WTDahab AssayOne Lab

05/03/2025

‘Experienced’ small-scale CIP operators usually inquire how to calculate the results of the silver nitrate titration of cyanide. If you bought the standard silver nitrate solution from us, we provide 2 sets of standards: the high standard which is used for “linang” and other high grade materials but can also be used for tailings provided the volume of sample is greater than 10 ml, and the low standard for “lugaba” and tailings materials. Most, if not all, prefer to use the high standard.

So the calculations will be:
For the high standard: (using 10 ml filtered sample solution)
Ppm Sodium cyanide = Volume of silver nitrate used x 1000/2
= Volume of silver nitrate used x 500
For the low standard: 10 ml sample
Ppm Sodium Cyanide = Volume of silver nitrate used x 100

Ppm = parts per million = milligrams per liter. It is the preferred unit for the concentration of sodium cyanide in the CIP circuit rather than the percentage because we are dealing with minute quantities.

To make it simpler (using the high standard and 10 ml filtered sample);
For tailings and regular ore: typical target* sodium cyanide concentration will be 300 to 500 ppm
Your reading (volume of silver nitrate) should be 0.6 to 1.0 ml
For “linang” and high grade ore: typical target sodium cyanide concentration will be 600 to 1000 ppm
Your reading (volume of silver nitrate) should be 1.2 to 2.0 ml
*Typical target concentration is the usual dosage employed by most CIP plants but if you have access to metallurgical testing so much the better. A metallurgical test will determine all the process parameters for a given type of ore or tailings.

Let us improve our small-scale process by monitoring our cyanide concentration in our plant. With this, excess usage of cyanide is avoided hence unnecessary expense on cyanide and ill-effects on our rivers and environment will be minimized. Let us be responsible operators.

