AU537831B2 - Manganese nitrate splash/spray decomposition - Google Patents

Description

MANGANESE NITRATE SPLASH/SPRAY DECOMPOSITION

FIELD OF THE INVENTION

The invention relates generally to a process for the thermal decomposition of manganese nitrate to manganese dioxide in a solution which contains alkali and/or alkaline earth nitrates, and more particularly to thermal decomposition of man¬ ganese nitrate obtained from a hydrometallurgical leaching process for the selective extraction and concentration of manganese, copper, nickel and cobalt from marine nodules or terrestrial ores con¬ taining alkali and/or alkaline earth compounds.

BACKGROUND OF THE INVENTION

Underwater deposits of manganese-bearing ores have been investigated for many years, but only recently have economic, strategic and environmental concerns encouraged the development of these materials as a viable alternative to conventional terrestrial mining techniques. The underwater deposits, variously referred to as marine nodules, sea nodules, ocean- floor nodules, manganese nodules, or maritime man¬ ganese ores, are distributed throughout the open oceans of the world and have also been discovered on the floor of inland lakes such as Lake Michigan.

Although the distribution of marine nodules is broad, it is also irregular, with areas of substan¬ tial concentration occurring both at great depths and on the shallower continental shelf. The characteris¬ tics of the marine nodules vary from one deposit to

another, showing considerable differences in size, shape, internal structure, and composition. In general, the marine nodules are primarily agglom¬ erates of manganese and iron oxides, containing lesser proportions of copper, nickel and cobalt compounds, and compounds of alkali and alkaline earth metals, along with traces of some two dozen metals such as molybdenum, lead, barium, vanadium, chromium, titanium, etc. The metal content of marine nodules is generally dependent upon the region from which the nodules are obtained, and economically important components vary in content within broad ranges. For example, manganese content may range from about 5 to 50 percent by weight, iron from about 5 to 40 percent by weight, copper from about 0.03 to 2 percent by weight, nickel from about 0.01 to 2 percent by weight, and cobalt from about 0.01 to 3 percent by weight.

This wide variation in metal values has complicated the refining of marine nodules and has led to the development of numerous processes for extracting the desirable materials. Straightforward physical methods for separating the metal components have not been successful due to the extremely fine- grained nature of the primary manganese oxide and iron oxide constituents. Pyrometallurgical techni¬ ques have inherent difficulties in achieving clean separations, probably due to the formation of complex metal alloys which are difficult to separate. Con- sequently, most effort has been directed to hydro- metallurgical processes.

-ζ^{JllE^m OMPI A hydrometallurgical process which pro¬ vides a simple, direct method for selectively separating iron and manganese from marine nodules and from each other, without high processing costs, while at the same time efficiently recovering other desirable metal components, such as copper, nickel and cobalt from marine nodules, is set forth in my U.S. Patent No. 4,123,499 entitled "Recovering Metal Values from Marine Manganese Nodules" which is here- by incorporated by reference. In accordance with the hydrometallurgical process described in my U.S. Patent No. 4,123,499, and illustrated in Figure 5, manganese-bearing ores such as marine nodules can be refined and various metal values efficiently recovered by first leaching the ore in an aqueous solution of nitrous and nitric acids to selectively solubilize the manganese, copper, nickel and cobalt as their respective nitrates, then separating the leach slurry from the insoluble iron oxide and gangue. The solu- tion is then treated with.manganous hydroxide in an amount at least equal to the stoichiometric value of metals other than manganese present in the solu¬ tion. This treatment precipitates substantially all of the copper, nickel, and. cobalt as a mixed metal hydroxide. If desired, the filtrate may optionally be further treated by the addition of hydrogen sulfide, which precipitates the remaining traces of copper, nickel, and cobalt as sulfides and assures virtually complete separation of these metals from the nitrate solution.

After filtration to remove the mixed metal hydroxide and sulfide precipitates, the solution con¬ taining manganese nitrate and alkali and alkaline earth nitrates is subjected to a thermal decomposi¬ tion process to form high purity manganese oxide. It is important that this thermal decomposition process be carried out so that the manganese nitrate decomposition is complete, and so that the manganese oxide product is of high purity and is easily separated from the alkali and alkaline earth nitrate present in the mother liquor.

The present invention is directed to an im- proved process for the thermal decomposition of man¬ ganese nitrate to high purity manganese oxide, which is particularly advantageous in the process for re¬ covering metal values from marine manganese nodules or terrestrial ores containing alkali and/or alkaline earth compounds as described in my issued patent No. 4,123,499.

