SE430979B - PROCEDURES FOR THE PREPARATION OF PHOSPHORIC ACID AND / OR PHOSPHATE FROM CHLORINE ROSE PHOSPHATE - Google Patents

Description

In addition to HCl, a number of other strongly corrosive compounds will be formed, in addition to HCl, including chlorine, nitrosyl chloride, nitric acid and nitrous acid, as well as nitrophosphate processes. chlorine-rich crude phosphate was used. In the nitrophosphate process, high chlorine content in the crude phosphate can cause stability problems in the post-digestion process steps and in the products, since the stability of the mixtures formed is to some extent a function of the chlorine content.

In the known nitrophosphate processes, chlorine added with the crude phosphate will be released during the digestion and the bulk of the chlorine compounds thus formed will remain in solution and cause corrosion problems both in the digestion step and in the later process steps. Furthermore, the chlorine compounds will be present in the end products and give them an undesirably high chlorine content.

It is previously known to pre-treat crude phosphate before digestion with small amounts of mineral acid and thus calcine the mixture to expel the contaminants. Norwegian patent specification 52 319 thus states that pretreatment with phosphoric acid and calcination to 1000 ° C lowers the chlorine and fluorine content. However, the method is considered uneconomical due to the large amounts of energy included in the calcination.

A special problem that exists with nitric acid digestion of crude phosphate is the emission of nitrogen oxides (NOX) during the digestion.

The content of reducing constituents in the crude phosphate, such as sulphides, organic pollutants, etc., causes the evolution of nitrogen oxides by reducing some of the nitric acid. " As disclosed in U.S. Pat. No. 3,528,797, urea will react with nitric acid to nitrogen, carbon dioxide and water.

Upon digestion of chlorine-rich crude phosphate with nitric acid in the presence of urea, some attenuation of nitrous evolution will be obtained, but the problems caused by chlorine are not solved by 'On the contrary, chlorine seems to reduce the effect of urea by would be added urea. on the prevention of nitrous emissions. close to expecting that urea could expel chlorine, experiments have shown that urea does not remove chlorine to the desired degree.

The main object of the present method is to remove chlorine, which is added with the crude phosphate, in a simple and economical way and at an early stage in the process. Another object of the invention is to provide a process which, in addition to removing chlorine, also enables nitros emission in the case of nitric acid digestion of chlorine-rich crude phosphate to be maintained.

It has now been possible to provide a process in which chlorine in the crude phosphate can be easily removed from the solution already during digestion. It has also been possible to provide a process in which chlorine-rich crude phosphate can be digested with nitric acid without problems of nitrosation and without the use of urea during digestion, while removing chlorine supplied with the crude phosphate from the resulting solution on a early stage in the process.

The special feature of the process according to the invention is that during the digestion or in a subsequent process step at least one strong, inorganic oxidizing agent is added in amounts sufficient to completely oxidize the organic and sulphidic constituents of the crude phosphate and in addition sufficiently for oxidation of chlorine compounds present to elemental chlorine and that the chlorine formed thereby is removed through the stripping system of the process.

The present method has proved to be particularly well suited for the digestion of chlorine-rich crude phosphate with nitric acid.

Preferably a chlorate, eg sodium chlorate, is used as the oxidizing agent.

Manganate or permanganate are also well suited as oxidizing agents.

In some cases it may be advantageous to use several oxidizing agents in successive order, for example a chlorate and a permanganate.

The optimal choice of oxidizing agent depends on several factors. In general, it can be said that the oxidizing agent must be strong enough to be able to oxidize chloride in acidic solution. Experiments have also shown that it is thus present during digestion. can not use any oxidizing agent. It has been found that the addition of H 2 O 2 during digestion does not sufficiently remove chlorine from the solution. HZO2 can, as is known, also act as a reducing agent and this may possibly be the explanation for the bad effect.

Upon digestion with nitric acid, which is also an oxidizing agent, the organic and sulphidic constituents of the crude phosphate will be oxidized and nitrose evolution is obtained. However, the nitric acid will only slightly oxidize the chlorides present.

By using strong oxidizing agents together with nitric acid, these will first oxidize the organic and sulphidic constituents and reoxidize any reduced nitric acid.

