Podcast
Questions and Answers
What is one significant advantage of fire assaying over wet methods in the analysis of ores?
What is one significant advantage of fire assaying over wet methods in the analysis of ores?
What primarily allows for the separation of precious metals from slags during fire assaying?
What primarily allows for the separation of precious metals from slags during fire assaying?
In fire assaying, which reagent type is NOT classified under fluxes?
In fire assaying, which reagent type is NOT classified under fluxes?
What is cupellation primarily used for in the fire assay process?
What is cupellation primarily used for in the fire assay process?
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Which class of furnace is primarily designed for melting ores and metallurgical products?
Which class of furnace is primarily designed for melting ores and metallurgical products?
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What is the primary purpose of cupellation in metallurgy?
What is the primary purpose of cupellation in metallurgy?
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What happens to lead during the cupellation process?
What happens to lead during the cupellation process?
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What material is considered optimal for making modern cupels?
What material is considered optimal for making modern cupels?
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Which of the following slag colors indicates the presence of cobalt?
Which of the following slag colors indicates the presence of cobalt?
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What critical factor primarily affects the loss of precious metals during cupellation?
What critical factor primarily affects the loss of precious metals during cupellation?
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In the context of cupellation, what is meant by 'shotting'?
In the context of cupellation, what is meant by 'shotting'?
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What kind of acid is primarily used for the parting of gold and silver?
What kind of acid is primarily used for the parting of gold and silver?
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What should the ratio of silver to gold be to effectively allow dissolution of the bead during cupellation?
What should the ratio of silver to gold be to effectively allow dissolution of the bead during cupellation?
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What is the consequence of using a cupel that is too dense during cupellation?
What is the consequence of using a cupel that is too dense during cupellation?
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What is 'matte' in the context of metals processing?
What is 'matte' in the context of metals processing?
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Study Notes
Fire Assaying: Principles and Methods
- Fire assaying is a quantitative chemical analysis method using heat (900°C) and dry reagents to determine metals in ores and metallurgical products, primarily for precious metals like gold, silver, and platinum group metals.
- It's faster and cheaper than wet methods, allows for larger ore samples, and quickly eliminates associated minerals.
- The process has been modernized but remains a centuries-old technique.
- Used to determine gold content in various samples like drill cores, soils, and chips.
Fire Assaying: Furnaces
- Furnaces are broadly categorized into crucible (pot) furnaces and muffle furnaces.
- Crucible furnaces are commonly used for melting ores and metallurgical products.
- Muffle furnaces hold a charge within a 'muffle' for heating.
Fire Assaying: Principles of Silver and Gold Ores
- Silver and gold have high solubility in molten lead but are insoluble in specific slags.
- Distinct specific gravities allow separation of precious metal alloys from slags.
- Lead is removed from precious metals through oxidation and absorption in cupels.
- Silver dissolves in dilute nitric acid while gold remains insoluble.
Fire Assaying: Fluxes and Reagents
- Fluxes are reagents classified as acid, basic, or neutral, and as oxidizing, reducing, sulphurizing, or desulphurizing agents.
- A crucial step is removing the button's gangue, followed by precious metal extraction using cupellation.
Cupellation Process for Precious Metal Separation
- Cupellation is an oxidizing fusion of lead alloys in a porous cupel (bone ash, cement, etc.) at 1000°C.
- Lead rapidly oxidizes (PbO) and absorbed or vaporizes partially leaving gold and silver as a bead.
Cupels: Characteristics and Preparation
- Cupels are shallow, porous dishes made of refractory and non-corrosive materials.
- Sheep bone is considered the best material for modern cupels.
- Cupels need proper porosity: too dense leads to increased silver losses, and excessive porosity leads to higher alloy absorption.
- Crucial to accurately air dry cupels before use.
Fire Assaying: Importance of Silver in Gold Determination
- Adding a silver-lead rod (e.g., Herman Inquarts) to the charge helps reduce gold losses during cupellation.
- This creates a silver-gold bead that easily dissolves for analysis, with a minimum 3:1 silver-to-gold ratio.
Slag Analysis and Identification
- Slag colors provide insights into metallic composition.
- Green slags often indicate ferrous silicates or copper. Brick-red indicates copper without iron, etc.
- Examination for 'shotting' (entrained lead particles) is crucial for ensuring accuracy.
Impurities in the Assay Process
- Shotting, matte (sulfide of metals like iron and copper), and speiss (Fe-As combination) can affect results and may require adjustments in fluxes or procedures.
- Most base metal impurities cause cupellation issues, excess litharge is recommended when unsure.
Precious Metal Losses During Cupellation
- Cupellation losses depend on cupel characteristics, temperature, lead and silver ratios, impurities, and air supply to the muffle.
- Higher temperatures generally lead to higher losses, so using the lowest possible temperatures is ideal.
- Silver loss intensifies with increased lead amounts.
Parting Process of Precious Metals
- Parting separates gold and silver using acids (nitric, sometimes sulphuric).
- Nitric acid is typically used, requiring a minimum 3:1 silver-to-gold ratio, and chlorine-free.
- Porcelain crucibles, capsules, and flasks are common parting vessels.
Inquartation
- Inquartation involves adding more silver if the bead contains too little to ensure a resolvable ratio (e.g., >3:1).
- This often involves wrapping the bead in silver foil and recupellation.
- Silver addition is often useful when a bead contains too little silver.
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Description
Explore the intricate principles and methods of fire assaying, a crucial technique for analyzing precious metals like gold and silver. This quiz covers various aspects, including furnace types and the unique properties of metals involved in the process. Test your knowledge and understanding of this centuries-old yet modernized analytical method.