02/11/2024

THE IMPORTANCE OF ASSAY
What Is an Assay?
An assay is a process of analyzing a substance to determine its composition or quality. The term is often used in the mining industry to refer to tests of ore or minerals. Assaying is performed by an Assay or Mineralogical laboratory.
Assaying starts from the first phase of exploration for the estimation of reserves, as well as for the optimization studies of the ore treatment process up to the rehabilitation of the mining sites after the mine closure. Gold assays for exploration and mining projects provide a range of information: on gold grade; on the different status of gold (free, attached, included or refractory); and on its speciation (under what mineralogical form gold occurs, native gold, or alloy).
A Mineral Assay Laboratory is usually composed of 4 sections but not limited to: a sample preparation section, a section which specializes primarily in the analysis of rock and mineral resource samples over the life of a mine, a section devoted to monitoring the environmental status of mining sites, mainly air, water and soil monitoring, and a section for bullion analysis. Attached to the assay laboratory is a metallurgical laboratory,
where metallurgical testing and researches are undertaken.
Assay results provide an early indication of the potential value of a mineral or ore body, and therefore they are closely monitored by investors in mining companies. An exceptional assay result can trigger a sudden sharp rally in the stock of a company that holds the mineral rights of the property. Conversely, poor assay results may lead to a significant decline in a stock that has run up on speculation about promising results.
Assaying is also important in futures markets. Metals that are used to meet delivery requirements of futures contracts must be assayed to ensure that they meet the stringent quality and purity requirements mandated by the futures exchange for the particular contract in which that metal is traded. The existence of the physical delivery of metals helps in the convergence of prices between futures and spot markets.
Assaying enables investors to see the true value of their gold. For example, if an investor purchases gold that is advertised as 24-karats, but after the assay result, the piece turns out to be comprised of only 75% gold, the investor would have paid a higher price for less gold. Assaying one’s gold provides a quality check to make sure that the gold purchased is not only at the purity level that it claims to be, but also that it meets the standards set by the bar’s mint. Assaying allows for transparency and honesty about the gold that investors are looking to buy.
For us small-sale miners, the primary role of the assaying is to ascertain that our ore contains gold especially if its occurrence is not free, monitors plant performance and losses in the tailings. Summing up, assaying defines the head grade, monitors the dissolution of gold, loading of gold complex unto carbon. With these, process parameters are optimized and cost of treatment or processing will be greatly reduced.
What Is an Assay Value?
An assay value is the quantity of an ore's valuable elements such as gold, calculated by multiplying its assay grade or percentage of valuable elements by its dimensions. For precious metals, this figure is generally set out in troy ounces per ton or grams per metric ton. Assay value can also refer to the monetary value of an orebody, determined by multiplying the amount of its valuable constituents by the market price of the precious metal.
Who performs assays?
Anyone can assay their gold, but certain methods will be easier and more cost-effective than others. Electronic gold testers are relatively cheap, are sold commercially, and do not require prior knowledge. Therefore, anyone can test their gold piece using this method. XRF and laboratory processes, however, require highly-trained specialists or chemists but are much more precise.
Beyond individual assayers, there are many companies that specialize in assaying. Oftentimes, refiners and mints will offer gold assay services to their investors, or even have a trusted third-party assayer complete the process for them.
How is gold analyzed?
To meet the varied requirements of the mining industry, the assay laboratory must be able to provide rapid and reliable results by handling a variety of complex analytical instruments. Thus, the choice of the most appropriate method to determine the nature and gold content of a sample depends on its physical, chemical and especially mineralogical properties as well as the purpose of the analytical results (mining research, optimization of the treatment process, sale of concentrate, etc.).
Before the decisive step of the analysis, it is important to underline that gold ores are characterized by a great variability and that a great vigilance should be taken during the preparation of the samples. This being said, the most commonly used methods are:
1. Fire Assay
Fire assay is a standardized technique for concentrating precious metals. The sample is first mixed with a fluxing agent, then heated to high temperature (melting) with collection by lead. A mixture of precious metals is obtained which are then extracted by a process called cupellation. The metals are then solubilized for determination by atomic absorption spectrometry or inductively coupled plasma spectrometry.
It is important to note that fire assay is frequently used as the first choice when analyzing large quantities of gold. However, the presence of sulfides, chromium metal oxides and other elements can decrease the recovery rate of precious metals and interact with the fire assay process. Indeed, to improve recovery, mining laboratories adapt the nature of the flux according to the mineralogy of the samples.
2. Acid digestion with aqua regia followed by analysis by Atomic Absorption Spectroscopy
The determination of gold by chemical digestion can be a reliable alternative to fire assay. This digestion is performed by aqua regia, which is a mixture of concentrated hydrochloric acid and nitric acid. The results of this analytical method for gold correlate fairly well with those obtained by fire assay, and if the samples have been properly pretreated. Indeed, samples that contain carbon in the form of graphite or charcoal must be roasted before digestion because gold can be adsorbed when dissolved, which affects the analytical results.
3. Cyanidation
Cyanidation is an indirect method of assaying gold. It consists of dissolving gold when it is only in the form of free or attached grains, thus accessible to a dilute cyanide solution. This solution loaded with dissolved gold is then analyzed to determine its concentration.
4. Mineralogical analysis
Mineralogical analysis of gold ore by microscopic observations (optical and electron microscope) and by elemental microanalysis (electron and ion microprobe, Laser ablation coupled with ICP-MS) is paramount to understand in what form is present the gold in the ore, unlike the previous analyses which only give its total content. Microscopic observations allow us to know the different statuses of gold: Free gold is that which can be recovered gravimetrically; Free and attached gold is that which can be recovered by cyanidation; Included gold cannot be recovered by cyanidation, but can be recovered gravimetrically if its size is significantly larger than that of the host mineral; Refractory (or invisible) gold is gold that is embedded in the crystal lattice, and is not observable by microscopy. It is the elemental microanalyses that allow it to be detected and measured.

A rough assay can be done by crushing ore samples and panning the results. With some experience and a careful examination of the amount of gold present, it is possible to make decent visual estimates of the amount of free gold in the ore.

10/10/2024

27/08/2024

Understanding Gold Purity
When it comes to buying gold, understanding its purity is crucial. The purity of gold is measured in ‘karats’, which refers to the amount of pure gold in a piece of jewelry or bullion. Simply put, the karat value is measured on a scale of 24. The higher the karat, the higher the purity. The karat value is represented by a number followed by the letter “K”. Pure gold is labelled as 24K.
However, pure gold is too soft and malleable to be used in jewelry, which is why it is often mixed with other metals to create an alloy. The amount of gold content in an alloy determines its karat value. For example, 18K gold contains 75% gold and 25% other metals, usually copper and zinc.

