Metal Technology laboratory crucibles are especially adaptable to the various needs and applications of the analytical chemistry laboratory, such as in strong alkaline fusions employed to reduce refractory samples to soluble form and for high temperature ignition and ashing purposes. Zirconium, a particularly difficult metal to deep draw with precision, is one of the most efficient all-around crucible materials for fusions employing either sodium carbonate, or sodium peroxide. Nickel, preferable in certain analytical processes, offers a very low cost per crucible. Inconel excels in high temperature applications such as ignition and ashing of samples, and exhibits high resistance to many corrosive media. Tantalum, one of the most corrosion resistant materials available, exhibits good resistance to acid attack. Molybdenum, due to its high melting point, is able to withstand temperatures as high as 2100oC (3800oF) in inert atmospheres. Every laboratory has its own specific needs and accepted procedures with equipment that has proven satisfactory. MTI offers high-quality products that can contribute to the efficiency and cost effectiveness of your facility. In addition to the standard sizes and materials listed, MTI can supply custom-made crucibles of many metals and shapes to exacting specifications. A fully qualified technical department is also avail-able to offer recommendations, or answer questions regarding the use of any of our products.

Zirconium

Zirconium has many unique properties. It is used as a fuel cladding in nuclear reactors because of its low thermal neutron capture cross-section and its unique corrosion resistance. It was for this purpose that a reduction method was developed to turn zircon sand into zirconium metal. The original application for reactors is now just one of the many uses for this metal. The remarkable corrosion resistant qualities of zirconium are increasingly evident in the Chemical Processing Industry. Zirconium will withstand a wider range of alkalis and acids than any other commonly used metals. As a result, the reduction of downtime for equipment repair or replacement has proved zirconiumís superiority and cost-effectiveness. Zirconium crucibles cost more than porcelain, steel or nickel, but based on an average number of fusions, which can be made in a zirconium crucible, as opposed to those of nickel, the ratio of longevity stands at 20:1. Therefore, the higher cost of zirconium is recovered three times over. In addition to cost-effectiveness, zirconium crucibles hold several advantages over other materials. If compared to platinum, several distinct advantages are readily apparent:

Every crucible is handmade to an exacting tolerance in order to achieve uniform wall thickness. They are also made from high purity zirconium material which is produced under the most stringent requirements.

Sample Disolution

Because of the varying compositions of ores, alloys and other materials requiring chemical analysis, it is impossible to have one set procedure that is applicable to all conditions. Sometimes it is necessary to combine two or more methods to effect solution of certain refractory materials. Acid extractions may be followed by fusion of the acid-insoluble material and fused samples then require extraction by water followed with various acids. Residues or ash-material remaining after incineration of combustible samples may re-quire fusion, acid extraction, or both. Some samples may require only sintering with a reagent at moderate temperatures to solubilize the desired element to be analyzed. In general, ores should be ground to pass an 80- or 100-mesh sieve before treatment by either wet or fusion methods.

Probably the simplest way to prepare a sample for analysis is complete decomposition and dissolution by direct fusion followed by acidification. The resulting solution can then be diluted to a known volume and aliquots taken for analysis by atomic absorption, volumetric Redox, gravimetric or spectrophotometric methods.

However, this presents several problems as to selection of the proper flux for fusion and a crucible that will withstand the effects of high temperature and chemical reaction, and yet be safe, reliable, relatively economical, and will not contribute undue amounts of contamination to the sample. Fusions are made in crucibles of silica, iron, nickel, silver, palau (80% gold / 20% palladium), platinum, graphite, porcelain, fireclay, fused quartz, or glass. None of these, however, are satisfactory for all types of fusions.

For example, hydroxide, carbonate, or peroxide fusions may be made in iron, nickel, silver or porcelain be-cause large amounts of crucible material will be taken into solution which contaminates the sample and shortens crucible life. Carbonate-potassium nitrate fusions may be made in platinum, but hydrochloric acid must not be used to dissolve the melt because the liberated chlorine will attack the platinum. Platinum, palau and silver crucibles are very expensive and easily ruined by samples containing lead, tin, antimony, sulfur, etc., if reduction is allowed to occur during fusion. They also are not very strong for constant use.

Potassium bisulfate fusions can be made in quartz, glass, or platinum, but not many samples are completely attacked by this fusion, since any silica present always remains insoluble along with the sulfates of barium, calcium, strontium and others. Since most fusions should be done by hand over the burner, there is always the danger of an accident due to the sudden cracking of porcelain crucibles.

