PRODUCTION OF MAGNESIUM NITRATE SOLUTIONS

Abstract: A magnesium nitrate solution (32-40 % w/v) is prepared by suspending a magnesium compound in water, adding 50-60 % w/w nitric acid in such an amount that the final pH is from 5,0 to 6,0, and filtering the product so as to separate magnesium nitrate as clear filtrate. The very pure product obtained is preferably used for thermal stabilization of ammonium nitrate-containing com­ positions.

Description: PRODUCTION OF MAGNESIUM NITRATE SOLUTIONS This invention relates to a process for preparing magnesium nitrate solutions and to ammonium nitrate-containing compositions, especially fertilisers, containing magnesium nitrate additives and to processes for their production. Conventional prilled and granulated fertilisers consisting essentially of, or comprising significant quantities of, ammonium nitrate suffer from two problems which render their storage and application to the land more difficult. Firstly, because of a change in the crystal structure of ammonium nitrate which occurs at about 32 "C, they tend to break down to form very fine particles which are difficult to handle, and secondly, they tend to cake on prolonged storage because of water solubility of ammonium nitrate. A method of allieviating these two difficulties which has been widely adopted is the addition of magnesium nitrate to the fertiliser which additive acts as a stabiliser for the ammonium nitrate and as an internal dessicant since it absorbs water to form a dry hydrate and thus reduces the tendence of the fertiliser to cake. The magnesium nitrate additive has conventionally been made by neutralising nitric acid with a commercially available grade of magnesium oxide or magnesium carbonate. It has been discovered that the addition of such additives to fertiliser plant is disadvantageous in that increased fouling of the heat exchange surfaces and the spray devices is encountered. These problems have now been found to be associated with the presence of impurities especially iron and aluminium compounds in the commercial grades of magnesium compounds employed in the neutralisation step. Since the use of purified magnesium compounds in the production of the nitrate additive is prohibitively expensive, the use of impure magnesium nitrate additives has continued. We have now discovered a process for the production of a magnesium nitrate additive of acceptable purity from the commercially available grades of magnesium salts which produces a substantially pure magnesium nitrate, the use of which as an additive to fertilisers does not lead to the difficulties outlined above and which is especially efficacious as an additive to the fertilisers. From one aspect our invention provides a process for the production of a magnesium nitrate solution which comprises the steps of (a) Adding a water insoluble magnesium compound to water so as to form an aqueous suspension thereof; (b) Adding 50-60 Z w/w nitric acid to the suspension formed in step (a) in such an amount that the final pH of the reaction medium is in the range 5,0 to 6,0; (c) Filtering the product from step (b) so as to separate an aqueous solution comprising magnesium nitrate as the filtrate from any insoluble material. We have discovered that these novel procedures lead to the separation of iron and aluminium salts during step (b) which are readily removed from the filtrate in step (c). Thus the process of the invention lead to the formation of a relatively pure magnesium nitrate from a commercial grade of magnesium compound. The process of the present invention may therefore utilise any grade.of magnesium compound as a starting material. The preferred compounds for present use are magnesium oxide, magnesium hydroxide and magnesium carbonate, the use of magnesium oxide being especially preferred. In particular, the commercial grades of magnesium oxide obtained by calcination of magnesite or of magnesium compounds precipitated from sea water with dolomite which contain about 93 to 96 % by weight of magnesium oxide and various impurities especially the oxides of iron, aluminium and silicon, may be employed. The magnesium compound is preferably in the form of finely divided particles since these are more easily dispersed in the water. The preferred particle size is in the range 50 to 200 micrometres. The magnesium compound is suspended in water. Sufficient water should be used to ensure that a homogenous suspension is formed upon agitation but the use of an excessive quantity of water should be avoided in order to minimise dilution of the magnesium nitrate product. In general, we prefer to use from 2.5 to 3.5 gms of water per gm of magnesium compound (expressed as magnesium oxide) which is added. The nitric acid is then added to the suspension at a controlled rate with thorough mixing. The actual temperature of the reaction medium is not critical, but temperatures which are sufficiently high to bring about the evolution of brown nitrogen oxide fumes produced by decomposition of the acid are undesirable. The concentration of the nitric acid reactant is also not crucial. Again, if too strong an acid is employed there will be a greater tendency to evolve nitrogen oxide fumes. If too weak an acid is employed, the product solution will be correspondingly more dilute. In general, we prefer to employ acids containing 55 to 58 Z wiw Yen03. The manner in which the reaction is carried out can have the significant effect upon the course of the reaction. If localised areas of high acid concentration and high temperature are allowed to form there is an increased tendency for any oxides of iron and aluminium which may be present to dissolve and on further mixing to reprecipitate in a form which is difficult to filter. It is therefore preferable to control the rate of acid addition and ensure that the reactants are thoroughly and rapidly mixed. The amount of nitric acid which is added should not exceed that required to neutralise completely the magnesium compound. Preferably not more than 99 Z of this quantity of acid is employed. Thus, preferably, the processes of the invention comprise the addition of from 90 to 99 Z, more preferably 96 to 99 %, of the quantity of nitric acid required to completely neutralise the magnesium salt. The control of the degree of neutralisation can conveniently be achieved by monotoring the pH of the reaction medium. The final pH is in the range 5.0 to 6.0, preferably 5.2 to 5.4. Lower pH1s indicate that excess acid has been added and increase the degree of contamination of the filtrate with soluble iron and aluminium compounds. The reaction product is then filtered so as to separate the insolubles and collect a clear filtrate which is a solution of magnesium nitrate. The impurities are readily filterable and can be separated using conventional filtration techniques e.g. a pressure leaf filter. The concentration of magnesium nitrate in the filtrate is below saturation so as to avoid any precipitation of magnesium nitrate, e.g. below 40 X w/w at ambient temperature. The quantity of water utilised- in the reaction is preferably adjusted so that the concentration of magnesium nitrate in the filtrate is in the range 32 to 40, e.g. 34 to 38 Z w/w. The production of more dilute solutions is less preferred since they are dried in the fertiliser production process and more dilute solutions thereby simply increase the evaporator load in the process. More concentrated solutions can be used but may require heating of pipelines and vessels to avoid crystallisation. The magnesium nitrate additive is added to the ammonium -nitrate composition, especially ammonium nitrate fertiliser in conventional quantities, normally about 0.5 Z by weight as MgO of the ammoniua nitrate. Preferably quantities of from 0.2 to 1.0, e.g. 0.3 to 0.7 Z by weight are added. The magnesium nitrate may be added to any convenient point in the ammonium nitrate manufacturing operation provided that it is subsequently dehydrated so that it is not in the fully hydrated form at the end of the manufacturing process and is thereby capable of functioning as an internal dessicant. Thus, in a typical process for the production of a prilled ammonium nitrate fertiliser the magnesium nitrate must be added prior to the final evaporation stage following which the ammonium nitrate is fed to the prilling tower. We prefer to add the nitrate solution to the ammonialnitric acid reaction vessel for reason of convenience, but equally it can be introduced at a later stage of the manufacturing procedure. Ammonium nitrate fertilisers comprising magnesium nitrate additives which are substantially free from iron and aluminium impuri ties are believed to be novel and constitute a further aspect of our invention. These additives are preferably produced from magnesium com- pounds which contain such impurities by the process hereinbefore described. The presence of some iron and aluminium impurities in the magnesium nitrate additive and the fertiliser may be tolerated. The proportion of these impurities is preferably less than 100 ppm, e.g. less than 10 ppm (as Fe203) of iron and less than 100 ppm, e.g. less than 20 ppm (as A1203) of aluminium in the nitrate additive. The invention is illustrated by the following examples: Example 1 To 300 ml water at 20 OC an amount of 100 g magnesium oxide was added, on which the mixture was stirred for 15 minutes. Use was made of a technical-grade product having an average particle size of 75 micrometres and a specific surface of about 50 m2/g which contained 2000 ppm Fe203, 1800 ppm A120.3, 8000 ppm Si02 and 1.3 wt. X CaO. Whilst stirring intensively, a 55 wt. Z HN03 solution was added to the suspension obtained, the metering rate being such that after 1 hour the pH of the mixture was 6.0. While the acid was being added, the temperature rose to about 105 OC. Subsequently, such an amount of 55 wt. % hun03 was very slowly added to the mixture, which was meanwhile being stirred, that the pR of the mixture was 5.3 after another hour. The solution obtained was filtered, the filtrate being a clear solution of magnesium nitrate which contained 1 ppm Fez3, about 2 ppm A1203 and 1 ppm Si02. Example 2 A mixture of 95.5. parts by weight of a 96 wt. % No403 solution and 4.5 parts by weight of a solution obtained according to the process of Example 1 was evaporated to a water content of 0.3 wt. % and subsequently sprayed in a tower in counterflow with a cool air stream. The prills obtained were cooled to about 45 0C in a rotary cooler-and subsequently subjected to 20 cycles between 15 and 50 OC. The product showed no deterioration in crushing strength and hardly any volume increase.