Another way to measure gold purity is through millesimal fineness, which is the proportion of pure gold in a product expressed in parts per thousand. For instance, 18K gold has a millesimal fineness of 750, meaning it contains 750 parts of pure gold out of 1000 total parts.
The table below shows the karat value, millesimal fineness, and gold content of different gold purities:
Karat Value Millesimal Fineness Gold Content
24K 999 99.9%
22K 916 91.6%
18K 750 75.0%
14K 585 58.5%
10K 417 41.7%
Millesimal fineness is one of the most popular ways that pure bullion is labeled. Often shorthand for the grade of bullion is “Four Nines” or “Five Nines” referencing the millesimal fineness of 999.9 and 999.99 respectively. The karat value and millesimal fineness are the two most common ways to measure gold purity. It is essential to note that when it comes to jewelry, gold purity standards vary by country. For instance, in the US, 10K gold is the legal minimum karat value for jewelry to be called “gold.” In contrast, in France, the UK, Austria, Portugal, and Ireland, 9K gold is the minimum karat value for jewelry to be called “gold”.
In conclusion, understanding gold purity is crucial when buying gold. The karat value and millesimal fineness are the two most common ways to measure gold purity. The higher the karat value or millesimal fineness, the higher the gold content in the alloy.

The Karat System
When it comes to measuring the purity of gold, the karat system is used, especially in jewelry. This system measures the amount of pure gold in a piece of jewelry or other gold item. The karat system ranges from 0 to 24, with 24 karat gold being the purest form of gold.
When it comes to gold jewelry, pure gold is not always the best option. Gold is a soft metal, and pure gold jewelry is easily scratched or dented. That’s why gold jewelry is often made from an alloy, which is a mixture of gold and other metals, making composites such as 10 karat, 14 karat, and 18 karat.

Understanding Karats
To the unfamiliar there is often confusion with gold ‘karats’ and precious gem ‘carats’. Precious gems such as diamonds use ‘carat’ as a unit of weight – 0.2 grams per carat – used to measure the size of a gemstone. Each gold karat represents 1/24th of the whole. For example, 18 karat gold contains 18 parts pure gold and 6 parts other metals or alloys. The higher the karat value, the higher the percentage of pure gold in the item.

Different Karat Values
The most common karat values used in jewelry are 10k, 14k, 18k, and 24k. 24k gold is the purest form of gold, but it is also the softest and most malleable. This makes it unsuitable for everyday wear.
18k gold is a popular choice for jewelry as it contains 75% pure gold and 25% other metals. It is durable enough for everyday wear and has a rich, warm color. Countries such as India, Pakistan, and Bangladesh often prioritize very high karats in their jewelry, often 18k or higher.
14k gold is one of the most popular gold alloys, a more affordable option that is made up of 58.3% pure gold and 41.7% other metals, such as copper, silver, or nickel. It is a popular choice for engagement rings and other fine jewelry that can see heavier use.
10k gold is the lowest karat value that can legally be called “gold” in the United States. It contains 41.7% pure gold and 58.3% other metals. While it is a more affordable option, it is also intrinsically less valuable than higher karat values.
Understanding the karat system is essential when purchasing gold jewelry or other gold items. The higher the karat value, the greater the pure gold content is in the item, but also the softer and less durable it may be. Consider your budget and lifestyle when choosing a karat value for your gold jewelry.

27/05/2024

Understanding Specific Gravity Method

One way of verifying the authenticity and purity of gold can be achieved by using the Specific Gravity Method. This method is a precise and scientific and is being used by the local gold buying station to determine the amount of gold present in an item.

Specific Gravity is a dimensionless quantity expressing the density ratio between one subject and a reference substance. The reference substance is typically water which has a density of 1.000 g/cc at 4oC. Hence, at 4oC, the specific gravity is equal to the density without the units. For example, the density of gold is 19.300 g/cc, so its specific gravity will be:
Specific gravity = 19.300 g per cc / 1.000 g per cc = 19.3
If measurement is done at different temperature, say 25oC, multiply 19.3 by the density of water at 25oC which is 0.9971 or 19.24.

