CS217953B2 - A method for obtaining manganese fractions from ores containing non-reduced manganese - Google Patents

Abstract

The invention relates to a method for obtaining manganese fractions from ores containing unreduced manganese, in particular to a method for obtaining high-purity manganese dioxide from said unreduced ores.

Description

The invention relates to a method for obtaining manganese fractions from ores containing unreduced manganese, in particular to a method for obtaining high-purity manganese dioxide from said unreduced ores.

For many years, a suitable method of utilizing poor ores containing manganese oxides to obtain manganese dioxide has been sought. Efforts have been made to find processes that correspond partially or completely to the course of the chemical equation ^{+ 2 n0} 2

The reversible nature of the reaction, which is the basis for the efficient operation of a batch or, in particular, a cyclic process, was the impetus for the initial development work. Because previous researchers did not know the exact conditions necessary for optimizing the chemical processes in the critical stages of the process, they were not successful in developing a practically feasible batch or cyclic process based on the reaction. The chemical processes in this process are described in a simplified diagram in the following diagram:

217933 . . (1 and cooling

NU* _{Water vapor'22HNO 2} * HNO^t ^water (Flammable!

manganese dioxide (as product) (2) (31 treatment filtration ore ⁺ tailings (feed material)

Ma(NO ₃ ) _z + H ₂ O (liquid -j~ tailings] tailings (waste)

Μα(Νθ^+ H _z 0 (liquid)

As can be seen from the above diagram, the process is theoretically completely cyclic. The manganese nitrate solution is decomposed by heat to form pure manganese dioxide as the product and nitrogen dioxide gas. This nitrogen dioxide gas, together with water vapor, condenses under conditions that produce nitrous acid and nitric acid. These then react with the manganese dioxide in the raw manganese ore to regenerate the manganese nitrate solution. The regenerated solution is separated from the impurities contained in the ore by adjusting the pH value and filtering, followed by feeding into the decomposition device to repeat the cycle.

Impure manganese nitrate solutions in the description and definition of the subject matter of the invention are understood to mean manganese nitrate solutions containing soluble impurities, as well as suspensions containing both soluble and insoluble impurities in the manganese nitrate solution.

U.S. Patent No. 1,287,041 describes the treatment of manganese dioxide with nitrogen dioxide to obtain a manganese nitrate solution which is then decomposed. In this process, powdered manganese ore is treated with nitrogen dioxide in the presence of water. The patent proposes the use of at least two vessels with stirrers through which nitrogen dioxide is passed together with a certain amount of air. The function of the air is to oxidize any nitrogen oxide present to nitrogen dioxide. It is stated that this nitrogen oxide is formed due to the presence of lower manganese oxides. It also proposes the possibility of using columns containing crushed ore through which nitrogen dioxide is passed together with recycled water. Although the patent specification does not describe in sufficient detail the parameters for controlling and managing the process, including pH control, the formation of nitric oxide is considered to be a major problem in ore beneficiation, and it is proposed to use air to convert the resulting nitric oxide to nitrogen dioxide. A batch process for the decomposition of manganese nitrate is described, which produces a hard, massive block of manganese dioxide that is difficult to pulverize. The inventive contribution of this patent specification is that it provides a process in which an alkali metal nitrate is added to the manganese nitrate solution prior to decomposition, causing the resulting block of manganese dioxide to disintegrate upon boiling with water. The purity and physical properties of the final product are not disclosed.

In the administration of the US Department of the Interior, Bureau of Mines (Technical Paper 674, 1945, US Department of the Interior, Bureau of Mines), entitled "Semi-pilot-plant Investigations of Nitrogen Dioxide Process for Be.neficiation of Manganese Ores" (Semi-pilot-plant Investigations of Nitrogen Dioxide Process for Be.neficiation of Manganese Ores) a method of leaching poor manganese ore with nitrogen dioxide and decomposition by leaching the obtained manganese nitrate solution is described. Three tanks equipped with stirrers and containing ground ore are used, into ^which nitrogen dioxide is passed countercurrently. Air is introduced into the gas stream between the penultimate tank, thereby oxidizing the excess nitric oxide formed during countercurrent leaching. The manganese dioxide content in the tank into which the nitrous oxide is first introduced is apparently kept low or adjusted to a low value so that the suspension contained in the tank can be discharged to filtration without excessive loss of manganese dioxide. There is no indication in the above report as to how the pH is controlled during the leaching of the water. This report and U.S. Patent No. 2,374,674 describe attempts to decompose the manganese nitrate solution by indirect heating. However, this procedure is difficult to carry out in practice because foaming and excessive formation of deposits on the surface of the heat exchange surfaces occur, thereby leading to poor heat transfer. To overcome these difficulties, the heating method described above is carried out in such a way that the decomposition products, i.e. water and nitrogen dioxide, are recycled through a high-temperature heat exchanger and fed back over a shallow layer of manganese nitrate solution. This results in heat transfer at the surface and partially eliminates the above-mentioned difficulties. However, it should be noted that a more detailed analysis of this process, contained in the above-mentioned report, shows that deposits are formed in the decomposition vessel and on the rakes with which the product is removed. This type of process is apparently not feasible on an industrial scale.

In assessing the significance of the prior patents, it is necessary to take into account the differences in the overall ore recovery process. Only the processes of U.S. Patent Nos. 1,287,041 and 2,374,674 use nitrous oxide directly as a leaching agent to recover a manganese nitrate solution from the raw ore feed, while the processes described in U.S. Patent Nos. 1,761,133 and 2,779,659 convert the nitrogen dioxide produced by the decomposition of manganese nitrate to nitric acid by direct oxidation, usually ^using excess air. This approach requires three separate stages in the leaching of the ore instead of one, making the overall process much less efficient and more expensive. These stages are:

1. nitrogen dioxide is oxidized to nitric acid by oxygen or in the presence of water;

2. the raw ore is reductively calcined, whereby the manganese dioxide usually contained is converted into manganese dioxide or some intermediate lower oxide;

3. The reduced ore is then leached with nitric acid to obtain a manganese nitrate solution.

There is little information in the literature regarding the treatment of manganese nitrate solution to remove impurities originating from the ore, especially as this treatment relates to the preparation of the solution for subsequent decomposition. In the current methods, working with the decomposition of manganese nitrate as a means of obtaining manganese dioxide of improved quality, the decomposition is carried out under extreme temperature conditions and a massive and relatively impure product is obtained.

U.S. Patents 1,761,133 and 2,779,659 describe the step of removing tailings from the resulting manganese nitrate solution, but do not disclose the pH value. The above-cited report of the U.S. Department of the Interior, Bureau of Mines, describes the removal of solid impurities by filtration after partial neutralization of excess nitric acid with calcium oxide. No specific procedure or pH value is given. U.S. Patent 2,737,441 describes a leaching process in which the pH is adjusted to 3.0 before filtering off the solid impurities. None of the sources of information cited mentions a relationship between the impurities present in the manganese nitrate solution and the type of decomposition product that is or may be formed.