Additional amounts of strong oxidizing agent will thus be able to oxidize the chlorides of the crude sophate to elemental chlorine.

The amount of oxidizing agent required will consequently depend on the total content of reducing components of the crude phosphate. It is not only the chlorine content, but also the content of sulphides and organic substances, which varies from one type of crude phosphate to one. The amount of oxidizing agent must therefore be adjusted accordingly. Due to analysis problems and variations in the raw material. turn the crude phosphate. the composition of the phosphate, it may be desirable to add oxidizing agents in excess, so as to ensure the oxidation of chlorine compounds to elemental chlorine. The required amount of oxidizing agent will in most cases be adjusted by means of analyzes of both crude phosphate and digestion solution. When chlorate is used as an oxidizing agent, the amount of it must be carefully adjusted as residual chlorate could end up as a harmful contaminant in the product or in the best case be reduced to chlorides in later process steps and thus give the inconveniences stated. . Excess permanganate, on the other hand, does not seem to cause serious complications. A combined use of these two oxidizing agents can therefore be advantageous. V To obtain a qualitative impression of the effect of the addition of oxidizing agent, a series of experiments were carried out with the digestion of chlorine-rich crude phosphate with nitric acid and the simultaneous addition of sodium chlorate in varying amounts. The chlorine content of the solution and the evolution of nitros were measured to ascertain how these varied with the amount of chlorate added.

For oxidation of Cl- + C12 with chlorate, the following reaction can be set up in an acidic environment: 1.5 HCl + HCl + + 3 C12 + 5 H2 O Since it has been shown that the evaporation of chlorine becomes less efficient when the water content of the digestion solution high, it is thus important to work in as concentrated a solution as possible and to avoid unnecessary dilution. Nitric acid can be oxidized with chlorate according to reaction formula 2: 10 20 25 50 55 40 I? Äš. n S + HCIO + HCl 2 3 + S HNO 2. 5 HNO 3 The results of the experiments are shown in Fig. 1, which gives a qualitative picture of the measurement.

From 0 + A, the concentration of Cl- in the solution increases at the same time as it is seen that the nitrous evolution decreases already with little addition of chlorate. From A + B, the concentration Cl- in the solution falls to a minimum, while the nitros evolution is stabilized at a low, constant level, ie the nitros evolution is no longer visible. From A »C the concentration Cl- increases again slowly.

Since many competing reactions are conceivable under these conditions, the explanation of what is registered must only be a hypothesis for what is assumed to take place. However, the studies done indicate that with an increasing amount of added oxidizing agent, the easily oxidizable contaminants, such as sulfides, organic contaminants, etc., are first oxidized, after which the chlorides are oxidized to chlorine and released. The evolution of nitrogen oxides decreases slightly, because the easily oxidizable impurities, which can also reduce nitric oxide. As a result of this reaction, at A, all of the tartaric acid is oxidized by added chlorate. increases the chlorine content of the solution correspondingly. The pollutants, with the exception of chloride, oxidize and further add chlorate oxidize the chlorides and the concentration of Cl- in the solution decreases. At B, all chloride will be converted to chlorine and further addition of chlorate will remain more or less unreacted in the solution and result in the measurement of Cl-, as this measurement will register ions of both Cl- and C105-.

The invention will be described in more detail in the following examples: Of the crude phosphates which have not previously proved useful in the nitrophosphate process, there is, for example, the crude phosphate which follows the ore in LKAB's plant in Malmberget. This phosphate has a chlorine content of 0.4-0.5%.

In some initial experiments and analyzes, it was found that by adding chloride (CaCl2) to a chlorine-poor crude phosphate, so that the total chlorine content corresponding to chlorine-rich crude phosphate, eg LKAB phosphate, obtained approximately the same ratio during the digestion of chlorine-rich phosphate and artificial, chlorine-enriched phosphate.

This enrichment was carried out by adding CaCl 2 (which was added as a 37% solution) during the digestion. During the further experiments, therefore, Florida and Kola phosphate were used, which were enriched with chlorine to a chlorine content corresponding to 0.2 U chlorine in the crude phosphate. 10, _.

