Manganese dioxide (MnO2) is a jet black powder found in pyrolusite, nature’s most prevalent manganese ore. They are several different crystalline forms of MnO2 and most are not nearly perfectly stoichiometric. Manganese compounds exist in oxidation states from +2 to +7 so with a +4 oxidation state manganese dioxide is already sufficiently electron hungry to serve as an oxidizer in many applications. Its main use by far is in dry-cell batteries, both carbon-zinc and alkaline, where it is reduced by zinc powder to manganese oxide hydroxide. The typical chemical reaction given to describe the processes of such batteries is:
Zn + 2MnO2 + 2H2O --> 2MnO(OH) + Zn(OH)2
This diagram serves as my map when mining old batteries for chemicals. The manganese dioxide is located along the outer walls of the batteries and is simply scraped off. Unfortunately, the manganese dioxide is not nearly pure and is also mixed with graphite and no doubt also has various zinc impurities. It also probably contains a host of lower oxides/hydroxides that need to be removed. This information from the MSDS of Duracell alkaline batteries should give you a good idea of just how rife such impurities are.
Cleaning up this mixture of chemicals turns out to be much more complicated (and more intriguing) than your standard textbook filtration and recrystallization processes. The problem lies in the fact that both carbon and manganese dioxide are insoluble in water and all other solvents as far as I know. Therefore, I have developed a way to separated manganese dioxide chemically from the graphite and subsequently regenerate with a series of reactions.
First the mixture is reacted with an excess of strong hydrochloric acid to give water-soluble manganese chloride and chlorine gas. This reaction is fairly slow so it must be heated and refluxed. This is the same reaction Carl Scheele first used to isolate chlorine gas in the eighteenth century.
MnO2 + 4HCl --> MnCl2 + Cl2 + 2H2O
Next the insoluble graphite is filtered away and sodium bicarbonate is added to neutralize the acid and to precipitate manganese carbonate.
MnCl2 + 2NaHCO3 --> MnCO3 + 2NaCl + H2O + CO2
The manganese carbonate is filtered, dried, and weighed and then reacted with a slight excess of nitric acid to form a solution of manganese nitrate.
MnCO3 + 2HNO3 --> Mn(NO3)2 + H2O + CO2
Lastly, the manganese nitrate solution is boiled to dryness and heating is continued until it completely decomposes into black manganese dioxide.
Mn(NO3)2 --> MnO2 + 2NO2
By no means am I a stickler for reagent grade chemicals, but let me explain. I need manganese dioxide to synthesize potassium permanganate, a potent oxidizer (its Mn is at +7) used widely in all fields of chemistry. To make potassium permanganate, one must first make the analogous manganate (+6) salt by fusing potassium hydroxide with manganese dioxide at about 500C or so.
The complete set of reactions I might use (there are tons of variations) to eventually manufacture potassium permanganate on a decent scale is outlined below.
Since this reaction is all about increasing the oxidation state of manganese, no reducing agents can be present! When I’ve tried this reaction with unpurified battery sludge, I can visibly see the carbon reducing the manganese dioxide to lower oxides (it embers and “catches on fire”). This wrecks any chance of what is already a very lower yield of potassium manganate. Thus, I embarked on the journey to purify manganese dioxide from batteries.
Most of the battery sludge came from a lantern battery I took apart for its graphite rods. Its approximate impure weight was 120g. This was added to water and boiled for about 10 minutes to dissolve any soluble impurities. The mix was then decanted and filtered and reacted with 10% sulfuric acid at room temperature to get rid of insoluble zinc compounds like zinc oxide. Upon adding the acid, I detected the “smell” of hydrogen (presumably due to sulfur impurities) and heard a substantial amount of gas formation. The partially spent acid was then filtered off from what was now mostly manganese oxides and carbon.
I assembled a setup for simple distillation and had it lead into a solution of sodium hydroxide to neutralize the chlorine that would form lest I gas myself World War 1 style. About 50mL 29% hydrochloric acid was added to the wet manganese dioxide sludge although 500mL was required stoichiometrically. I will add another 500mL HCl to the rest of the sludge later, but for now I just wanted to make sure that the entire purification process worked. The mixture was reacted in a 500mL round-bottomed flask in a water bath for about 2 hours.
The mixture was then filtered in two coffee filters and a yellow solution of what seemed to be mostly unreacted hydrochloric resulted. It took an awfully large amount of sodium bicarbonate to neutralize the hydrochloric acid and then produce a tan precipitate of manganese carbonate. I ended up with several grams of tan manganese carbonate precipitate which settled from solution rather quickly.
The semi-dry precipitate of manganese carbonate weighed 13.3 grams. This was reacted with 12mL of 15.1M nitric acid which reacted vigorously with it. The solution was then boiled down in a 150mL beaker. First it turned yellow and smelled of acrid nitric acid fumes. Specks of manganese dioxide began to form almost immediately. Once most of the water boiled off, lots of deep orange fumes of nitrogen dioxide were given off and the beaker turned completely black. I ended up with 3.3 grams of dry manganese dioxide which is pretty much a 33% yield because I only could expect 10 grams of the manganese dioxide to react because I only used 50mL of hydrochloric acid. With this experiment, I have definitely proven that this a route to pure manganese dioxide, but it wastes too much precious nitric acid. So I am currently researching possible alternatives. Some ideas:
Manganese dioxide is formed when hydrogen peroxide is added to manganese hydroxide at high pH levels (in acidic solution hydrogen peroxide is catalytically decomposed). An unverified source says that manganese dioxide also is formed when a “strong solution of hypochlorite” is heated with manganese hydroxide. I once added bleach to manganese chloride and due to its brown appearance I now believe I somehow obtained the hydroxide. Also, decomposition of manganese carbonate yields an amorphous mixture of manganese oxide and manganese dioxide which can be further oxidized completely to the latter with sulfuric acid and potassium chlorate. Manganese dioxide can also most likely undergo a thermite reaction with aluminum to form manganese metal, which could then be converted to a soluble manganese salt with the desired acid.