U.S. Patent No. 2,779,659 describes the decomposition of a manganese nitrate solution under pressure in the presence of oxygen to directly convert nitrogen dioxide to nitric acid. This batch process produces manganese dioxide in the form of a solid block. U.S. Patent No. 2,681,268 describes the decomposition of a manganese nitrate solution in a spray dryer. A solution containing 390 g of manganese nitrate per liter is fed into it and the temperature in the dryer is in the range of 2(00 to 309 °C. Air is used as the gaseous medium to oxidize nitrogen dioxide to nitric acid. The method of regeneration of nitric acid from a relatively large volume of air, nor the type of manganese dioxide obtained as a product, is not stated. US Patent No. 2,737,441 describes the decomposition of a manganese nitrate solution on the surface of a heated drum by feeding it with a stream of excess air. The layer of manganese dioxide that forms on the surface of the drum is scraped off with a scraper in the usual way. The manganese dioxide formed is not in the form of high-purity pyrolusite crystals.

U.S. Patent No. 1,761,133 describes the decomposition of preconcentrated "solid" manganese nitrate into manganese dioxide at temperatures between 120 and 200 °C. There is mention of stirring, but it is not stated whether it is a batch or continuous process. The physical form of the product is not given, nor is the method of heating. The inventive contribution of this patent is doubtful, since it is stated that solid manganese nitrate (anhydrous) is obtained by heating a neutral solution to boiling "until solid manganese nitrate is excluded", which is impossible. The known solid forms of manganese nitrate contain several moles of water; for example, the hexahydrate is known with a melting point of 25.8 °C and the trihydrate with a melting point of 35.5 °C. The existence of an anhydrous form in neutral solution is not known, and the known solid forms can only be prepared by cooling the solution, not by evaporation on heating.

The decomposition processes described in U.S. Patents Nos. 1,287,041, 2,779,659 and 176|11313 are so vague and ill-defined as to be of no use. The decomposition processes described in U.S. Patents Nos. 2,374,674 and 2,737,441 have significant shortcomings. U.S. Patent No. 2,374,674 partially addresses problems that must be overcome to make the process commercially viable, such as the formation of deposits on the heat exchanger surface, foaming and dirt accumulation. To overcome these problems, U.S. Patent No. 2,374,674 proposes a mechanical device that requires very significant maintenance and is also very difficult to seal to prevent nitrogen dioxide loss. A particularly serious drawback is that it is not possible to eliminate the caking and the formation of deposits. It has only been possible to prevent the formation of deposits from interfering with the required heat exchange. According to the method of U.S. Patent No. 2,737,441, instead of eliminating the undesirable formation, it is simply used to advantage by allowing it to run on the surface of a heated drum or belt. The deposit is then separated as a product from the hot surface by a scraper. This process also requires considerable maintenance, especially since the manganese dioxide deposit is very abrasive. It is also difficult to seal this device against air and to prevent nitrogen dioxide losses. An excess of air is required, but this is achieved by blowing air into the device where the manganese dioxide layer is formed. However, in the process of the present invention, due to the efficient and direct use of nitrogen dioxide in the leaching step, it is necessary to prevent the entry of air or other inert gas in order to maintain the equilibrium of the leaching system.

According to U.S. Patents Nos. 2,374,674 and 2,737,441, the manganese dioxide product is prepared under extremely non-equilibrium conditions, which result in the retention of impurities in the product. Furthermore, the necessity of grinding or crushing the product in the processes of both of these patents results in a material containing a wide range of particle sizes with a high percentage of fines, which is difficult to wash and handle.

The purpose of the invention is to provide an improved method for recovering manganese fractions from ores containing unreduced manganese by an improved process for leaching these ores with an aqueous solution of nitrous acid.

Another purpose of the invention is to provide the above method which can be successfully carried out on an industrial scale.

Another purpose of the invention is to provide an economical cyclical process for recovering manganese fractions from ores containing unreduced manganese in the form of high purity pyrolusite crystals.

Further details and advantages of the method according to the invention will be apparent from the following description of an embodiment of the method according to the invention.

The subject of the invention is therefore a method of obtaining manganese fractions from ores containing unreduced manganese by leaching these ores with an aqueous solution of nitric acid to form manganese nitrate. The essence of the invention lies in the fact that a charge of ore containing unreduced manganese, which contains a stoichiometric excess of manganese dioxide, based on the content of nitric acid in the leaching aqueous solution of nitric acid, is treated at a temperature in the range of 35 to 80 °C and at a pH in the range of 0.5 to 4 at a lower equilibrium partial pressure of the mixture of nitric oxide with water vapor than atmospheric pressure, with an aqueous leaching solution of nitric acid to form a suspension containing manganese nitrate.

In a preferred embodiment, the ore containing unreduced manganese is contacted with an aqueous solution of nitrous acid and leached therewith in an aqueous medium comprising an aqueous solution of manganese nitrate, at a pH in the range of 0.5 to 3.0.

Leaching with an aqueous solution of nitrous acid is preferably carried out at a temperature in the range of 40 to 50°C.

To remove impurities, the pH of the manganese nitrate suspension is adjusted to a value in the range of 4.0 to 5.5, the suspension is heated to a temperature in the range of 70 to 130°C, and then filtered.

Manganese dioxide is added to the filtered suspension containing manganese nitrate, the resulting suspension is intensively stirred and heated by controlled heat supply to decompose manganese nitrate to form freely flowing pyrolusite crystals of manganese dioxide, nitrogen dioxide and water vapor.

Manganese dioxide is added to the manganese nitrate solution in an amount sufficient to form a suspension with a solids content by weight in the range of 5 to 25%.

The filtered suspension containing manganese nitrate is preferably concentrated to a manganese nitrate concentration of 50% by weight before decomposition.

Preferably, the decomposition of manganese nitrate is allowed to proceed in a controlled manner at a rate at which no more than 1.2 kg of manganese dioxide is produced per liter of suspension per day. The preferred rate of this decomposition is such that 0.24 to 0.6 kg of manganese dioxide is produced per liter of suspension per day.

Preferably, the process according to the invention is carried out in such a way that the nitrogen dioxide and water vapor formed during the controlled decomposition of manganese nitrate are returned to the leaching aqueous nitric acid solution used in the first stage, whereby the nitrogen dioxide and water vapor are dispersed and dissolved in this leaching aqueous nitric acid solution, thereby forming a cyclic process.

As already mentioned above, the process can be carried out batchwise or, preferably, continuously in all stages.

In the process according to the invention ^, manganese dioxide is obtained as a product in the form of free-flowing crystals of the pyrolu.s type, in "cement pure" quality with a particle size ranging from about 46 μm to about 260 μm, with about 80% of the particles being in the range of 74 μm to 177 μm. No deposits, scales or lumps of decomposition product are formed. Soluble impurities from the ore are conveniently removed with continuous removal of a portion of the mother liquor, and the gases resulting from the decomposition are removed in the leaching circuit without the risk ^of escape or air pollution. The decomposition is continuous and very efficient in terms of material losses and heat conservation. The equipment is simple, common and practically maintenance-free.

To make the improvement of the chemical processes in the process according to the invention more obvious, it is advantageous to summarize the individual chemical reactions that take place in it as follows:

heat

Mn(NO3)2 —---** aqueous solution

Quantity -j- 2NOŽ (+ water vapor)

AND

-» MnOr + 2ίΝΟζ ( + water vapor)

MnOz + HNOz + HN0 ₃ -> MnjNOsjz + H2O

III

HNOz ~

H+ + NO3- -|- 2NO + I-I2O

IV

NO + O2 -> 2 NO2 V

MnO + 2HNO3 -> Mn(NO3)2 + H2O VI

K2O (MgO, CaO, NazO, BaO, etc.) + 2HNO3

- 2KNO3 + H2O

VII

It is clear that in the leaching according to the invention, the ideal prerequisite is that reactions II and

III occurred simultaneously with the smooth and complete conversion of manganese dioxide to manganese nitrate using only 2 moles of nitric oxide. However, in practice, side reactions occur,

In the process according to the invention, the formation and presence of nitric oxide is not a problem in itself; nitric oxide is a normal and, at controlled concentrations, a beneficial component of the leaching system. However, in order to achieve an efficient, commercially viable process, uncontrolled, non-equilibrium formation of excess nitric oxide must be avoided. It should be borne in mind that any attempt to oxidize nitric oxide with air may not only result in an imbalance in the entire process, but also cause unavoidable losses of active and environmentally harmful nitrogen through escape into the atmosphere.

