Manganese in Mineral Chromite, Ferrochrome Slags, and Ferrochrome

Manganese commonly occurs in mineral chromite as an impurity, and the amount can range from trace quantities to a few percent. In ferrochrome smelting operations, the manganese should find its way into the slag rather than the metal. Data from these analyses have most often been used to trace the partitioning of the manganese through the smelting process and for material balance calculations.

At this Center, the method of choice for slags and ores is decomposition by peroxide fusion, followed by a separation of interfering elements and oxidation of the manganese to permanganate, to be measured by molecular absorbance in an ultraviolet- (UV) visible spectrophotometer.

Metallic ferrochrome samples may be decomposed by fusion if the sample is ground to 100 mesh or finer and the sodium peroxide flux is moderated by the addition of 10 to 20 pct of sodium carbonate. Acid digestion beginning with approximately 5-pct sulfuric acid and a trace of hydrofluoric acid is the alternative.

Equipment

• Zirconium crucible (approximately 30 mL capacity).
• Meker or similar gas-air mix burner.
• 150-mL beaker.
• 250-mL beaker and cover.
• Stirring rod.
• Magnetic stirrer and stirring bar.
• 200-mL volumetric flask.
• 100-mL volumetric flask
• Rubber policeman.
• Funnel.
• Filter paper.
• Assorted pipettes.
• UV-visible spectrophotometer.
• Wash bottle.

Materials

• Sodium peroxide, reagent grade, granular, 20 mesh or finer.
• Sodium carbonate, reagent grade, anhydrous powder.
• Sulfuric acid, reagent grade, diluted 1:1 with distilled water.
• Hydrogen peroxide, reagent grade, 30-pct solution.
• Zinc oxide, reagent grade, slurry.
• Iodate oxidant solution.
• Standard manganese solution.

Procedure

1. Weigh the sample into a zirconium crucible.
2. Add 5 to 10 g of sodium peroxide (plus 1 to 2 g of sodium carbonate for ferrochrome metal), and stir until the sample and flux are thoroughly mixed.
3. Fuse over a burner until all sample particles are dissolved. Swirl and inspect occasionally to keep unattacked particles dispersed.
4. When the fusion is complete, allow the crucible and melt to cool.
5. When they are cool, tap gently to free the melt from the bottom of the crucible.
6. Place the solidified melt in the 250-mL beaker, add about 20 mL of distilled water, and cover immediately.
7. Place the crucible in front of its beaker, and add to the crucible about 5 mL of distilled water and about 2 mL of 1:1 sulfuric acid.
8. Police the crucible thoroughly, and slowly rinse its contents into the beaker with a small amount of distilled water.
9. Remove and rinse the beaker cover and the beaker sides. Place a stirring rod in the beaker.
10. Slowly and with vigorous stirring, acidify the leach in the beaker with 1:1 sulfuric acid.
11. Add 30-pct hydrogen peroxide dropwise with vigorous stirring to reduce the Cr+6 to Cr+3 and bring the beaker to a boil to decompose any excess peroxide.
12. When the solution is clear, remove from the hotplate and cool to room temperature.
13. Place a stirring bar in the beaker and begin stirring at a moderate rate.
14. Add zinc oxide slurry in small (3- to 5-mL) portions, allowing for dispersion between additions, until all iron and chromium are precipitated and a small excess of zinc oxide is apparent.
15. Remove and rinse the stirring bar, and transfer the sample slurry to a 200-mL volumetric flask. Use a wash bottle to ensure complete transfer.
16. Cool to room temperature and make up to volume with distilled water. Stopper, mix, and let settle for a time.
17. Using a dry funnel, dry paper, and a dry beaker, filter a portion of the supernatant liquid through a medium- speed qualitative paper (such as S&S 597).
18. Pipette an aliquot of dry filtered solution into a 150-mL beaker.
19. Add 20 mL of the iodate oxidant solution, and set the beaker on a hotplate at low to medium heat.
20. Leave the beaker on the hotplate for about 15 min after the first appearance of the permanganate purple.
21. Remove the beaker from the hotplate, and cool to room temperature.
22. Transfer the solution to a 100-mL volumetric flask, make to the mark with distilled water, stopper, and mix.
23. Measure the absorbance of the solution with a UV-visible spectrophotometer at 545 nm.
24. Prepare a calibration curve by pipetting appropriate aliquots of a standard manganese solution into 150-mL beakers and following steps 19 through 23.