SUMMARY OF THE INVENTION

It has been discovered that manganese ni¬ trate in the presence of alkali and/or alkaline earth nitrates can be advantageously thermally decomposed to high purity manganese oxide by a two-step thermal decomposition process. In the first thermal decom¬ position step, which may be called a splash decomposi¬ tion, heat is exchanged between hot gases and a dis- persed manganese nitrate-containing liquid phase consisting of drops or sheets of liquid, as opposed to a fine spray or thin liquid film. A relatively slow rate of thermal decomposition of manganese nitrate to manganese oxide is maintained during the first thermal decomposition step, allowing high

OMPI . IPO purity manganese oxide crystals to form in the presence of alkali and alkaline earth compounds under approximate equilibrium conditions as des¬ cribed in my earlier U.S. Patent No. 3,780,158, which is hereby incorporated by reference. It is also a feature of the present invention that this first decomposition step, or splash decomposi¬ tion, may accomplish the complete decomposition of manganese nitrate to high purity manganese dioxide crystals in the presence of alkali and/or alkaline earth compounds.

Preferably, however, a second thermal de¬ composition step is used which ensures the decomposi¬ tion of any manganese nitrate remaining in the mother liquor after the first thermal decomposition step. In the second thermal decomposition step, which may be called a spray decomposition, heat is exchanged between hot gases and a spray of very small, uniform liquid droplets of the manganese nitrate-containing solution. The second decomposition is very rapid and produces a reactive manganese oxide product which is deficient in oxygen relative to manganese dioxide.

It is an advantage of the present invention that it provides an improved process for the complete thermal decomposition of manganese nitrate in an aqueous solution containing alkali and/or alkaline earth compounds.

It is a further advantage of the present invention that it provides an improved process for preparing high purity manganese oxide by the thermal decomposition of manganese nitrate in an aqueous solution containing alkali and/or alkaline earth compounds.

It is another advantage of the present invention that it provides an improved process for preparing a reactive manganese oxide product which is deficient in oxygen relative to manganese dioxide by the thermal decomposition of manganese nitrate in an aqueous solution containing alkali and/or alkaline earth compounds.

Another advantage of the present invention is that it provides a process for preparing and easily separating a manganese oxide product in the form of a slurry. Yet another advantage of the present inven¬ tion is that it permits a much lower ratio of manganese to total alkali and alkaline earth compounds to be maintained in the decomposition chamber than can be maintained in my earlier U.S. Patent No. 3,780,158.

Still other advantages of the invention will be readily apparent to those of ordinary skill in the art from the Figures and detailed description of the invention which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a flow diagram illustrating a splash decompositon according to the present in¬ vention.

FIGURE 2 is a flow diagram illustrating a spray decomposition according to the present in- vention.

_ OMPI FIGURE 3 is a graph illustrating the ratio of soluble manganese to total alkali and alkaline earth compounds present versus time in a splash decomposition.

FIGURE 4 is a flow diagram illustrating a two-step thermal decomposition according to the present invention.

FIGURE 5 is a flow diagram illustrating a process for recovering metal values from manganese- containing marine nodules or terrestrial ores con¬ taining alkali and/or alkaline earth compounds to which the manganese nitrate decomposition process of the present invention may be applied.

While the invention will be described in connection with preferred procedures, it will be understood that it is not intended to limit the in¬ vention to those procedure's. On the contrary, it is intended that the present invention encompass all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the claims.