The leaching process according to the invention is associated with a group of reactions, designated as reactions VII, which are unavoidable side reactions requiring a precisely defined amount of nitric acid. As will be apparent from the following discussion, the required amount of nitric acid for reaction VII is to be added during leaching directly in the form of nitric acid.

Reaction VI represents a special case which will be discussed in more detail below. In order to establish the essential features of the process according to the invention, it is assumed that reactions VI and VII occur by adding a stoichiometric amount of nitric acid from an external source. It is also assumed that the leaching takes place in a closed apparatus which substantially prevents the access of air during the leaching. In accordance with this analysis, the leaching of manganese dioxide as an ore component with nitric oxide is completely defined by reactions II, III and IV.

The common feature of reactions II, III and IV is nitrous acid, and it is indeed the chemical behavior of nitrous acid that largely defines the critical parameters of the leaching system. Of the three reactions mentioned, the reaction

IV is most crucial for determining leaching conditions. Since reaction IV is an equilibrium reaction with equilibrium constant K = — 20 at 20°C, K = 40 at 30°C, and extrapolated values of K = 80 and 160 at 40° and 50°C, respectively, it can be expected that the partial pressure of nitric oxide will vary with temperature, hydrogen ion concentration, nitrate ion concentration, and nitrous acid concentration in a manner that can be predicted for a leaching system using water. At high concentrations of manganese nitrate in solution, some shift of the equilibrium position can be expected, but there will still be an equilibrium state at which the interdependence of the vapor pressure of nitric oxide, acid concentration, temperature, and nitrous acid concentration can be expected. It is known from the literature that the equilibrium rate is relatively low at lower acid concentrations and temperatures, but high at higher temperatures and acid concentrations. The general characteristics of reaction IV have been verified here in the case of an aqueous system containing manganese nitrate. Two characteristics of the equilibrium state of the reaction in an aqueous solution of manganese nitrate are of particular importance for this process and have been studied in detail: the vapor pressure of nitric oxide under various conditions and the rate of the reversible reaction as measured by the absorption rate of nitric oxide. 7, the data obtained in the pilot process lead to the following important generalizations:

1. If the value of other parameters, such as temperature, hydrogen ion concentration and nitric acid concentration, is kept unchanged, the partial pressure of nitric oxide increases with increasing concentration of manganese nitrate in the solution. The partial pressure of nitric oxide over a 30% manganese nitrate solution is more than twice its value over pure water;

2. Under normal operating conditions, the rate of the reverse reaction IV is less than the rate of the forward reaction and depends strongly on the acid concentration. The rate of nitric oxide absorption therefore approximately always doubles with a decrease in pH of half a unit between pH values of 1.5 and 0. The effect of temperature on the rate of the reverse reaction is relatively small.

When the specific characteristics of reaction IV are applied to an operational leaching system comprising only reactions II, III and IV, certain operational parameters are now clear. It should be noted that an "operational leaching system" is understood to mean a system in which reactions I and II proceed at a controlled and nominal rate. It should also be emphasized again that in this discussion it is assumed that the leaching system is closed and allows only negligible air ingress and/or loss of nitric oxide or nitrous oxide in gaseous form. These parameters are:

1. The nitrous acid produced by reaction II must be rapidly dispersed in the reaction medium to avoid high local concentrations. If this does not happen, the forward rate of reaction IV, caused by high local concentrations, causes nitric oxide to be formed faster than the entire amount formed can be re-dissolved in the system, and the equilibrium state cannot be maintained. The result is uncontrolled, non-equilibrium and continuous nitric oxide formation;

2. the pH of the system must be maintained below about 4.0 so that the rate of the IV reverse reaction is significantly affected to maintain the equilibrium state of the system;

3. manganese dioxide as a component of the ore must be present in sufficient stoichiometric excess or must have a sufficient excess surface area relative to the nitrous acid content of the leaching solution so that reaction III can proceed at a rate that keeps the nitrous acid concentration below the critical value associated with the maximum allowable vapor pressure of nitric oxide in the equilibrium state;

4. The upper temperature limit is approximately 80°C for these three reasons:

a) the value of the equilibrium constant K increases rapidly with temperature, which results in an ever lower equilibrium concentration of nitrous acid and at the same time a clearly increasing partial pressure of nitric oxide;

b] the rate of the forward reaction IV increases with temperature, resulting in increasingly shorter time intervals between the formation of nitrous acid by reaction II and its dissipation (see cdst. Ij;

,c) the water vapor pressure increases rapidly with temperature, which limits the allowable partial pressure of nitric oxide.

It should be noted that, with the possible exception of parameter 2 above, the limit value of any of the above variable parameters is dependent on the limit value set for the other parameters. Thus, the dispersion rate (parameter

1.) dependent on the limiting temperature (parameter 4.) and the minimum excess of manganese dioxide contained in the ore will depend on pH and temperature.

One limitation can be established for the system, which will allow the above-mentioned limit values to be determined. In order not to blow up the pressure leaching system, the sum of the water vapor pressure above the leaching system and the partial pressure of nitric oxide should not exceed atmospheric pressure. This limitation, together with pH and temperature, are the only parameters that can be determined independently with acceptable accuracy. The temperature should be maintained within a range given by the highest upper limit of about 83 °C, and will preferably be in the range of about 40 to about 50 °C.

At this point it is appropriate to describe the effect of the pH value of the system on reaction III. This reaction is an oxidation-reduction reaction in which hydrogen ions play a role and is therefore dependent on the pH value. The oxidizing agent is manganese dioxide, the reducing agent is nitrous acid. Basic thermodynamic data give the oxidation potentials of both relevant electrode reactions as follows:

HNO.I + HzO = NOa + 3H+ + 2e 0.94 V Mn ²⁺ + 2 HzO = MnOz + 4Η ^Ψ + 2e 1.2(3 V

Under standard conditions, the value of Λ V, i.e. the driving force of the oxidation-reduction reaction, is D.29 V. The oxidation-reduction values under actual operating conditions can be approximately expressed as follows:

O 050 'ΉΝΟ2 = 0.94 +-~^log ^ν Μηθ2 = 1.23 -)0.0)59 ,

--i _O g [Mn ²⁺ ] [·3η+ ?* where the values of [NCtr] and [Mn ²⁺ ] can be approximately expressed as 4.0 and 2.0, respectively, for a 30% solution of manganese nitrate· and the value of [a _H+ ] can be related to pH. The value of [HNiOz] can be approximately derived from the steady-state operating conditions given in the following examples. For an efficient leaching system where nitric oxide gas is not produced, the concentration of nitrous acid must be considerably lower than 0.1 g/l, and probably in the range of 00.2 to 0.05 g/l. The molar concentration of nitrous acid can be considered for calculation purposes as 0.031 M (0.0)5 g/l. Using these values, the following data can be calculated from the above equations:

pH ( ^and H + ) ΉΝ02 0 1 1.05 1 0.1 0.9)3 2 0.01 0.86 3 0.001 0.78 4 0.0001 0.69 5 0.00001 0 60

^ν Μηθ2 Δ v (driving force]

1.22 0.17 3.10 0.14 0.98 0.11 0.86 0.08 0.74 0.05 0.62 0.02

These data clearly show the dependence of the critical value Δν on the acid medium. The driving force around pH 1.0 is about twice that around pH 3.0 and about three times that around pH 4.0. Indeed, as the nitrous acid concentration gradually decreases, the driving force of the leaching reaction decreases, reaching zero at pH 5, which means that leaching cannot proceed with high efficiency at higher pH values.

