there was obtained 420 grams of FeCl3-solution, which was 15% with respect to iron. The density was 1.45. Example 18 Example 17 was repeated, but with the difference that the sodium chlorate was added discontinuously, in seven batches of 4 grams per batch over a period of 16 hours. Consequently, 28 grams of NaC13 were required. It was noted, however, that discernible quantities of chlorine dioxide were driven off with each addition. In other respects the yield was the same. Although sodium chlorate was used in the above working examples, it will be obvious to all of normal skill in this art that any chlorate can be used, such as potassium chlorate, (calcium chlorate) ammonium chlorate and barium chlorate, although sodium chlorate is to be preferred for reasons of cost. The chlorates given in parenthesis are not preferred, although they are able to provide a sulphate precipitation (CaS04, BaSO4). Hypochlorite can be used instead of chlorate, although in this case larger quantities must be used. As with chlorate, corresponding salts can also be used.

Claims: CLAIMS 1. A method for manufacturing a water-purifying chemical containing trivalent iron by oxidising a compound containing divalent iron in the presence of hydrochloric acid, characterised by oxidising divalent iron in hydrochloric acid by means of chlorate and/or hypochlorite ions to obtain ferri-ions in solution, the hydrochloric acid preferably being present in an amount beneath the soichiometric amount. 2. A method according to Claim 1, characterised by oxidising ferrosulphate-heptahydrate in hydrochloric acid, the hydrochloric acid being present in an amount corresponding to 5-8% of the weight of the ingoing ferrosulphate-heptahydrate and calculated as 100% HC1, and the chlorate ions being present in an amount corresponding to 3.5-5.5% of the weight of the ingoing ferrosulphate-heptahydrate and calculated as sodium chlorate. 3. A method according to Claim 1, characterised by dissolving metallic iron in concentrated hydrochloric acid, whereafter resultant ferro-ions are continuously oxidised to ferri-ions with chlorate ions to form a ferri-chloride solution, the hydrochloric acid being present in an amount corresponding to 2.3-2.6 moles per mole of ingoing iron, and in that the chlorate ions are added in an amount corresponding at least to the stoichiometric amount. 4. A method according to Claim 2, characterised in that water is introduced into the reaction mixture in an amount such that the ingoing amount of ferrosulphate-heptahydrate constitutes two thirds of the total reaction mixture. 5. A method according to Claim 3, characterised in that the water is introduced into the reaction mixture in order to increase the solubility of the ferri-chloride formed, the water being added in an amount corresponding to 7-10% of the total reaction mixture.

Inventors: HELLSTROM, BENGT LOUIS CHRISTIAN, STEEN, BILLY DENNIS, STENDAHL, KJELL ERIK, Hellström, Bengt Louis Christian
Applicants: BOLIDEN AKTIEBOLAG
Application year: 1983
Main classification: C01G49
IPC codes: C01G49, C02F1