DETAILED DESCRIPTION OF THE INVENTION

Turning first to Figure 1, there is shown a flow diagram illustrating a splash decomposition process for the thermal decomposition of manganese nitrate to high purity manganese dioxide from a solu¬ tion containing alkali and/or alkaline earth compounds, such as nitrates. The splash decomposition illustrated in Figure 1 is the first step of the two-step thermal decomposition of the^" present invention. In the splash decomposition process illustrated in Figure 1, a manganese nitrate solution or slurry is dispersed into droplets or sheets of liquid, as opposed to a fine spray or thin liquid film, and decomposed by heat exchange with hot gases such as steam or hot nitrogen dioxide. The droplets or sheets of liquid which characterize a splash decom- position process are typically formed by vigorous agitation of the surface of the manganese nitrate solution or slurry contained in a closed heat ex¬ change and decomposition chamber. This vigorous agitation of the surface of the solution or slurry propels droplets or sheets of liquid solution or slurry into the hot atmosphere above the bulk liquid slurry of manganese nitrate solution containing man¬ ganese dioxide. The closed heat exchange and de¬ composition chamber comprises an upper heat exchange zone above the liquid slurry, into which the liquid slurry is propelled as droplets or sheets of liquid as described above, and a lower decomposition zone which contains the bulk liquid slurry of manganese nitrate solution containing manganese dioxide. The droplets or sheets of liquid slurry which charac¬ terize a splash decomposition process are sufficiently massive such that only a portion of the manganese nitrate contained in each droplet or sheet of liquid is thermally decomposed in the hot atmosphere of the closed heat exchange and decomposition chamber before the droplet or sheet of liquid falls back into the manganese nitrate slurry. Decomposition continues in the lower decomposition zone due at least in part to heat exchange between the hot gases and the surface of the bulk liquid slurry. Consequently, the product manganese dioxide crystals are formed under almost equilibrium conditions and are of high purity in a splash decomposition process.

In the splash decomposition process illus¬ trated in Figure 1, a closed heat exchange and splash decomposition chamber is illustrated. A feed solution containing manganese nitrate enters the closed de¬ composition chamber through a liquid-gas separator. This manganese nitrate solution may, for example, be a manganese nitrate solution to be thermally de- composed to manganese dioxide as described in my U.S. Patent No. 4,123,499 entitled "Recovering Metal Values from Marine Manganese Nodules. "

Two propellers are mounted on a single shaft extending vertically into the closed decomposi- tion chamber. One propeller is mounted in the lower section of the decomposition chamber to suspend parti¬ cles of manganese oxide product in the slurry of man¬ ganese nitrate and manganese oxide in the closed de¬ composition chamber. Another propeller is mounted at the liquid-gas interface to cause splashing of the manganese nitrate slurry into the gas phase. It will of course be understood that other means of vigorously agitating the surface of the manganese nitrate slurry to cause splashing of the slurry into the gas phase may also be used.

The splashed manganese nitrate slurry in Figure 1 comes into contact and heat exchange with the heated recycle gas which is shown entering the upper portion of the closed decomposition chamber. The heated recycle gas supplies the heat of reac¬ tion for the thermal decomposition of manganese nitrate to form manganese dioxide. The heated recycle gas, along with gases generated in the thermal decomposition (such as nitrogen dioxide) , are discharged through the liquid-gas separator to remove any carryover of manganese nitrate. These gases are then fed to a scrubber system

(not shown) to collapse the steam and absorb the nitrogen dioxide gas and produce manganese nitrate from the nitrogen dioxide gas and manganese bearing ore. It is a feature of the present invention that the manganese nitrate solution is fed into the closed decomposition chamber through a gas discharge port in the liquid-gas separator, to permit preheating of the feed solution and to ensure that no decomposi¬ tion products build up at the gas discharge port. The liquid level of the manganese nitrate slurry in- the closed decomposition chamber may be controlled by a constant level overflow system. The constant level overflow system may also serve as a product discharge means. For example, the over- flow from the closed decomposition chamber may be discharged into a thickener tank by a constant level overflow system. In the thickener tank, the high purity manganese dioxide product is separated from the mother liquor, and may then be collected in a product tank.

The splash decomposition process of the present invention is particularly adapted for the. thermal decomposition of manganese nitrate solution- obtained from my process for recovering metal values from marine manganese nodules as described in my U.S. Patent No. 4,123,499 and illustrated in Figure 5. The application of the splash decomposition process of the present invention to the thermal de¬ composition of manganese nitrate solution containing alkali and/or alkaline earth nitrates obtained from my process for recovering metal values from marine manganese nodules comprises the following steps:

(a) Leaching manganese-bearing marine nodules or terrestrial ores containing alkali and/or alkaline earth compounds with an aqueous solution of nitrous and nitric acids having a pH below about 2.5 to selectively dissolve manganese, copper, nickel, and cobalt, while retaining substantially all iron oxide in the gangue;

(b) Separating the insoluble iron oxide and gangue from the leach solution at a pH of less than 2.5;

(c) Adding to the leach solution a quantity of manganous oxide or manganous hydroxide to neutralize the solution and selectively preci¬ pitate copper, nickel, and cobalt;

(d) Removing precipitated copper, nickel, and cobalt hydroxides from the manganese nitrate- containing solution; and

(e) Thermally decomposing the manganese nitrate solution containing alkali and/or alkaline earth nitrates according to the process of the present invention.