From this it is apparent that the chemistry of reaction III clearly requires a maximum pH value in the range of 3 to 4, above which the leaching process will not proceed.

The introduction of air into the leaching system can now be conveniently considered in connection with the leaching system. It has been stated in the preceding discussion that leaching cannot proceed effectively unless the system reaches a certain degree of acidity. The following data will be presented from which it will be seen that practically usable leaching rates cannot be achieved at pH values above about 2. If gaseous nitrogen dioxide is introduced into an aqueous suspension of manganese dioxide in the form of ore without controlling the pH value, then the initial reactions are only reactions II and IV, resulting in the following overall reaction:

NOz + H2O -- 2 HNO3 + NO Fairy

Reaction VIII, as can be seen, produces nitric acid with the loss of nitric oxide. When a sufficient excess of nitric acid is produced to reduce the pH to a working level of about 2 or less, reaction III begins to proceed. During the leaching of the ore, both the manganese dioxide and the metal oxides contained in the ore react with the excess nitric acid (reactions VI and VIII) and the system reverts to reaction VIII to reestablish the desired concentration of nitric acid. It is therefore clear that in the absence of any additional or external source of nitric acid for the reaction of the impurities, i.e., the metal oxides, nitric oxide will inevitably be produced. If the manganese dioxide content in the leaching tank is reduced by countercurrent leaching to a level at which the excess manganese dioxide is no longer sufficient to keep up with reaction III, nitrous acid will accumulate and reaction _IV will produce nitric oxide and nitric acid. In other words, reaction VIII may again play an important or even predominant role. Therefore, although a conventional countercurrent leaching system can reduce the to a minimum of manganese dioxide loss, will increase nitrogen loss very noticeably. A third source of nitric oxide may be formed as a result of the equipment used. If an inlet tube extending below the surface of the suspension is used to introduce water vapor and nitrogen dioxide into the leaching tanks, the inlet tube acts as a cold-walled condenser. The nitrogen dioxide and water vapor introduced through this tube partially condense on the inner surface of the inlet tube to form a liquid film containing a significant proportion of nitric acid. In this liquid film, reaction VIII will continue to complete. Either of these last two sources of nitric oxide may cause an excess of nitric oxide to form. By using air to oxidize the excess nitric oxide to nitrogen dioxide according to reaction V, an inert gas, i.e. nitrogen, is introduced into the system, which will have the effect of remove nitric oxide from the system, thus forcing reaction IV to always proceed in the forward direction, thereby eliminating any possibility of establishing an equilibrium state that would prevent the irreversible formation of nitric oxide.

Without temperature control in the leach tanks, the temperature can be expected to approach or reach the boiling point. It has already been stated that high temperatures accelerate the rate of the forward reaction IV in addition to shifting the equilibrium far to the right. At temperatures close to or at the boiling point, the high vapor pressure of water, together with the increase in the boiling point, can cause the reaction to proceed at a constant temperature.

An increase in the partial pressure of nitric oxide can cause excessive expulsion of nitric oxide, thereby disturbing the equilibrium state of the system.

In order for the overall process to be in equilibrium, it is necessary that each unit mass of manganese nitrate decomposed into manganese dioxide as a product and nitrogen dioxide be completely regenerated with nitrogen dioxide in the leaching reaction. Any metal nitrates formed from the nitric acid formed in the combined reactions IV and V mean a loss of nitrogen dioxide from the circulation, since this nitrogen can no longer be recovered by the decomposition of the alkali metal nitrate. In this case, it is therefore necessary to add nitrogen dioxide to the circulation from an external source in order to maintain the overall equilibrium balance during the aspiration and decomposition. In practice, nitrogen dioxide is an economically difficult chemical to replenish. A suitable solution, ^as mentioned above, is to add nitric acid as a chemical that must be replenished so that reaction VII does not lose the nitric oxide needed for a stoichiometrically balanced state from the overall process.

Before defining the optimal leach system with the parameters previously given, it is useful to discuss the special case represented by the reaction VI of manganese dioxide with nitric acid. Unlike reaction VII, in which it is practically impossible to regenerate nitrogen dioxide from alkali metal nitrates, the nitrogen contained in the nitric acid required for reaction VI is recovered as nitric oxide. This means that the nitric acid required for reaction VI can in fact be regenerated by a combination of reactions II, IV and V without loss in the overall process. In fact, the use of an external source of nitric acid for reaction VI, as assumed above for the purpose of more clearly defining the basic parameters, leads to a theoretical imbalance in the process by producing an excess of manganese nitrate solution, usually about 5%, but the actual amount depends on the oxidation state of the ore. Oxidation of nitric oxide, as indicated by reaction V, cannot be effectively carried out with air because the inert nitrogen in the air entrains and carries nitric oxide from the system and it is therefore not possible to establish an equilibrium state for reaction IV. Oxidation can be conveniently carried out with pure oxygen, but taking into account the operating conditions.

The use of oxygen in industrial operations depends on two general factors

1. on the percentage of manganese in the form of compounds with lower valence in the ore; if manganese in the form of compounds with lower valence is abundant, the use of oxygen is appropriate;

2. on the extent of losses of manganese nitrate solution in other parts of the process.

In all processes, some material loss usually occurs, and in the case of the process according to the invention, this loss is almost entirely in the form of manganese nitrate solution. It is therefore usually necessary to make up for some loss caused by spillage of manganese nitrate. In addition to spillage, some manganese nitrate solution may be lost with the alkali metal nitrates when they are removed from circulation. It follows that the need for additional manganese nitrate for make-up, together with the quality of the ore and economic factors, will determine whether a certain proportion of the reaction of manganese oxide VI with nitric acid is to be carried out with nitric acid produced by introducing a controlled amount of pure oxygen into the leaching system.

The leaching system according to the invention, based on the above parameters, includes:

,a) introducing nitrogen dioxide and water vapor from any source or from the decomposition of a manganese nitrate solution into an aqueous leaching medium, preferably containing manganese nitrate, in such a way that the resulting nitrous acid is rapidly dispersed in the liquid medium to prevent the formation of high concentration gradients,

b) immediately thereafter, the solution from step a) is brought into contact with the ore, thereby extracting the manganese dioxide contained therein from the ore, while maintaining a sufficient excess of ore (sufficient surface area of the ore relative to the dynamics of the system) to maintain a sufficiently low concentration of nitrous acid so that the equilibrium partial pressure of nitric oxide is maintained in the range of about 2.7 kPa to about 26.6 kPa, with the practical upper limit of the sum of the partial pressure of nitric oxide and the water vapor pressure above the extracting suspension being less than 101.3 kPa, i.e. higher than atmospheric pressure,

c) wherein the temperature of the slurry is maintained below about 80°C, preferably in the range of about 30 to 55°C, and the pH is maintained in the range of about 0.5 to about 1.5, with an upper limit preferably of about 3.0, and

d) the weight concentration of manganese nitrate in the aqueous leaching system is preferably maintained in the range of from about 2.5 to about 35%, usually up to about 50%.