Extracted NOX and HNO3 from the digestion solution were absorbed in a 3% H2O2 solution. NO3-N was thus determined titrimetrically. What is measured is thus the sum of NOX and HNO3 in the exhaust gases.

The total chlorine content (chlorine, chlorate and chloride) in the digestion solution was determined potentiometrically after reduction with SO 2.

Example 1

In a test plant on a semi-technical scale consisting of two series-connected vessels, each with a liquid volume of 52 l and equipped with propeller tubes (rotational speed 600 r / min), Cola phosphate (4.1) was fed continuously for about 5 h. kg P) and 56 kg / h 58% HNO3 (% by weight). The digestion ratio was 1.35 kg HNO5 / kg phosphate. The phosphate was enriched to a chlorine content of 0.1%. The containment tanks were equipped with pull-off hoods and about Nmj air / h was sucked off. The temperature in the tank was measured at 611100.

The product, a phosphate and nitrate solution, was removed from the test facility through an overflow tube in vessel 2] After about 5 hours of operation, corresponding to 3-U changes of the total liquid content of the reaction vessel, the chlorine content of the product solution and the total nitrogen oxides and nitric acid were measured. in the exhaust gases. The chlorine content was 0.11% Cl or 92% of added chlorine. The total content of nitrogen oxides and nitric acid vapor in the exhaust gases was measured: iii 9.5 g HNO3 / kg P.

Example 2.

An experiment similar to that of Example 1 and under the same experimental conditions, except that 120 g of NaClO 3 / h (supplied as a 37% solution), which constitutes 0.5% of the phosphate supplied, was also continuously added to vessel 1, was carried out under about 5 h.

Measurements corresponding to those shown in Example 1 showed a chlorine content in the production solution of 0.012% Cl, which corresponds to 10 ß of added chlorine and a content of nitrogen oxides and nitric acid vapor in the exhaust gases corresponding to 3.1 g HNO3 / kg P. Example 5.

An experiment similar to that of Example 2 and under the same experimental conditions, with the exception of the temperature which was 59 ° C, was performed. In this way, 5 kg / h of a 25% H3® solution was continuously added, which gave an extra water supply of about 0.9 kg of H2O / kg of added P.

After about 1.5 hours of operation, the Cl content of the production solution was measured to be 0.021 Z Cl, which corresponds to 17% of the chloride added. The nitric oxide and nitric acid content of the exhaust gases were the same as in Example 2. H0 Example U.

In an experiment under the same experimental conditions as in Example 1 except that the digestion temperature was 65 ° C, 192 g of KMnOu / h mixed continuously in the phosphate were continuously added. This amount is oxidized to Example 2.

After approximately 4.5 hours of operation, the chlorine content of the production solution was measured at 0.007% Cl, which corresponds to 6% of added chlorine. The total content of nitrogen oxides and nitric acid vapor in the exhaust gases corresponded to 3.5 g HNO3 / kg P.

Example 5.

In the same experimental plant, approximately 2U kg of Florida phosphate / h and 56 kg / h of 58% HNO 3 were continuously added, the digestion ratio was 1.36. The crude phosphate was enriched to a chlorine content of about 0.4% by continuous addition of 150 g of CaCl2 / h in the form of a 57% solution. The other experimental conditions were the same as in Example 1.

After about 5 hours of operation, the chlorine content of the product solution was measured to be 0.070% Cl, which corresponds to 58% of the chlorine added.

The total content of nitrogen oxides and nitric acid vapor in the exhaust gases corresponded to ü9.7 g HNO / kg P. 3 Example 6.

In an experiment similar to that of Example 5, 216 g of NaClO5 / h were continuously added to the vessel in the form of a 37-Drocentig solution, which constitutes 0.9% of the phosphate added.

After about 5 hours of operation, the chlorine content of the production solution was measured at 0.010% Cl, which corresponds to 8% of added chlorine.

The total content of nitrogen oxides and nitric acid vapor in the exhaust gases corresponded to 3.1 g HNO5 / kg P.

Example 7.

However, an experiment similar to that of Example 2 was performed. Now, however, 26 H g of NaClO 3 / h, corresponding to 1.3% of phosphate added, were introduced into vessel 1. Measurements corresponding to those in the previous example showed a chlorine content in the production solution of 0.03%, corresponds to 28.3% of added chlorine.