Further features of the present invention which are related to my process for recovering metal values

OMPI from marine nodules illustrated in Figure 5 include the transferring of nitrogen dioxide gas obtained from the thermal decomposition of manganese nitrate according to the present invention to the aqueous leaching solution of my process for recovering metal values. Yet another feature of the present invention which is related to my process for re¬ covering metal values from marine manganese nodules illustrated in Figure 5 is the separation of unde- composed alkali and alkaline earth metal nitrates remaining after the decomposition of manganese ni¬ trate according to the present invention, and treat¬ ment of said alkali and alkaline earth metal nitrates with a mineral acid such as sulfuric acid to produce nitric acid which may be used in the leaching solu¬ tion in my process for recovering metal values. Still other advantages of the present invention which are related to my process for recovering metal values from marine manganese nodules will be readily apparent to those of ordinary skill in the art from the complete specification and disclosure of my process for re¬ covering metal values which is set forth in my U.S. Patent No. 4,123,499, which is hereby incorporated by reference. It is a feature of the present invention that the splash decomposition chamber, -as well as the recycle gas pipes, may conveniently be insulated to prevent excess heat losses.

The splash decomposition system illustrated in Figure 1 is found to produce a high purity man¬ ganese dioxide product by allowing manganese dioxide crystals to form under almost equilibrium conditions, as described in my earlier U.S. Patent No. 3,780,158. This is accomplished because the droplets or sheets of liquid in a splash decomposition according to the present invention are sufficiently massive so that in a single cycle only a portion of the water con¬ tained in the liquid phase is removed. Consequently, the temperature change in the liquid phase is small. These characteristics define a splash decomposition according to my invention. A splash decomposition is further defined by the thermal balance in the closed chamber, viz. , there is not sufficient heat in the upper heat exchange zone of the closed chamber at any dynamic moment to effect complete thermal de¬ composition of the manganese nitrate in that zone at that moment. As described in my earlier U.S. Patent No. 3,780,158, the growth of manganese dioxide cry¬ stals under near-equilibrium conditions results in a well-defined, free-flowing, uniform and inclusion- free crystalline product of exceptional purity as described in Example 2 of U.S. Patent No. 3,780,158. Such high purity manganese dioxide crystals are characteristic of the splash decomposition process of the present invention. In addition, because the droplets or sheets of liquid in the splash decomposi- tion process of the present invention have sufficient surface area and are sufficiently small in size or thickness, the splash decomposition of the present invention permits product gases such as nitrogen dioxide to disengage from the relatively viscous mother liquor without causing foaming, which is a significant advantage of the splash decomposition process of the present invention.

OMPI « Another significant advantage of the splash decomposition process of the present invention is that it permits a very low ratio of soluble manganese to total alkali and alkaline earth compounds to be maintained in the manganese nitrate solution in the splash decomposition chamber. In my earlier U.S. Patent No. 3,780,158, it was necessary to maintain the ratio of soluble manganese to total alkali above _about 1 to 1 to prevent the viscosity of the solution from interferring with the release of nitrogen diox¬ ide gas and water vapor. In sharp contrast, in the splash decomposition process of the present invention, the manganese content in the manganese nitrate solu¬ tion in the splash decomposition chamber can be re- duced to as low as about 0.5 weight units of soluble manganese to one weight unit of total alkali and/or alkaline earths without causing foaming as the ni¬ trogen dioxide gas and water vapor disengages from the relatively viscous mother liquor. It is an im- portant feature of the present invention that it permits the manganese nitrate solution in the de¬ composition chamber to be reduced to about 0.5 to about 0.8 grams of soluble manganese per gram of total alkali and/or alkaline earths present after the splash decomposition process is complete, as illustrated in Figure 3. This low concentration of soluble manganese relative to total alkali and/or alkaline earth content in the final nitrate solu¬ tion after the splash decomposition is complete in turn permits a fluid product slurry to be formed in the second step of the decomposition, which is called a spray decomposition, and is now described.