The atmosphere above the liquid surface should be closed so as to prevent the entry of air and/or the loss of nitric oxide or nitrogen dioxide. From a design point of view, for practical purposes, this closed space need not be designed as a pressurized space, although a pressurized system is theoretically possible. If a pressurized space were used, the limitations on the sum of the partial pressure of nitric oxide and the water vapor pressure, which, ^as mentioned above, should not exceed atmospheric pressure, as well as the upper temperature limitations, could be appropriately revised and the pressure could be increased accordingly.

In general, all control of operating parameters can be carried out by conventional equipment. Cooling coils or a heat exchanger are suitable for temperature control; a pH meter can be used together with an acid pump to maintain the pH value; as for other control equipment, these are completely conventional devices.

However, the mechanical devices to meet the requirements of paragraphs aj and b) require more detailed analysis. The requirement for stage aj that the nitrous acid from the 'reaction of nitrogen dioxide with water is immediately dispersed in the aqueous leaching medium clearly indicates the use of some type of stirred system as the most common dispersing device. If a stirred tank is considered, there still remains the problem of introducing the nitrogen dioxide and water vapor into the stirred liquid suspension in the tank. It should be noted that any area of the gas supply equipment which serves as a 'cold-wall condenser' will cause excessive nitric oxide formation. If spraying is used, the walls of the spraying device must be insulated against heat transfer, or the uninsulated spray pipe must pass through the side wall of the tank. In both cases, the introduction of gases below the liquid level requires that the gases be produced under pressure or supplied by a blower. Problems also arise when withdrawing the suspension by the suction effect of condensation of nitrogen dioxide and water vapor. All of these aspects are inconvenient from a design point of view. A much more suitable solution is to use a contact condenser as a device for bringing the gases into contact with the liquid medium of the leaching system. If the liquid leaching medium is recycled through a constant condenser at a high flow rate, the concentration and temperature gradients within the contact condenser can be kept very low and if the condenser opens directly into the stirred tank, the time interval before the condensate in the stirred tank is completely dispersed is negligible. The contact condenser eliminates the problems of the cold-wall condenser as well as the pressure-related problems. All pipes carrying nitrogen dioxide and water vapor should be insulated to prevent condensation on the walls. Another advantage of using a contact condenser to introduce nitrogen dioxide and water vapor into an aqueous leaching system is that heat exchange equipment can be incorporated into the recycled stream passing through the contact condenser to control the temperature of the leaching system.

In terms of equipment and general process requirements, the requirements of reaction III and the equilibrium conditions of reaction IV can be

HNO ₂ ^H ⁺ + NOf 2 NO + H _Z O be satisfied simultaneously. These requirements include a practical factor which has not been mentioned as one of the critical chemical parameters of the process, namely that the solution contained in the leaching system, which contains the regenerated manganese nitrate solution, must be discharged to further processing stages ^, otherwise there would be a more appreciable loss of unleached ore. The preferred solutions to this problem will be discussed individually for the various categories, taking into account the physical character and cost of the raw ore used. The simplest solution is in the case of very cheap raw material which is naturally finely powdered. The preferred leaching plant consists of a large tank provided with a device for stirring the suspension, which is connected to at least one contact condenser, which flows directly into the tank. The leaching is preferably carried out continuously ^. If the tank is large enough, it can contain a sufficient excess of ore to maintain a suitable reaction rate in the presence of a relatively small amount of unreacted ore per unit volume. Losses of ore as the leached slurry is carried to the next stage of processing are therefore minimal. In a specific experiment with the process of the invention with ore containing 76% by weight manganese dioxide and with particles up to 74 μm in size, 2270 kilograms of manganese dioxide as product per day were obtained in a 9500 liter leaching tank, or 0.24 kg of manganese dioxide as product per liter of desired slurry per day, with an unreacted ore concentration corresponding to 0.0198 kg of manganese dioxide per liter. Since the mass concentration of manganese nitrate in the aspirated suspension is 30%, which corresponds to 0.19 kg of manganese dioxide as product per liter, the loss in the form of unreacted residue from the leaching tank is only 10.4% of the ore fed. This loss is not excessive in the case of very low-priced ore. Other critical factors observed in the test were: a temperature of 45°C and a pH in the range cd about 1.0 to about 1.3. The aspirating efficiency, based on nitrogen dioxide, is greater than 99.8%.

In the case where the raw ore is in its natural state finely powdered but sufficiently valuable that a loss of 10 to 12% is considered unacceptable, a different leaching system is recommended. In such a case the leaching system consists of two to three stirred tanks, each equipped with a contact condenser as described above. The tanks should be of nominal size and the charge should be treated in series, not countercurrently. The stream of gases produced during the decomposition should be split, with the larger portion of these gases being passed to the tank which is filled with fresh ore. The second tank or two tanks are then "flushed" with a decreasing portion of the total stream of gases produced by the decomposition until a suitable concentration of the remaining manganese dioxide is reached. In this way a leaching efficiency of at least 97% can be achieved. ' If the raw material is not finely powdered in its natural state, grinding to a fine grain size is an undesirable expense. In contrast, crushing the ore to a maximum size of about 1.5 to 1.3 mm is a relatively inexpensive procedure. A leaching system has also been designed for the use of ore crushed in this way as a raw material, without having to sacrifice compliance with critical chemical parameters. The leaching system in this case, similar to the first case, consists of a single stirred tank, equipped with at least one contact condenser. Leaching is also carried out continuously. However, the internal equipment of the tank is not of the usual type and requires a more detailed description.

In this case, there are two basic problems that need to be solved ^by suitable equipment design. One of them is that ore particles of 1.5 to 1.3 mm are very abrasive and can cause moderate wear of the heat exchanger or even the cooling coils. The second problem concerns the small surface area to weight ratio of the coarse particles. The amount of excess ore required to maintain an effective reaction rate III depends in fact on the surface area of the ore rather than on its mass. In the case of coarser ore, the mass of ore contained in the leaching tank must therefore be many times greater than in the case of finely powdered ore if the same reaction rate is to be maintained. Furthermore, due to the small surface area, the larger particles of the fed ore tend to accumulate in the leaching tank, so that the excess ore in the leaching tank will contain particles of a coarser size compared to the fed material. A tank of design to meet the above requirements includes a still settling zone or still compartment from which the stream of the recycler flows to the heat exchanger and to the contact condenser. The leach solution, proceeding to the next stage, is also removed from this still zone. This forces the coarse, unreacted and abrasive ore particles to remain in the agitated zone of the leaching system. In addition to the still zone, the design can be supplemented by modifying the shape of the tank and the agitator to obtain an intensively agitated zone near the bottom of the tank and a relatively quiescent zone near its top. This modification must also include a pipe which allows the condensate from the contact condenser to be led directly to the intensively agitated zone. This leaching system therefore operates as follows:

Crushed ore with a particle size of at least 1.5 millimeters is fed to the leaching tank; the coarser particles fall from the still zone at the top of the tank, but remain in a stirred suspension in the zone at the bottom. A relatively large excess of ore is still maintained at the bottom of the tank by stirring, so that by bringing the current from the contact condenser into contact with the ore, the concentration of nitrous acid is rapidly reduced to almost the equilibrium value. The fine fractions of the ore fed tend to remain in suspension at the top of the tank, but due to their large surface area they react rapidly with the nitrous acid remaining after the main reaction at the bottom of the tank, so that the top of the tank is essentially free of suspended ore particles containing manganese dioxide. However, the top of the tank contains all the tailings in suspension, since these solid particles are very fine.