Most of these problems can be avoided by making fusions in zirconium crucibles. Zirconium is an ideal crucible material for most fusions because it is only very slightly attacked by melts of alkali (Na, K, Li) carbonates, hydroxides, peroxides, borates, nitrates, chlorides and some fluorides, or combinations of the above, when done in full heat of the Meeker burner. Bisulfate fusions can be made in zirconium, but some attack may occur if the fusion process is carried out at very high temperatures for long periods of time. High temperature reactions with concentrated phosphoric acid are not recommended, but sample containing high phosphate can be fused safely with alkaline fluxes.

Other advantages of zirconium are its complete resistance to the action of most solvents of all concentrations such as nitric, hydrochloric, sulfuric and perchloric acids. This allows the fusions to be dissolved out of the crucible completely with little or no contamination of the sample. Alkaline solvents such as ammonium hydroxide, acetate, sulfide, carbonate, (NA, K) hydroxide and cyanide are not detrimental to zirconium. This resistance to most chemical solutions also permits direct chemical attack on sample materials not requiring fusion.

NOTE:

1. Hydrofluoric acid in all concentrations will attack zirconium and should not be used.
2. Fusions should not be done in a muffle furnace at high temperature unless provision is made for the exclusion of air. Zirconium is readily oxidized at temperatures exceeding 500oC when exposed to air for extended periods of time. This shortens the life of the crucible. However, ignitions of organic mate-rial such as petroleum products, vegetable matter and food products can be done in zirconium if the temperature does have to exceed 55oC, but the residues may have to be removed for separate weighing. Tare weighing in zirconium is not recommended for very accurate work. The material may gain weight even at lower temperatures due to oxidation. However, initial charring and decomposition can be done in zirconium and then transferred to fused quartz or other material for final ignition at temperatures in the range of 800oC to 1100oC.

As aforementioned, fusions are best made in a reducing flame of the burner. When done in this manner, there is little or no attack or oxidation of the crucible regardless of the sample material, or flux mixture. In making fusions, the sample is mixed with 4 to 10 times its weight of flux and placed over a thin bed of flux in the crucible. The burner set up should be under a fume hood and positioned under a suitable tripod with a nichrome or pipe-stem covered wire triangle across the top. The air-gas mixture and volume should be ad-justable to vary the flame temperature. Initially, the crucible and contents should be gently introduced over the flame moving it in and out as melting begins.

When the mass has become molten, the heat can be raised and the contents swirled to keep the sample off the bottom. The crucible should be grasped at the opposite side with stainless steel tongs to accomplish the swirling action. The swirling action should be continued throughout the fusion process. When the molten mass has become clear and homogeneous, or very bright red, the fusion is complete. The fused mass can then be poured on a stainless steel plate, or allowed to solidify in the crucible. The latter is preferable. The crucible and contents can then be placed in a beaker covered with water and a suitable solvent added to re-move the fused mass from the crucible. Any adhering material can be policed out or simply dissolved with more solvent. Under these conditions, only a few milligrams of zirconium will be introduced into the sam-ple.

For most atomic adsorption, redox, colorimetric, or spectrophotometric methods of analysis, it can be ignored. However, if it needs to be removed this can be accomplished by several methods. In strong acid solutions (HCl, H2SO4) zirconium can be precipitated by mandelic acid, or sodium phosphate reagents. Zirconium is also precipitated by cupferron in acid solution along with iron, titanium, etc. Extraction of the cupferrates by chloroform is an excellent separation from aluminum in acid solutions. Zirconium is also precipitated by ammonium, or sodium hydroxides.

Fluxes

The following fluxes can be used in zirconium.

Sodium Peroxide: Use with very refractory materials such as chromite, magnetite, illmenite, rutile, silicon carbide, certain alloys and steels, etc. ó an excellent general flux for most any material. There are one or two precautions to be taken when fusing chromite. When these materials are fused with peroxide, the chromium is oxidized to chromate which will tend to leave a yellow film on the inside of the crucible which will be unnoticed until the crucible has been removed from the subsequent dissolving operation, rinsed and dried. This can be prevented by adding a few milliliters of hydrogen peroxide to the acid solvent (H2SO4) while the crucible is still immersed. The peroxide in acid reduces the chromate to chromic chrome which goes readily into solution. The excess peroxide can be eliminated by boiling. Chrome can then be deter-mined by the usual persulfate oxidation followed by a reduction titration.