OMPI The second thermal decomposition step of the present invention completes the decomposition of the manganese nitrate remaining in the mother liquor from the first thermal decomposition step. The second thermal decomposition may be called a spray decomposition. The second thermal decom¬ position, or spray decomposition, is accomplished by heat exchange between hot gases and very small, uniform liquid droplets of the manganese nitrate feed solution. In a spray decomposition, the very small liquid droplets of manganese nitrate solution are sufficiently small that in a single cycle all of the manganese nitrate contained in an individual droplet is decomposed. A spray decomposition is further defined by the thermal balance in the spray decomposition chamber, viz. , there is sufficient heat in the spray decomposition chamber at any dynamic moment to effect complete thermal decom¬ position of the manganese nitrate in the airborne droplets. These characteristics define_, a spray de¬ composition according to my invention. As mentioned above, by maintaining the soluble manganese concen¬ tration in the feed solution in the spray decomposi¬ tion at a concentration between about 0.5 to about 0.8 grams of soluble manganese per gram of total alkali and/or alkaline earth compounds, a fluid product slurry is obtained from the spray decomposi¬ tion which is easily and efficiently separated from the gas stream. .The fluid, or liquid, manganese oxide product slurry obtained is an important ad¬ vantage of the spray decomposition of the present invention. Turning now to Figure 2, there is illus¬ trated a spray decomposition process according to the present invention. As illustrated in Figure 2, a feed solution containing manganese nitrate, which is preferably the manganese nitrate solution after the splash decomposition process described above is complete, is fed into a spray chamber 3_ via a spray head 2_. Spray head 7_ may be powered by com¬ pressed air from a compressor or an electric motor (not shown) . The spray head _ disperses the mangan¬ ese nitrate feed solution into very small, uniform liquid droplets which come into contact with hot recycle gases S_ in the spray chamber 3_. As can be seen from Figure 2, recycle gases 9_ are heated by recycle heat exchanger _[ and fed into the spray chamber at an inlet port adjacent to spray head 2_. The heat exchange between the hot recycle gases 9_ and the very small, uniform liquid droplets of manganese nitrate solution produces a very rapid thermal decom- position of the manganese nitrate to a "manganese dioxide" product which may be nonstoichio etric, e.g. , which is deficient in oxygen. It is reported in the Encyclopedia of Chemical Technology, 2d Ed. , at Volume 3, page 112, that in nonstoichiometric forms of man- ganese dioxide, the Mn/O ratio may range from about

1:1.7 to about 1:2. This nonstoichiometric "manganese dioxide" product will henceforth be called the man¬ ganese oxide product (of the spray decomposition) . It is found that if the manganese nitrate feed solu- tion in the spray decomposition contains between about 0.5 to about 1.0 grams of soluble manganese per gram of total alkali and/or alkaline earths, the product of the spray decomposition will maintain its fluid, liquid character. It is found that the manganese

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OMPI oxide product of the spray decomposition is in fact a droplet of a slurry of manganese oxide in a liquid melt of alkali and alkaline earth compounds, such as nitrates. It is an important feature of the spray decomposition of the present invention that this flow- able liquid slurry of manganese oxide in a liquid melt of alkali and alkaline earth compounds may be easily removed from the bottom of the spray chamber by product discharge means 4_. It is, of course, neces- sary to maintain the temperature in spray chamber 3_ at a temperature sufficient to prevent the alkali and alkaline earth melt from solidifying. Suitably the spray chamber 3_ may be heated by a heating jacket 10. The decomposition product gases which are to be recycled, such as nitrogen dioxide gas and water vapor, leave spray chamber 3_ by exit port 5_ in the lower portion of the spray chamber. The recycle gases then pass through impingement separator 6_ which separates any manganese oxide product which may have been carried with the recycle gases. Thereafter the recycle gases, drawn by recycle fan 7_, are passed through the recycle gas heat exchanger 8_^ and then come into contact with the very small, uniform liquid droplets of manganese nitrate solution in spray chamber 3_ to provide the required heat exchange.

Another feature of the spray decomposition process is that it permits decomposition gases con¬ taining nitrogen dioxide to be removed from the re¬ cycle gas, for example after passing through recycle • fan 7_. ^Tne nitrogen dioxide gas obtained from the thermal decomposition of manganese nitrate in the spray decomposition process may be usefully returned to the aqueous leaching solution of nitrous and

-^j -EATr nitric acids in my process for recovering metal values illustrated in Figure 5.

The use of a recycle gas system to supply heat for the thermal decomposition in the spray de- composition process is advantageous because it avoids the introduction of additional gases into the recycle gas from which nitrogen dioxide is to be extracted and used as described above.