The streams discharged to the heat exchanger and to the contact condenser are taken from the top of the tank, so that there are no defects caused by abrasive particles or particles that could clog the pipes. The regenerated manganese nitrate solution discharged to further processing is also taken from the top of the leaching tank, in a suitably simple case; it carries with it practically no particles of unreacted manganese dioxide.

In addition to the systems described above, numerous physical variations of the leaching system are possible. For example, a combination of a tank equipped with agitating equipment, a contact condenser and a heat exchanger in conjunction with columns of coarsely crushed ore can be used. The agitated leach solution can then be recycled from the tank through the ore column. However, regardless of the equipment used, the chemical controls described above must be applied if the leaching process is to achieve the efficiency required for practical use.

In order to adequately define a leaching process suitable for practice, it is also necessary to mention a possible small backwash circuit. This circuit serves as a gas expulsion system. It is desirable to maintain a slight negative pressure in the upper part of the tank in the main leaching process, approximately 2.50 Pa, so that nitrogen oxides cannot escape into the atmosphere through any leaks around the agitator shaft seal or other seals. Any defect in the various seals will therefore cause a small amount of air to enter the system. Another source of air is the raw ore being fed; even if the ore is fed into the leaching system via a liquid seal, air can still enter the leaching system, trapped in the gaps between the ore particles. Due to these various sources of air, there is a small amount of nitrogen oxide being expelled from the leaching system. This small amount of nitric oxide can be effectively removed from the inert gas in which it is carried in a specially designed scrubbing circuit. This scrubbing circuit with gas expulsion, to be effective, must operate under different chemical parameters than the main leaching circuit, including a large surface area of the liquid in contact with the gas, such as in a scrubbing tower, a large excess of finely divided manganese dioxide with vigorous agitation, a pIT value of less than about 0.5, a relatively low concentration of manganese nitrate, and moderate temperatures in the range of about 25 to about 35°C.

Under such conditions, the concentration of nitrous acid will be practically zero at any time and nitric oxide will be easily absorbed. The course of chemical processes in the scrubber can best be expressed by the equation

NO + 2 HNO ₂ + 2 Mn0 ₂ (H ₂ O

Mn(N0 ₃ ) _ζ

The manganese nitrate solution leaving the leaching system with washing can of course be returned to the main leaching system to avoid losses of manganese or nitrogen.

It has been found that in order to obtain highly pure, crystalline manganese dioxide as a powder product from a manganese nitrate solution obtained by ore distillation, it is necessary to remove certain acid-soluble impurities which adversely affect the growth of manganese dioxide crystals.

It has been found that one of the common impurities present in manganese ore is aluminum, which can pass into solution during leaching and, if not sufficiently removed by suitable treatment, can prevent the growth of well-defined manganese dioxide crystals during the subsequent decomposition of the manganese nitrate solution. The effect of aluminum contamination on the decomposition operation in a stirred system can be summarized as follows:

ý improperly processed feed solution temperature during decomposition

139 to 146 °C (depending on «Wdecomposition rate]. Stable conditions within the specified range, smooth continuous decomposition.

product

Suitability for technical use

Well-defined crystals · similar to sugar crystals, 80% at least 74 pm in size. Very easy to isolate from concentrated mother liquor.

Yes

145 to 158 °C. Unstable conditions. Decomposition begins at high temperature, then the temperature begins to decrease; as it approaches the lower limit, decomposition ceases, and the process repeats.

Very fine particles, low degree of crystallinity. More than 95% below 46 μm. Very difficult to isolate from mother liquor.

No

The maximum allowable dissolved aluminum content in a manganese nitrate solution is about 606 ppm and in practical operation this content should be kept below 220 ppm. It has been found that the pH of the regenerated manganese nitrate solution must be adjusted to a value in the range of 4.0 to 5.5 in order to properly control the amount of soluble aluminum. Preferably, the pH is adjusted to a value in the range of about 4.5 to about 5.0, thereby substantially removing the contaminating aluminum.

Since the aluminum and other impurities contained in the regenerated manganese nitrate solution must be removed in solid form by filtration, it is important that the colloidal precipitates, such as aluminum hydroxide and ferric hydroxide, are in a suitable physical form. It has been found that heat and pH adjustment are important in preparing the suspension for filtration. The purpose of the heating is to flocculate the colloidal precipitates formed. A temperature range of about 76 to about 105°C is suitable, and a temperature of about 90 to about 100°C is advantageous in combination with the pH value indicated for satisfactory removal of impurities including aluminum from the contaminated manganese nitrate solution.

In the process according to the invention, the decomposition of the manganese nitrate solution is controlled so that

a) as a product, the manganese dioxide obtained is produced in the form of free-flowing crystals of the pyrolusite type, in "chemically pure" quality, with particle sizes ranging from about 46 μm to about 256 μm, with about 80% of the particles being in the size range of 74 μm to 177 μm,

b] no deposits, scales or lumps of decomposition product are formed, •c] soluble impurities from the ore are suitably removed with continuous removal of part of the mother liquor,

d) gases resulting from decomposition are discharged in a leaching circuit without the risk of leakage or air pollution,

e) decomposition is continuous and very efficient in terms of material losses and heat conservation,

f) the device is simple, common and practically maintenance-free.

The conditions of the process according to the invention, given for the decomposition of manganese nitrate, are based on the simple and logical consideration that the decomposition reaction should be carried out under conditions as close as possible to the equilibrium state. The generalized conditions which promote the achievement of the equilibrium state in a two-phase system of this type are well known and can be summarized as follows:

A. good mixing to avoid high concentration and temperature gradients,

B. large interface area between the solid and liquid phases,

C. controlled uniform heat exchange rate,

D. when a continuous process of constant and uniform feed of raw material and product discharge is required.

The characteristic features of the decomposition of manganese nitrate solution have been carefully studied, and it has been found that there are some superficial details with the conventional crystallization system. Of particular interest here is a phenomenon similar to supersaturation. In commercial crystallization plants, supersaturation is advantageously used, since a controlled degree of supersaturation can be achieved in a unit volume of solution when briefly exposed to heat exchange surfaces without immediate crystallization occurring on the heat exchange surfaces themselves. The supersaturated solution then slowly reaches an equilibrium state inside the crystallization plant, with crystals growing under conditions close to equilibrium.

In contrast to the high decomposition temperatures reported in the literature, decomposition in the system occurs at equilibrium at approximately 137 °C and smooth decomposition, occurring at a suitable operating rate, can be achieved at a temperature in the range of 138 to 142 °C, indicating a small but significant degree of supersaturation. In the absence of an equilibrium solid phase, formed by free-flowing pyrolusite crystals with an active surface of pure crystals, decomposition may not begin until temperatures in the range of 158 to 160 °C. Once decomposition begins and the solid manganese dioxide phase is formed, the system is able to quickly return to the normal decomposition temperature.

From an operational point of view, it is clear that superheating a manganese nitrate solution to approximately its equilibrium decomposition temperature is similar to supersaturation in a conventional crystallization system, and the attempt to effect decomposition under conditions close to equilibrium leads to the same type of operational measures. Thus, a certain degree of superheating can occur in a unit volume of manganese nitrate solution when briefly exposed to heat exchange surfaces, without immediate decomposition on the heat exchange surfaces, and the decomposition can then be achieved within the stirred mass under conditions close to equilibrium conditions.