Peroxide fusions of silicon carbide and other finely pulverized materials are another matter. These materials tend to react violently at very low temperatures with oxidizing fluxes and will often burn right through iron or nickel crucibles on their first use. However, these can be safely fused in zirconium if the sample is first mixed with about 4 to 6 times its weight of powdered anhydrous sodium carbonate (º gram sample is usually more than enough), then add about twice the sample weight of sodium peroxide. Mix well. The crucible and contents are then gently moved toward a fairly cool flame and moved cautiously in and out of the flame until melting around the edges begins. It must not be put in the flame and held there unless all spattering, if any, has ceased. When the mixture appears to be melted and quiet, the temperature can be increased and fu-sion continued as usual. The fusion should be kept swirling and finished at a red heat.

• Sodium Carbonate: M.P. about 850oC. Decomposes most silicates of aluminum, calcium, chro-mium, nickel, etc. Also halides of silver and sulfates of barium and lead.
• Potassium Carbonate: M.P. about 910oC. Acts the same as sodium carbonate and can be mixed with it.
• Sodium and Potassium Carbonate: Equimolar mixture acts as either one alone, but melts at a lower temperature.
• Na,K Carbonate-Sodium Fluoride: Flux used on silicates to detect uranium by fluorimitry.
• Na, K Carbonates plus Oxidizing Agent: KNO3, KClO3, Na2O2, MgO, ZnO. Used on sulfide ores of arsenic, antimony, iron, nickel, molybdenum, etc.
• Sodium Hydroxide: M.P. about 320oC. Basic flux for oxidized ores of tin, zinc, antimony, etc.
• Potassium Hydroxide: M.P. about 360oC.
• Sodium Chloride: M.P. about 804oC. Neutral flux. Can be used as a cover for fusion mixtures.
• Na, K Bicarbonates: Desulfurizing flux. Rarely needed.
• Potassium Nitrate: M.P. about 339oC. Powerful oxidizing agent and basic flux. Used as a mixture with carbonates.
• Sodium Nitrate: M.P. about 316oC. Acts the same as KNO3.
• Lithium Metaborate: M.P. about 840oC. Flux for various oxide and silicate materials when sodium and potassium need to be determined, or would interfere with atomic absorption procedures.
• Lithium Carbonate: M.P. about 620oC. Can be mixed with lithium borate.
• Lithium Hydroxide: M.P. about 445oC. Can be added to other fluxes to help lower the melting point.
• Lithium Fluoride: M.P. about 870oC. Added to Na, K Carbonates for fluorimetric analysis.
• Calcium Carbonate-Ammonium Chloride: A sintering flux used to solubilize alkalis for analysis of sodium and potassium.
• Sodium Borate (Borax Glass): M.P. about 742oC. Used with Na, K Carbonates to give a lower melting flux for refractory silicates and oxides of aluminum, iron, nickel, etc. This list of fluxes can be used in most any combination in zirconium crucibles so long as the fusions are made in the reducing flame of the gas burner, or in a furnace equipped to provide an inert atmosphere such as argon, or helium.

Inconel

InconelR 601 is a nickel-chromium general purpose alloy for applications that require resistance to heat and corrosion. This alloy has excellent resistance to oxidation in the 1000oC to 1200oC temperature range and also has good corrosion to many acid and aqueous salt solutions. The properties of Inconel 601 make it a material of broad utility in such fields as thermal processing, chemical processing, aerospace and power generation. It has been used for baskets, trays and fixtures for annealing, carburizing, carbonitriding, nitriding and other heat-treating operations. Industrial furnace applications include radiant tubes, muffle retorts, shields, burner nozzles and electrical resistance heating elements. In the pollution control arena it is used for combustion chambers in solid waste incinerators. The alloy is also used for jet engine igniters and containment rings in gas turbines for aircraft.

Metal Technology offers Inconel alloy 601 laboratory ware in a wide variety of standard shapes and sizes. Inconel alloy 601 may be your answer to high temperature applications requiring resistance to oxidation and spall-ing where platinum has traditionally been relied upon. In addition to its resistance to corrosive oxidation, the alloy is also unaffected by rapid changes from hot to cold and it retains its mechanical strength at elevated temperatures. The high resistance of Inconel to oxidation, carburization, or sulfidation make it well suited for vessels used in determining moisture, volatiles, fixed carbon and ash in most coal and coke products, or wood products, e.g., pulp or fiber. It has also been recommended for use in drying and ashing biological materials whose residues are soluble in dilute acid or alkali for subsequent analysis. Trace-level determinations of principal constituent elements are excluded. Smoothing and reshaping after use is not necessary. Uniform heating is assured since the strength of alloy 601 precludes the necessity of reinforced rims and thicker bottoms as is the case with platinum in some instances. The vessels can be cleaned simply by scouring with seasand, or some other mild abrasive.