The total content of nitrogen oxides and nitric acid vapor in the exhaust gases corresponded to 3.3 g HNO5 / kg P.

Example 8.

At a test plant, as set forth in Example 1, about 2U kg of Carbon Phosphate and 5U, 6 kg of 53% nitric acid per hour were introduced. To mimic a chlorine content of 0.1% in the phosphate, 150 g of CaCl2 / h (added as a 37% solution) were added continuously. Furthermore, 50 kg / h of 25% HBPOÄ solution was added, which gave an extra water supply of about 0.9 kg of H 2 O / kg of added P. The digestion temperature was measured at 57f1 ° c.

120 g / h of NaClO 3 (37% solution) were added to digestion vessel 1 and 2Ä g / h of KMnOu (5% solution) were added to vessel 2.

After about 5 hours of operation, the Cl content of the production solution was measured to 0.0lü% Cl, which corresponds to 12% of added Cl. The total content of nitrogen oxides and nitric acid in the exhaust gases was measured at 2.16 g HNO3 / kg P.

The examples show that 83-9% of the chlorine added with the crude phosphate can be removed from the digestion solution by adding oxidizing agent during the digestion. The chlorine content of the product solution from the digestion step is also below that obtained with ordinary digestion of many of the crude phosphate mixtures used today, when no additional oxidizing agent is added during the digestion.

The experiments show that permanganate gives slightly better results with respect to the removal of chlorine than chlorate, but since the latter is cheapest, it is most suitable to use chlorate in any case to remove most of the chlorine. As shown in Example 8, by carrying out the chlorine removal in two steps with chlorate in the first and permanganate in the last, 88% of the added chlorine is added and at the same time ensuring that the product solution contains significant amounts of chlorate by accidental overdose of oxide. agents.

Comparison of Examples 3 and 8 shows that the removal of chlorine in two steps, as in Example 8, gives the best results even when the reaction solution is diluted with water by adding H PO 4 solution.

Example 7 shows that the measured chlorine content in the production solution in the case of overdose of the oxidizing agent chlorate will be higher than in the case of a better adapted amount of oxidizing agent, as in Example 2.

Example 7 thus corresponds to that indicated under point C in Fig. 1.

When using chlorate as oxidizing agent, the dosage is thus very important for achieving a minimum content of chlorine compounds. Since it is difficult to obtain a continuous 1 J in the product solution. 10? RU2 @ ëå ~ 5 9 measurement of the chlorate excess, a procedure such as that in Example S will be able to eliminate this problem.

Furthermore, the experiments show that the saturated sum of nitrogen oxides and nitric acid vapor in the exhaust gases is greatly reduced by the addition of oxidizing agents during the digestion. However, the amount of evaporated HNO3 will be approximately the same whether or not oxidizing agents are used. By comparing the results from example and 2, it will be seen that NOX + HNO3 in the exhaust gases is reduced by 70 "nä I-J consequently, the reduction in NOX emission will be significantly greater than 70%, in fact up to 100%. This again says that the NOX emission is reduced by the process according to the invention to at least as great an extent as in the known method with urea addition. __ ---.-__ - __ ---- _---

Claims (4)

??: “% Ü2ë« f ~: ~ f =; = - 5 m v / vr fi nrkyn / 1. A process in the preparation of phosphoric acid and / or phosphate from chlorine-containing shale phosphate, which is first digested with nitric acid or a nitric acid-containing mineral acid mixture, after which this mixture is further processed into desired products, characterized in that during the digestion or in subsequent process steps add at least one strong, inorganic oxidizing agent in amounts sufficient to completely oxidize the organic and sulfidic constituents of the crude phosphate and in addition sufficient to oxidize the chlorine compounds present to elemental chlorine and that the chlorine formed thereby is removed via the process deduction system. 2. A process according to claim 1, characterized in that a chlorate is used as the oxidizing agent. 3. A process according to claim 1, characterized in that a manganate or permanganate is used as the oxidizing agent. 4. A process according to claim 1, characterized in that the digestion is carried out in at least two steps and that in the first step or steps a chlorate is used and in the last step or steps a permanganate is used as oxidizing agent.