In the spray decomposition of the present invention, it is important to limit the manganese level in the manganese nitrate feed solution as described above to obtain a liquid or fluid manganese oxide-containing product, which is easily separated and removed from the recycle gas stream.. It is found that, if the soluble manganese level in the manganese nitrate feed solution is too high, the manganese oxide-containing product acts like a dry particulate material, which is difficult to separate from the recycle gas stream in the spray chamber. The parti- culate manganese oxide material which results from too high a manganese level in the feed solution also tends to form thin films in the spray chamber causing problems of scaling, and surface abrasion in recover¬ ing the manganese oxide product. It is therefore an important advantage of the present invention that the manganese oxide product of the spray decomposi¬ tion is a fluid or liquid slurry which may be easily separated from the recycle gas stream and is able to flow into a product container. Other problems in collection and separation of a dry particulate product material, which are avoided by the spray decomposition of the present in¬ vention, include producing an appreciable quantity of

OMPI manganese oxide particles which are too fine to separate in a centrifugal device. Furthermore, if the soluble manganese level in the manganese ni¬ trate feed solution is permitted to become too high, i.e. , above about 1 gram per gram of total alkali and alkaline earths, the alkali and alkaline earth melt in the nonfluid manganese oxide product can cause the manganese oxide particles to block or even absorb water if the environmental gas temperature drops below about 350°C. All of these conditions render conventional separation and collection methods virtually nonoperative. In sharp contrast, the spray decomposition of the present invention produces a fluid or liquid manganese oxide-containing product which is easily separated from the recycle gas stream by a cyclone separator or an impingement separator, as illustrated in Figure 2.

The spray decomposition process is particular¬ ly adapted to follow the splash decomposition process in the thermal decomposition of manganese nitrate solution as illustrated in Figure 4. As discussed above, the splash decomposition process of the pre¬ sent invention is particularly adapted for the ther¬ mal decomposition of manganese nitrate solution ob- tained from my process for recovering metal values from marine manganese nodules as described in my issued U.S. Patent No. 4,123,499 and illustrated in Figure 5.

When the spray decomposition process of the present invention is used as the second step of a two-step decomposition process as illustrated in Figure 4, the feed material of the spray decomposi¬ tion process is the product manganese nitrate solu¬ tion after treatment in the splash decomposition described above. Although the splash decomposition - process can accomplish the complete decomposition of manganese nitrate to manganese dioxide, it is pre¬ ferred to discontinue the splash decomposition pro- cess when the manganese nitrate solution in the splash decomposition chamber in Figure 1 is reduced to about 0.5 to about 0.8 grams of soluble manganese per gram of total alkali and alkaline earths present in the manganese nitrate solution. It has been found that approximately 80% by weight of the man¬ ganese nitrate contained in the original feed solu¬ tion obtained from my process for recovering metal values from marine manganese nodules is then decom¬ posed in the splash decomposition process. The re- maining manganese nitrate, constituting about 20% by weight of the manganese nitrate in the original feed solution is decomposed in the spray decomposi¬ tion process of the present invention.

Both a splash decomposition process and a spray decomposition process will now be illustrated by an example. These examples are, of course, only illustrative, since alterations and modifications thereof will be apparent to those of ordinary skill in the art.

EXAMPLE 1

A splash decomposition was conducted using the apparatus illustrated in Figure 1 and described above. The composition of the manganese nitrate feed solution and of the product manganese nitrate solution in the splash decomposition chamber after the splash decomposition process was discontinued are set forth below in Table. The temperature of the manganese nitrate solution in the decomposition chamber during the decomposition process was approximately 142°C. Solution temperatures in the range from about 140°C to about 150°C are suitable during the de¬ composition process. During the decomposition pro¬ cess, the temperature of the heated recycle gas (at the gas exit) in the decomposition chamber was approximately 145°C. Gas temperatures in the heat exchange zone of the decomposition chamber range from about 142°C to about 160°C are suitable during the decomposition process. The pressure of the re- cyle gas in the decomposition chamber during the decomposition process was approximately one atmos- phere. Pressures somewhat above or below one at¬ mosphere are suitable during the decomposition pro¬ cess.

The decomposition chamber had a diameter in its upper portion of 45", while the agitator propeller at the gas-liquid interface had a diameter of 8.9" and rotated during the decomposition process at approximately 50 to 100 rpm to splash the man¬ ganese nitrate solution. Suitable splashing condi¬ tions will be readily apparent to those of ordinary skill in the art bearing in mind the required ther¬ mal balance in the splash chamber, described above.