NOTE: Strong alkaline or oxidizing fusions are not recommended with Inconel 601 laboratory ware.

Nickel

In the laboratory, nickel crucibles offer high resistance to dilute alkalis at a very low cost per unit. In some in-stances, nickel crucibles are preferable to zirconium. For instance: sodium peroxide fusions in which zirconium itself is to be determined; also in analysis for niobium, tantalum, or low phosphorous. Although significant amounts of nickel can be introduced into samples, it can be removed by several ammonia separations. Life expectancy of a nickel crucible is from 4 to 6 fusions. They present an advantage, other than cost, if small amounts of zirconium are present, or its removal with Mandelic Acid is unsatisfactory. If small amounts of phosphorous are to be determined because of extremely low solubility of zirconium phosphate, then nickel must be used. Nickel is completely resistant to phosphoric acid as well as being highly resistant to the corrosive effect of the strongest alkalis. However, it is less than satisfactory when used in salt solutions containing oxidants such as ferric chloride, or solutions of mineral acids containing oxidizing salts. Nickel should not be used for hypochlorite solutions when the available chlorine is over 3 grams/liter. It should not be exposed to strongly oxidizing acids like nitric, or sulfurous acid and ammonium hydroxide in concentrations over 1%. In choosing crucibles for laboratory work, nickel can be effective with regard to cost per unit and for use in fusions where other metals cannot be used. It is even a cost effective replacement for platinum in those applications for which zirconium must not be used.

Molybdenum

Molybdenum, a refractory metal recognized for its excellent strength at high temperatures, its high melting point of 2622oC and its resistance to corrosion, serves a definite purpose in the laboratory. This high melting point makes molybdenum excellent for use as in vapor deposition boats and dishes. Vessels of molybdenum have also been used for such applications as processing nuclear fuel pellets at temperatures up to 1650oC, and molybdenum crucibles are durable and will withstand repeated rough handling. In air, or oxygen-containing atmospheres, molybdenum is not oxidized to any considerable degree at temperatures below 400oC. At 400oC and up, oxide forms and begins to sublimb. It is recommended that for high temperature applications, except for brief periods, fusions be performed in a vacuum, or inert atmosphere. The crucibles could then be heated up to about 2100oC. Molybdenum has the ability to withstand the corrosive attack of many mineral acids including sulfuric, hydrochloric, hydrofluoric and most organic acids as well as many molten metals, glasses, etc. Molybdenum is corroded by alkalis in the presence of oxidants.

Tantalum

Exhibiting a melting point of 3007oC among the refractory metals, tantalum is outranked only by tungsten (3410oC) and rhenium (3167oC). Tantalum is one of the most corrosion resistant materials available, exhibiting an excellent resistance to acid attack. Due to these qualities, laboratory crucibles fabricated from tantalum offer a wide variety of applications. Tantalum has been used widely in the electronics, nuclear, aerospace and chemical indus-tries in such areas as heat exchangers, where heat must be transferred to, or from acids and other corrosive fluids and vapors. It is also a superior material for the fabrication of heat shields, heating elements, etc. It is inert to most organic and inorganic compounds up to temperatures of about 150oC. The metal displays almost complete immunity to attack by most acids and is impervious to liquid metals up to 900oC. Like glass, one of the few exceptions to tantalums general acid resistance is hydrofluoric acid, which will attack tantalum readily. Alkalis, oxalic acid and fuming sulfuric acid should also be avoided when using tantalum, as well as any solution containing fluoride ions. Concentrated alkaline solutions will attack tantalum at room temperature. The degree of attack is somewhat dependent on temperature and concentration, but in general, strong alkalis above room temperature should be avoided. Most gases, including either wet or dry chlorine, or bromine are not reactive with tantalum at temperatures below 150oC. As temperature and concentration of such gases as oxygen, nitrogen, hydrogen chloride and ammonia are in-creased, oxidation becomes more rapid. Fluorine, hydrogen fluoride and gaseous SO3 attack tantalum at all temperatures. Salts and their solutions generally do not attack tantalum unless they are prone to alkaline hydrolysis, or contain fluoride ions. Chlorides and bromides such as ferric chloride, mercuric and stannous chloride up to 175oC are satisfactory for use with tantalum. Heating and vaporization elements made of tantalum are frequently used in flameless atomic absorption equipment, thus eliminating the ìcarry overî of ions found when using graphite elements.

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