Specific Gravity is locally referred to as SP instead of SG.

Specific gravity (SG) is, usually, determined based on Archimedes' Principle (or the Law of Buoyancy) which states that: the upward force on an immersed object is equal to the weight of the displaced fluid. It is determined by attaching the sample to a string hung from a specially designed balance. The mass*or weight of the sample is determined, then a beaker of distilled water is raised from below the sample until the sample is submerged, and the sample is reweighed. The sample weighs less in water by an amount equal to the mass of the water displaced by the sample. This is Archimedes Principle. The SG is calculated with the equation:
SG = weight in air/(weight in air – weight in water)

The denominator of the equation represents the buoyancy, which is the mass equivalent of water displaced.

Suppose a mineral sample weighs 33.93 g in air and 32.01 g when suspended in water. The SG is determined as follows:
The denominator (weight in air - weight in water): 33.93 – 32.1 = 1.92 g. This is the weight of water displaced. Hence,
SG = 33.93/1.92= 17.67

The SG of a substance depends mainly on its composition, but is affected by certain conditions. The effect of temperature has been already considered. Air holes and empty spaces lessen the specific gravity of solid bodies; and metals, which after fusion become imperfect solids, have their density increased by hammering or rolling. But metals when free from pores have their density diminished when rolled, without annealing. The effects of these conditions are slight when compared with those due to the presence of impurities.

To determine the purity of the mineral, its specific gravity thus obtained is then compared to the specific gravity of pure gold which is 19.3 and to the karatage of pure gold which is 24.

% Purity = 17.67/19.3 x 100 = 91.55%

In terms of karat: 91.55/100 x 24 = 21.97 = 22K

*The mass of the sample does not change, so it would be more appropriate to say weight. Here "mass" should be understood as "mass equivalent" of the weight, since balances are calibrated in grams, a unit of mass. Weights are technically measured in Newtons.

14/05/2024

Understanding Gold Assaying

An assay is a process of analyzing a substance to determine its composition or quality. In the mining industry, assay is a chemical analysis that identifies the amount of metallic or mineral elements in a rock sample. Gold assaying is the process of determining the amount of gold in rock located within a mineral deposit. The findings from assays help gold exploration companies determine the quality and potential of a mineral deposit and assist in mapping out targets for future drill programs. To meet the varied requirements of the mining industry, the assay laboratory must be able to provide rapid and reliable results.
Gold assays conducted on a regular basis provide a range of information:
1. On gold grade (define ore reserves, optimize mill feed).
2. On the different status of gold (free, attached, occluded or refractory) to define plant performance and loss in tailings).
3. On its speciation (under what mineralogical form gold occurs, native gold, or alloy).
Thus, the choice of the most appropriate method to determine the nature and gold content of a sample depends on its physical, chemical and especially mineralogical properties as well as the purpose of the analytical results (mining research, optimization of the treatment process, sale of concentrate, etc.) and the type of the sample to be analyzed.
Before the decisive step of the analysis, it is important to underline that gold ores are characterized by a great variability and that a great vigilance should be taken during the preparation of the samples. This being said, the three most commonly used methods are:
1. Fire Assay - Fire assay is a standardized technique for concentrating precious metals. The sample is first mixed with fluxing agents with a lead collector. The mixture is then heated to high temperature (1150 –1450oC) in a furnace to melt and fuse. A ”button” of mixed precious metals is obtained in which the gold is extracted by a process called cupellation. The gold prill obtained after parting can be weighed gravimetrically or the button solubilized for determination by atomic absorption spectrometry or inductively coupled plasma spectrometry. This gives the total gold content.
2. Acid digestion followed by analysis by Atomic Absorption Spectroscopy - The determination of gold by chemical digestion can be a reliable alternative to fire assay. This digestion is performed by aqua regia, which is a mixture of concentrated HCI and HNO3. The results of this analytical method for gold correlate fairly well with those obtained by fire assay, and if the samples have been properly pretreated. Indeed, samples that contain carbon in the form of graphite or charcoal must be roasted before digestion because gold can adsorb (preg-robbing) when dissolved, which affects the analytical results.
3. Cyanidation - Cyanidation is an indirect method of assaying gold. It consists of dissolving gold when it is only in the form of free or attached grains, thus accessible to a dilute cyanide solution. This solution loaded with dissolved gold is then analyzed to determine its concentration.
4. Mineralogical analysis - Mineralogical analysis of gold ore by microscopic observations (optical and electron microscope) and by elemental microanalysis (electron and ion microprobe, Laser ablation coupled with ICP-MS) is paramount to understand in what form gold is present in the ore. Microscopic observations allow us to know the different status of gold:
-Free gold is that which can be recovered gravimetrically.
-Free and attached gold is that which can be recovered by cyanidation.
-Occluded gold cannot be recovered by cyanidation, but can be recovered gravimetrically if its size is significantly larger than that of the host mineral
-Refractory (or invisible) gold is gold that is embedded in the crystal lattice, and is not observable by microscopy. It is the elemental microanalyses that allow it to be detected and measured.
A rough assay can be done by crushing ore samples and panning. With some experience and a careful examination of the amount of gold present, it is possible to make decent visual estimates of the amount of free gold in the ore. Assaying by the blow-pipe furnace is another inexpensive path.
Aside from the AAS and ICP, instrumental methods include XRF, X-Ray Fluorescence; Atomic Emission Spectrometer; ICP-MS: Inductively Coupled Plasma – Mass Spectrometer; and INAA: Induced Neutron Activation Analysis and many others.