Table 1 ■Material Balances (Dr Basis)

Initial Feed

CMPD RATE (gm/min) Mn(N0₃)₂ 230 NaN0₃ 15 KN0₃ 8

Mg(N0₃)₂ 24 Ca(N0₃)₂ 16 Initial Product CMPD RATE (gm/min)

Mn(N0₃)₂ 147

NaN0₃ 15

K O-, 8

Mg(N0₃)₂ 24 Ca(N0₃)₂ 16

NO- 42

Figure 3 illustrates the concentration of soluble manganese in the manganese nitrate solution as a function of time of operation of the splash process. It was found that the process operated smoothly over a period of approximately 170 hours and that a ratio of soluble manganese to total alkaline earths of about 0.5:1 was easily obtained.

EXAMPLE 2

A spray decomposition was conducted using apparatus illustrated in Figure 2 and described above. A manganese nitrate feed solution containing 94.7 g/1 of Mn and 495 g/1 of alkali and akaline earth nitrates was subjected to a spray decomposition for 11 hours at an average of 11 1/hr. of feed solu¬ tion. The volume of the spray chamber was about 30 ft. 3. It was found that the spray decomposi■ti.on process was nearly 100% efficient. That is, only traces of manganese nitrate (less than 0.1% of the total Mn in the feed) was left undecomposed in the decomposition product and no nitrogen dioxide gas was thermally decomposed in the recycle gas heat exchanger. The temperature of the recycle gas en¬ tering the spray chamber was about 357°C at a flow

OMPI rate of approximately 300 cfm. Inlet gas tempera¬ tures between about 320°C and about 420°C are suit¬ able in the spray decomposition process. The flow rate of the recycle gas entering the spray chamber and the flow rate of the manganese nitrate feed solution entering the spray chamber are related. The recycle gas must enter the spray chamber at a sufficient rate and at a sufficient temperature to provide sufficient heat in the spray chamber at any dynamic or operating moment of the process to effect complete thermal decomposition of the airborne man¬ ganese nitrate droplets. The airborne manganese nitrate liquid droplets in the spray decomposition process are very small, typically about 50 microns in diameter. Mean droplet sizes from about 20 microns up to about 1000 microns in diameter may be used in the spray decomposition process, subject to the re¬ quirement of sufficient heat transfer to effect com¬ plete thermal decomposition, as described above. It is also observed that the airborne manganese nitrate liquid droplets are rather uniform in diameter, typi¬ cally varying only about 3 to 4 fold in diameter. The temperature of the manganese nitrate feed solu¬ tion entering the spray chamber was about 100°C. Solution temperatures between about 60°C and about 140°C are suitable in the spray decomposition pro¬ cess. The temperature of the recycle gas exiting from the spray chember was approximately 170°C. The spray head revolved at about 25,000 RPM. The jacket surrounding the spray chamber was maintained at a temperature of about 320°C. The nonstoichio¬ metric manganese oxide product of the spray decom¬ position had the following analysis:

- ϊLEAlT OMPI Product: 19.5% Mn_χO

(Mn = 59.8%)

(Mn0₂ = 87.2%)

(CaO = 0.53%) (MgO = 1.72%)

(Na₂0 = 0.16%)

(K₂0 = 0.11%)

(Chemical H₂0 = 3.75%)

80.5% alkali and alkaline earth nitrates and H₂0

It is understood that various other modi¬ fications will be apparent to and can readily be made by those skilled in the art without departing from the scope and spirit of this invention. Ac- cordingly, it is not intended that the scope of the ^' claims appended hereto be limited to the descrip¬ tion set forth herein, but rather than the claims be construed as encompassing all the features of patentable novelty which reside in the present in- vention including all features which would be treated as equivalents thereof by those skilled in the art to which the invention pertains.

Claims (11)

WHAT IS CLAIMED IS:

1. A method for decomposing manganese nitrate in an aqueous solution containing alkali and/or alkaline earth nitrates, comprising the steps of:

(a) splashing said aqueous solution containing manganese nitrate to form droplets or sheets of solution,

(b) heating said droplets or sheets of solution to partially decompose each droplet or sheet of solution to form manganese dioxide and nitrogen dioxide and water vapor, and

(c) reuniting said droplets or sheets of solution containing manganese dioxide with said aqueous solution containing manganese nitrate.