Claims: CLAIMS 1. A process for the production of a magnesium nitrate solution which comprises the steps of: (a) Adding a water insoluble magnesium compound to water so as to form an aqueous suspension thereof; (b) Adding 50-60 Z wiw nitric acid to the suspension formed in step (a) in such an amount that the final pR of the reaction medium is in the range 5,0 to 6,0; (c) Filtering the product from step (b) so as to separate magne sium nitrate as the filtrate from any insoluble material. 2. A process according to claim I, wherein the magnesium compound comprises particulate material having a particle size of from SO to 200 micrometres. 3. A process according to either of claims 1 or 2, wherein the magne sium compound is suspended in from 2.5 to 3.5 grams of water per gram of magnesium compound (expressed as equivalent weight of magnesium oxide). 4. A process according to any of claims 1 to 3, wherein the nitric acid comprises from 55 to 58 Z wiw of nitric acid. 5. A process according to any of the preceding claims, wherein the amount of nitric acid which is added in step (b) is from 90 to 99 Z of the amount of acid required to completely neutralise the magnesium salt. 6. A process according to claim 5, wherein the amount of nitric acid which is added in step (b) is from 96 to 99 Z of the amount required to completely neutralise the magnesium salt. 7. A process according to any of the preceding claims, wherein the final pR of the reaction medium is in the range 5.2 to 5.4. 8. A process according to any of the preceding claims, wherein the concentration of magnesium nitrate in the filtrate produced in step (c) is in the range 32 to 40 Z wiw. 9. A process for the preparation of stable ammonium nitrate containing compositions, wherein a magnesium nitrate solution obtained by the process according to any one of the preceding claims is added to the ammonia-nitric acid reaction mixture or to the ammonium nitrate-containing solution prior to the final eva poration stage. 10. A process according to claim 9, wherein 0,2 to 1,0 % by weight of the magnesium nitrate solution, calculated as magnesium oxide relative to the amount of ammonium nitrate, is added.

Inventors: BLUCKERT, NEVILLE PAUL, KELLY, WILLIAM JOHN
Applicants: STAMICARBON B.V.
Application year: 1983
Main classification: C01F5
IPC codes: C01F5