Procedure Notes

1. Sample size is estimated to yield 0.5 to 1 mg of manganese in the final aliquot for oxidation.
2. Increase the sodium peroxide as the sample size increases, but consider the final melt volume when the sample weight approaches 1 g. When metals are fused in sodium peroxide they tend to behave like a thermite mixture. Carbon will behave that way also but will not get as hot as a metal. The addition of sodium carbonate to the flux will slow the reaction of metal and peroxide. Iron burns very fast and very hot; chromium burns much more slowly. If a sample of ferrochrome were as high as 80 pct chromium, moderation with sodium carbonate would probably not be necessary as long as the sample and flux were well mixed. Efficient mixing of the sample and flux cannot be overemphasized. Several milligrams of sample left unmixed can produce a spot hot enough to burn through a zirconium crucible.
3. The burner should be capable of bringing the bottom of the crucible to red heat. When the flux and sample reach sintering temperature, sample particles may begin to burn. This produces reddish-orange flashes and some hissing and popping sounds. When this happens, remove the crucible from the flame and attempt to swirl the contents so that the heat of the burning metal will be absorbed by the remaining flux and sample mix. This is called autofusion and is common in samples with a very high carbon content and well-moderated flux. Rarely will any of the sample-flux mixture be ejected from the crucible if mixing has been thorough. Very rarely a sample-flux mix will “skyrocket” or be uncontrollably active. If this happens, while holding the crucible with tongs, remove the crucible from the flame and hold it as still as possible until the activity has subsided. Let the melt solidify and drop the crucible into a large beaker or sink partly filled with tap water. Spilled melt may sometimes be chipped off such materials as transite and stone counters, but thorough cleanup will require the use of dilute acid and plenty of water. Skyrocketing is nearly always caused by inaccurate sample estimates, very poor mixing of sample and flux, or lack of flux moderation; with care it is avoidable.
4. If the melt is to be leached soon, then the crucible may be placed on any heat-resistant material to cool. If the melt must sit for some time before leaching, the crucible can be placed on a low-heat hotplate to keep it from absorbing atmospheric moisture.
5. If gentle tapping does not free the melt, then stronger tapping should be tried. If the melt sticks stubbornly, then place the crucible upside down on a clean piece of some durable, nonbrittle, nonreactive material (such as a clean scrap of counter stone). Tap on the bottom of the crucible with a light hammer until the melt is broken free. Transfer the melt to a 250-mL beaker, and rinse any small melt particles into the beaker also. Proceed with the leach.
6. Often a small part of the melt will stick in the crucible bottom, and the melt will splatter on the crucible walls during fusion. These walls should all be policed. Pour the policing solution into the beaker slowly and carefully. The solution in the beaker is very basic, and the solution in the crucible is moderately acidic.
7. The leaching reaction is usually quite active and often splashes leachate on the cover and sides.
8. Considerable heat is evolved when acidifying the highly basic leachate with the highly acidic 1:1 sulfuric acid. Excess sodium peroxide will also be partially decomposed and give off oxygen. Vigorous stirring and slow acid addition are therefore imperative. Observe closely during the acid addition to keep the reaction from boiling out of the beaker. If sodium carbonate was added to the flux, then carbon dioxide will be evolved on acidification, causing considerable foaming and requiring even closer attention.
9. Ideally, just enough acid should be added to completely dissolve the melt. Practically, attempt to keep the excess of acid small; it will be neutralized in a later step. The solutions resulting from fused samples usually have most, if not all, of their chromium in the +6 state. It is necessary to reduce this chromium to the +3 state so that it will be precipitated by the zinc oxide since the Cr+6 will interfere in the absorbance measurement. The most often used method of reducing the Cr+6 to the Cr+3 state is to add small portions of 30-pct hydrogen peroxide until an addition produces no color change and little more effervescence. The solution is then boiled for several minutes to destroy excess peroxide. The addition of hydrogen peroxide to an acidic solution of Cr+6 first produces the purple Cr+5 ion, which then disproportionates to Cr+3 and Cr+6. This purple color is very intense; lack of it, upon a peroxide addition, is the prime indicator of complete reduction. Boiling will decompose excess hydrogen peroxide, but a small amount remains intact. This excess peroxide will consume the oxidizing solution by reacting with the oxidized manganese. Therefore, close attention must be given during the Cr+6 reduction to minimize the hydrogen peroxide excess.
10. The zinc oxide precipitation will cause heat to be evolved, so the solution should be cooled.
11. Stirring should be vigorous enough to completely disperse the added zinc oxide slurry but not vigorous enough to cause bubbles. For the best results, allow the first few additions to dissolve completely before continuing. When the precipitate becomes dark and begins to persist, make the additions slightly larger and more frequent until the precipitate no longer darkens but begins to lighten in color. The precipitation is complete when there are white particles of zinc oxide visible in the precipitate and the liquid looks slightly milky if the precipitate is allowed to settle for a minute or two.
12. When mixing the contents of the flask, shake vigorously enough to dislodge from the glass any precipitate that has caked while cooling.
13. Take an aliquot to approximate a manganese content of 1 mg if possible, but use 50 mL as a maximum.
14. All of the manganese present should be oxidized within 5 min of the first appearance of color; 15-min digestion allows a safety factor.
15. Measurements should be made the same day as the oxidation step. Checks made at this Center have indicated that the color is stable for at least 24 h, except in the case of very high manganese concentrations (more than 3 mg of manganese per 100-mL flask).