Why assay?
- Assay results provide an early indication of the potential value of a mineral or ore body, and therefore they are closely monitored by investors. An exceptional assay result can trigger a sudden sharp rally in the stock of a company. Conversely, poor assay results may lead to a significant decline in a stock.
- An assay test determines the impurities present in the ore before sending the ore to a treatment facility.
- It is not possible to tell how pure a gold bar is by simply looking at it. While a bar may look and feel like it is entirely made of gold, it is common for bars to be comprised of gold mixed with different metals called alloys. Purity is measured by karats: if a bar is 24-karat gold, then 99.99% of that bar is comprised of pure gold. If a bar is 12-karat gold, then it is a mixture of 50% gold and 50% other metals.
- Assaying one’s gold provides a quality check to make sure that the gold purchased is not only at the purity level claimed, but also that it meets the standards set by the bar’s mint. Assaying allows for transparency and honesty about the gold that investors are looking to buy.
- Assaying is also important in futures markets. Metals that are used to meet delivery requirements of futures contracts must be assayed to ensure that they meet the stringent quality and purity requirements mandated by the futures exchange for the particular contract in which that metal is traded.

who assays gold?
Anyone can assay their gold, but certain methods will be easier and more cost-effective than others. Electronic gold testers are relatively cheap, are sold commercially, and do not require prior knowledge. Therefore, anyone can test their gold piece using this method. XRF and other laboratory processes, however, require highly-trained specialists such as chemists, but are much more precise.
Beyond individual assayers, there are many companies that specialize in assaying. Oftentimes, refiners and mints will offer gold assay services to their investors, or even have a trusted third-party assayer complete the process for them.

Types of Assay
Many gold and silver pieces come with some proof of assay. Smaller ounces, for example, may come sealed in an assay card, which includes such information as the date of creation or assay. An assay certificate, though different from a card, is similarly designed to provide proof of purity and authenticity. In both cases, buyers, investors, and collectors have proof that their purchases are both pure and worthwhile.
The information included on an assay is just as important as the presence of a card or certificate. The assay tells recipients the serial number of the piece, which is further proof of its authenticity and integrity. Good assay cards also include the specific type of metal, thus proving that it is pure silver, gold, palladium, or platinum. It proves the promised purity of a piece and includes the mark of the mint where the piece originated.
A certificate of authenticity, or COA, is somewhat like an assay certificate or card. It is usually a sticker or a seal on a proprietary item. Usually the seal is on a piece of paper or the actual certificate, and it proves that the coin or bullion is authentic. Unlike an assay, however, there typically is no thorough process to prove the item’s purity, or at least it does not go to such great lengths. Similarly, a COA does not seal a piece of gold or silver, unlike an assay card. That being said, a COA does contain pertinent information like purity and a stamp of approval. It will generally include some sort of maker’s mark as well, along with information about its fineness and weight. Some products do not receive a COA for various reasons. They may come in bulk orders, or they may come with an assay, which can render a COA unnecessary.