To establish such a system, it is necessary to maintain a suspension of manganese dioxide crystals in the decomposition of decomposing manganese nitrate,

b) maintaining the suspension in active motion around the heat exchange surfaces and c) regulating the rate of heat input so that the degree of superheat is maintained within a metastable temperature range of about 138 to about 160°C.

In addition to the above conditions, there are two special limitations which result from the special characteristics of the manganese nitrate/manganese dioxide system. One of these important limitations is that the heat exchange surfaces must be kept below the liquid level at all times. If these surfaces are allowed to protrude above the suspension level, significant deposits will form at and above the level. The second limitation concerns the percentage of solid manganese dioxide contained in the decomposing suspension. It has been found that a high concentration of solids causes excessive formation of fines by friction of one part against another. For this reason it is desirable to keep the solids below about 25%.

A decomposition apparatus meeting the above limitations consists of a simple vessel with a blower and a heating jacket or heating coils. It must of course be closed at the top to prevent the escape of the nitrogen dioxide produced, and the heat exchange surfaces must be below the usual level of the liquid suspension. Arrangements must be made for adding fresh manganese nitrate solution, as well as for removing the product and for discharging the gases into a scrubbing circuit. In order to obtain a uniform and rapid flow along the heat exchange surface, it is preferable to use a tank with a heating jacket rather than with heating coils.

The control of the impurity content in the decomposition stage of the process according to the invention requires further explanation, since it is related to the overall upgrading of the raw ore. It has already been mentioned that the impurities contained in the raw ore are removed after leaching by pH adjustment, heating and filtration. Although the main proportion of impurities from the ore, such as silica, alumina, phosphorus, heavy metals, iron, etc., are removed in this stage, all the proportions of alkali metal oxides and alkaline earth metals contained in the ore remain in the decomposition of manganese nitrate in the form of soluble nitrates. In the usual, abundantly occurring poor ores (manganese content 35 to 45%) the proportion of soluble impurities is relatively low, and is almost exclusively potassium oxide. The highest weight ratio of potassium ions to manganese ions in such ores is approximately 1:20. It has been found that the relatively high percentage of potassium nitrate does not interfere with the decomposition of manganese nitrate nor with the purity of manganese dioxide obtained as a product which is designated as chemically pure. The weight ratio of potassium ions to manganese ions in the liquid phase of the decomposition system may reach up to 1:1, at which point the viscosity of the solution begins to interfere with the escape of gaseous nitrogen dioxide and water vapor. In normal operation of the process according to the invention, the potassium content in the liquid phase of the decomposition system will be maintained so that approximately 1 part potassium accounts for 1 to 3 parts manganese, simply by withdrawing a portion of the liquid phase of the decomposition suspension. It is clear that under these conditions manganese is also removed in the withdrawn portion in an amount equal to approximately 10% of the amount of manganese fed in the ore. It should also be noted that the extracted portion can be evaluated by several known processes and can optionally be used directly as fertilizer.

It has already been stated that the product obtained is "chemically pure"; the results of the chemical analyses are given in Example 3. The exceptional quality of the product apparently results from the almost equilibrium conditions under which the product is formed. The well-defined, free-flowing, uniform crystals of manganese dioxide, free from inclusions, which are obtained as the final product, have a pyrolusite structure and are characterized not only by their exceptional purity, but also greatly facilitate mechanical separation, i.e. filtration of the product and its washing from the mother liquor.

The following examples illustrate the invention in more detail.

Example 1

The leaching parameters were evaluated in an experimental pilot plant operating continuously after several months of operation at a production of approximately 2250 kg of manganese dioxide per day.

Lean, unreduced manganese ore in the form of particles up to 74 µm in size is fed into a closed tank of 10,800 liters, equipped with a stirrer, the shaft of which passes through a gland with a seal in the lid of the container and the turbine blades of the stirrer are arranged near the bottom of the tank. The ore is fed through inlet openings in the lid of the container, equipped with liquid seals, at a controllable speed. An aqueous solution of manganese nitrate, containing dissolved and dispersed nitrogen dioxide and water, is fed into the tank through the opening in the lid into the ore suspension near the zone with the least intensive mixing in an amount calculated to produce '22150' kg of manganese dioxide per day after decomposition of manganese nitrate obtained by contact leaching of the ore. The source of the nitrate leach solution containing dissolved and dispersed nitrogen dioxide is a liquid contact condenser having a disc and toroidal contact chamber through which the manganese nitrate solution is recycled and into which nitrogen dioxide and water vapor obtained from the decomposition of the manganese nitrate solution being processed are fed. The manganese nitrate solution obtained by leaching finely powdered ore in dynamic contact with an aqueous nitrate solution containing nitrogen dioxide is maintained at a mass concentration of about 30% manganese nitrate by adding water and withdrawing the solution from the tank. The nitrate solution obtained by leaching is discharged from the tank from an internally located still box at the top of the tank as an overflow stream generated by the feeding of the ore and the aqueous nitrate solution containing dispersed nitrogen dioxide. A quantity of about 760 l/min of the overflow solution is passed to a heat exchanger and then through a condenser chamber for contact and condensation of the hot nitrogen dioxide and water vapor supplied to the condenser in the decomposition of the manganese nitrate solution being processed. The remainder of the nitrate solution from the overflow is passed to a treatment tank for removal of impurities and subsequent decomposition. An additional quantity of nitric acid, which is required for the reaction and for transferring the impurities formed by the metal oxides contained in the ore into the solution, and further for maintaining the equilibrium reaction conditions, is added to the overflow stream to the condenser. The condenser is equipped with a vent pipe, connected to a washing and drainage device, designed to maintain a negative pressure of approximately 250 Pa under the cover of the leaching device and to allow air to pass through the lid of the leaching tank at a rate of approximately 28 1/min due to leaks in some of the seals and closures in the tank lid.

The constant feed rate of nitrogen dioxide and water vapor from the decomposition of the manganese nitrate solution obtained from the previous ore leaching is set at a rate calculated to produce approximately 2250 kg of manganese dioxide as product per day. The weight concentration of manganese nitrate is maintained at approximately 30% of the manganese nitrate in the leaching tank.

The above parameters are kept constant and in order to determine suitable and practically applicable operating parameters for contact leaching of ore, the temperature, pH value and the amount of ore in excess of nitrous acid contained in the suspension are varied. Failure of the leaching system is manifested by the loss of nitrogen dioxide leaving the outlet of the switching system. The partial pressure of nitric oxide and water vapor in the space above the suspension level in the leaching tank is monitored by chemical analysis of gas samples taken from the tank.

ore in stoichiometric excess to the nitrous acid contained in the suspension

PH temperature

The operating parameters are determined by keeping two of the parameters constant within the above ranges while varying one parameter, Loss of equilibrium ores in stoichiometric excess to the nitrous acid contained in the suspension pH temperature

The partial pressures of nitric oxide and water vapor have been found to be in the range of 6.7 to 20 kPa when all the above parameters are maintained within the ranges indicated. When the system is approaching failure, the exact values of the partial pressures cannot be determined, but they have been observed to exceed 40 kPa.