2. The method according to claim 1 including repeating said splashing, heating, and reuniting steps until approximately 80% by weight of said manganese nitrate is decomposed to form manganese dioxide.

3. The method according to claim 1 in¬ cluding repeating said splashing, heating, and reuniting steps until approximately 100% by weight of said manganese nitrate is decomposed to form manganese dioxide, and recovering said manganese dioxide.

4. The method according to claim 1 in¬ cluding repeating said splashing, heating, and re¬ uniting steps until a soluble manganese content in said aqueous solution of 0.5 to 0.8 grams of solu¬ ble manganese per gram of total alkali and alkaline earths is attained.

5. The method according to claim 1 in¬ cluding:

(d) heating said nitrogen dioxide and water vapor, and

(e) recycling said heated nitrogen dioxide and water vapor to heat said droplets or sheets of solution as set forth in step (b) .

6. A method for decomposing manganese nitrate in an aqueous solution containing alkali and/or alkaline earth nitrates, comprising the steps of:

(a) spraying said aqueous solution contain¬ ing manganese nitrate to form droplets of solution,

(b) heating said droplets of solution to fully decompose each droplet to form nonstoichio¬ metric manganese dioxide, and nitrogen dioxide and water vapor, and

(c) recovering said nonstoichiometric manganese dioxide.

7. The method according to claim 6, in¬ cluding:

(d) heating said nitrogen dioxide and water vapor, and

(e) recycling said heated nitrogen dioxide and water vapor to heat said droplets of solution as set forth in step (b) .

8. The method according to claim 1, in¬ cluding:

(d) separating said manganese dioxide from said aqueous solution containing manganese nitrate, and recovering said manganese dioxide,

■^ E

OMPI (e) spraying said aqueous solution con¬ taining manganese nitrate to form droplets of solu¬ tion,

(f) heating said droplets of solution to fully decompose each droplet to form non¬ stoichiometric manganese dioxide, and nitrogen dioxide, and

(g) recovering said nonstoichiometric manganese dioxide.

9. The method according to claim 4, including :

(d) separating said manganese dioxide from said aqueous solution containing manganese nitrate, and recovering said manganese dioxide,

(e) spraying said aqueous solution con¬ taining manganese nitrate to form droplets of solu¬ tion,

(f) heating said droplets of solution to fully decompose each droplet to form a nonstoichio¬ metric manganese dioxide, and nitrogen dioxide and water vapor, and

(g) recovering said nonstoichiometric manganese dioxide.

10. The method according to claim 9, including:

(h) heating said nitrogen dioxide and water vapor from step (b) ,

(i) recycling said heated nitrogen dioxide and water vapor to heat said droplets or sheets of solution as set forth in step (b) ,

(j) heating said nitrogen dioxide and water vapor from step (f) , and (k) recycling said heated nitrogen dioxide and water, vapor to heat said droplets of solution as set forth in step (f) .

11. A method of processing marine nodules or terrestrial ores to selectively recover manganese values, the nodules or ores comprising primarily oxides of Mn and Fe with lesser proportions of compounds of Cu, Ni, Co, alkali metals and alkaline earth metals, com¬ prising the steps of:

(a) washing the nodules with water until substantially all soluble chloride salts have been removed;

(b) leaching the nodules at a tempera¬ ture below about 80°C with an aqueous solution of nitrous and nitric acids having a pH of less than 2.5 under an equilibrium partial pressure of a combination of nitric oxide and water vapor less than atmospheric pressure, wherein said nodules contain a stoichiometric excess of man¬ ganese dioxide equivalent in relation to the nitrous acid content of the aqueous solution, to selectively dissolve Mn, Cu, Ni and Co, while retaining substantially all of the iron oxide in the gangue;

(c) filtering the resulting slurry con¬ taining Mn, Cu, Ni, Co, alkali metal and alkaline earth metal nitrates at a pH of less than 2.5 to remove insoluble iron oxide and gangue; (d) adding Mn(OH)₂ to the filtrate in a quantity at least equal to the^" stoichiometric value of the Cu, Ni and Co metals present in the solution to selectively precipitate said metals as hydroxides;

(e) adding the filtrate a quantity of H-.S in any proportion up to the Mn(OH)-, equivalent present to precipitate additional metal traces as sulfides;

(f) filtering the solution to remove precipitated metals and separate the manganese nitrate solution, and

(g) decomposing the manganese nitrate solution by the method of claim 1, 4, 5, 8, 9 or 10.