15/04/2024

Since many of our brothers in the small scale mining industry are still unaware of the importance of monitoring the cyanide concentration in their process by the titration of free cyanide, hereunder is a brief description of the method.

TITRATION OF FREE CYANIDE

In the mining industry, removing the precious metals from the ore bodies is achieved by having cyanide form complexes with gold and silver, resulting in their dissolution. The toxicity and the control of cyanide concentration in the extractions of gold and silver require precise monitoring and detection of this compound. The efficiency of the cyanidation processes and meeting the environmental regulations depend heavily on the measurement and control of free cyanides. Cyanides are classified into three categories: free, weak acid dissociable, and total cyanides. Free cyanides include cyanide ions CN- and hydrocyanic acid, HCN. Weak acid dissociable cyanide has the weakly complex forms of cyanide (Cu, Ni, Zn, and Cd cyanide complexes). Total cyanide consists of all forms of cyanide (free cyanide, weak and robust complexes such as iron and cobalt).
Titration is considered the most common method to determine the free cyanide concentration in the gold extraction industry. This technique consists of adding a titrant whose concentration is known to a known volume of a sample whose concentration is unknown. The completion of titration is marked by a change of color or the electrode's potential, and it is known as the endpoint. These changes can be detected either visually or instrumentally. Procedures involving titration are generally used for monitoring large cyanide quantities when no weakly complexed metal cyanides or other interferences are present.
Silver nitrate titration is considered the most common method for monitoring free cyanides based on the number of silver nitrates required to transform the entire cyanide present into silver cyanide or one of the derivative compounds. As usual, the titration is performed, introducing the silver nitrate standard solution to a known quantity of the material with cyanide. If the color indicator is potassium iodide, the silver nitrate would react with forming silver iodide, which causes yellowish turbidity that is easily recognizable. When using rhodanine, color change from straw yellow to salmon pink.
The reaction taking place between the silver nitrate and sodium cyanide may be represented by the following equation:
AgNO3 + 2NaCN = AgNa(CN)2 + NaNO

Silver nitrate titration is considered reliable for determining free cyanides when no other interfering compound is present. With the gold industry now treating more complex gold ores, interferences are introduced to the solution. Thiosulfate for one is known to cause overestimation of the concentrations of free cyanide. Sulfides produce black precipitates and make the detection of the end points difficult. The presence of metals such as Cu and Zn causes the quantification of both free cyanide and a portion of the cyanides complexed with metals. While zinc-cyanide complexes can be fully titrated using silver cyanide if the pH is higher than 12, copper-cyanide complexes can only be partially titrated by this method. These interferences can be overcome with silver nitrate titration with potentiometric end-point.

Titration using KI indicator:
Filter an aliqout portion of the slurry or pulp to be tested. Take 10 ml of the filtrate and place in an Erlenmeyer flask, add a few drops of potassium iodide indicator solution free from alkali, and slowly add in the silver nitrate from the burette, shaking the flask meanwhile, until a distinct yellow opalescence appears. Then read off the number of ml of silver nitrate used and multiply by 0.05, and the result will be the amount of free cyanide in the solution in terms of percentage. In terms of ppm, the volume of silver nitrate used times 500. (This is of course when using the high standard silver nitrate solution obtained from us) When using the low standard, the number of ml of silver nitrate used times 0.01 gives you the % free cyanide or multiply by 100 to give you the ppm free cyanide.
The low standard is preferred to the unexperienced guy because a drop or two excess of the standard solution will not affect the reading. Low standard is available upon request.

Titration with Rhodanine
Basically the same procedure with KI but the indicator used is rhodanine and the end point is more distinct from straw yellow to salmon pink. Also for higher expected free cyanide content, the sample may be diluted with water up to 50 ml before adding the indicator.

In practice, most cyanide plants treating gold ores use solutions containing less than 0.05% (500 ppm) free NaCN; the general average is probably in the neighborhood of 0.02 – 0.03% (200 – 300 ppm) free NaCN.
Hence the common practice of small scale operators of dumping one or more drums of cyanide per CIL batch without monitoring the cyanide concentration is a complete waste of money and unaware disposal of cyanide to the river stream.