Example 1 and 2

A 3800 liter tank, equipped with a steam heating jacket and a stirrer, is charged with the crude manganese nitrate obtained in the leaching system of Example 1. The suspension is heated to about 90°C and the pH is adjusted to a value in the range of 4.8 to 5.0 by slow addition of manganese dioxide obtained from reduced manganese dioxide-containing ore. During the heating and pH adjustment, the suspension is continuously stirred and after about an hour it is filtered on a prepared bed drum filter, after which the precipitate is washed with water spray.

The above processing is carried out in batches, but the same processing can be carried out continuously by using several tanks connected in series.

The manganese nitrate solution thus treated is then concentrated to a mass concentration of about 55% manganese nitrate before subsequent decomposition, as this concentration is advantageous for practical and economical decomposition of said solution.

The average results of chemical analyses of the filtered manganese nitrate solution after the above treatment are:

During operation of the leaching system, the following ranges of the listed operating parameters are maintained:

up to 50 g in 1 liter of suspension, with a particle size below 74 /m.

It is possible to use a larger amount, but this is unnecessary and economically disadvantageous.

0.8 to 1.2 to 50°C.

conditions will result in the release of nitrogen dioxide from the outlet of the washing device under the following conditions:

less than about 10 g/l of suspension with ore with a particle size below 74 μΐη,

3.0 or higher °C or higher

Mn 130.0 g/1 To 5.5 g/1 Fe 0.9 PPm Cu 0.8 ppm Cr 1.0 ppm Al 2.0 ppm Zn 7.0 PPm None 5.6 PPm What 13.0 PPm Mg 9.0 ppm Example 3

The data presented in this example were obtained during continuous decomposition of manganese nitrate in a pilot scale decomposition plant for six weeks over several months of plant operation.

At the start of operation, a concentrated manganese nitrate solution from Example 2 is introduced into a tank with a diameter of 2 m and a height of 2.5 m, equipped with a high-pressure steam jacket up to a height of 1.5 m of the side wall. The jacket provides 9.2 m ² of heat exchange surface. The inside of the tank is practically closed except for openings for the agitator shaft, for feeding the solution to be processed, for removing gases and for removing the manganese dioxide obtained as a product. Mixing is provided by a double turbine agitator driven by a 7.5 kW motor. Solid manganese dioxide obtained as a product is added to the solution, forming 6500 l of suspension in which the weight content of solid manganese dioxide is approximately 5%. The decomposition treatment is carried out continuously by feeding a manganese nitrate solution at a rate corresponding to 6500 -760 liters of suspension, in which the solid manganese dioxide content is maintained in the range of 5 to 20% by weight of the suspension during the decomposition. Continuous operation is thus carried out at a controlled rate of heat input so that 1600 to about 2600 kg of manganese dioxide are obtained per day. The decomposition temperature varies between 140 and about 146 °C. The manganese dioxide obtained as a product is continuously withdrawn from the bottom of the tank and substantially all of the mother liquor is returned to the decomposition tank without dilution. The weight ratio of potassium ions to manganese ions in the supplied manganese nitrate solution is 1:28 and is maintained in the mother liquor in the range of 1:2 to 1:3 by removing a portion of the mother liquor necessary to maintain this ratio. Occasionally, undesirable foam appears on the surface of the suspension and a silicone-type agent is added to reduce its amount.

The nitrogen dioxide gas and water vapor produced in the upper part of the tank are fed to a contact liquid condenser cooler, which is provided with discs and a toroidal chamber, in which they are immediately condensed into a manganese nitrate solution by dissolving and dispersing the gas and water vapor. The manganese nitrate solution is fed to the condenser from the still box of the leaching system of Example 1. The manganese dioxide obtained as a product is removed from the decomposition tank and dried by heating to a temperature of about 110 to 150 ° C. The dry manganese dioxide obtained as a product forms very well-defined and free-flowing uniform crystals of manganese dioxide of high purity, free of inclusions, pyrolusite structure.

As proof of the high purity of the manganese dioxide obtained as a product by the process according to the invention, the analysis of the starting ore used and the average chemical analysis of the product obtained during the operation of the decomposition system in Example 3 are given below for comparison. Average concentration analysis (%)

Mn

Fe

Al

Zn

Cu

Cr

Ni

SO ₄ ² insoluble fraction CO alkali and alkaline earth metals (as sulphates) NO3- less than As

P product, weight

63.0 (MNO2 - 99.5) 0.03 0.007 0.0001 0.0015 0.006 0.0025 0.05 0.02

0.007

0.15

0.05.

not detected

0.004

Analysis of the starting ore used, average values, mass concentration (%), converted to dry matter

Mn 46.4 ,Fe 6.3 AI2O3 5.9 SiO2 4.0 K2O 2.0 As 0.2 P 0.08 None 0.07 Zn 0.06 What 0.00 Cu 0.03 With 0.02

Although the invention has been described with reference to certain specific and presently preferred embodiments, it is not limited to these embodiments, as changes and modifications may be made therein which are within the scope of the invention as defined in the appended claims.

Claims (11)

Subject A method for obtaining manganese fractions from ores containing unreduced manganese by leaching these ores with an aqueous solution of nitrous acid to form manganese nitrate, characterized in that a charge of ore containing unreduced manganese containing a stoichiometric excess of manganese dioxide based on nitric acid content in the at a temperature in the range of 30 to 80 ° C and at a pH in the range of 0.5 to 4 at a lower equilibrium partial pressure of a mixture of nitric oxide and water vapor than atmospheric pressure with an aqueous leaching solution of nitrous acid. forming a suspension containing manganese nitrate. vynalezu 2. A process according to claim 1, wherein the ore containing non-reduced manganese is contacted with an aqueous solution of nitrous acid and is leached in an aqueous medium comprising an aqueous solution of manganese nitrate. 3. The method of claim 1, wherein the ore containing non-reduced manganese is contacted with an aqueous solution of nitrous acid and leached at a pH in the range of 0.5 to 3.0. 4. The process of claim 1, wherein the ore containing non-reduced manganese is contacted with an aqueous solution of nitrous acid and leached at a temperature in the range of 40-50 ° C. 5. The process of claim 1, wherein the pH of the manganese nitrate-containing slurry is adjusted to a value in the range of 4.0-5.5, the slurry containing manganese nitrate is heated to a temperature in the range of 70-105 ° C and filtered to removal of impurities. 6. The method of claim 5, wherein manganese dioxide is added to the filtered manganese nitrate slurry, the resulting suspension is vigorously stirred and heated by controlled heat input to decompose manganese nitrate to form free-flowing pyroluzite crystals of manganese dioxide, nitrogen dioxide, and nitrogen dioxide. water vapor. 7. The process of claim 6 wherein the manganese dioxide is added to the solution of manganese nitrate in an amount sufficient to form a slurry having a solids content of from about 5% to about 25%. fi. 6. The method of claim 6, wherein the filtered manganese nitrate containing suspension is concentrated to 50 wt% manganese nitrate concentration prior to decomposition. 9. The process of claim 6, wherein the manganese nitrate decomposition is controlled in a controlled manner at a rate of at most 1.2 kg of manganese dioxide based on 1 liter of suspension per day. 10. A process according to claim 6, wherein the manganese nitrate decomposition is controlled in a controlled manner at a rate of 0.24 to 0.6 kg of manganese dioxide, based on 1 liter of suspension and 1 day. 11. The process of claim 6, wherein the nitrogen dioxide and water vapor produced by the controlled decomposition of manganese nitrate is returned to the leaching aqueous nitrous acid solution used in the first step, wherein the nitrogen dioxide and water vapor are dispersed and dissolved. in this leaching aqueous nitrous acid solution to form a cyclic process.