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Tuesday, February 28, 2012

Gold Occurrences in the Dominican Republic

Coat of arms of the Dominican Republic
by Pavel D

The islands of the Greater Antilles were originally attached to the Yucatan Peninsula of Mexico and are composed of typical continental rocks.  These islands including the Dominican Republic were detached from the continent in the Eocene era.  It might be supposed that the Chixilub event when an asteroid crashed into the earth causing the extinction of the dinosaurs is somehow related to the islands subsequent drifting to their present location.  The geology of these islands is at best uncertain and inadequate the fossil records here are wretched causing a probable worst case scenario, and at best a very bad one.

The Spaniards began mining gold in the Dominican Republic in the 1520s to the detriment of the local Arawak Indians who they enslaved to work the mines, gold mining on a big scale is a recent development.  As recently as July 2001 Placer Dome Inc. of Canada announced that it had the right to negotiate an agreement for the Sulferos De Pueblo de Viejo gold project leaving the company a four year period to develop the mine after the award in March 2002.  By 2005 the company decided on a conventional open pit mine from which they anticipated production of about 800,000 ounces of gold for a period of five years and an additional 625,000 ounces per year for an additional eleven years.  The company estimated it would cost around $200 per ounce to recover the gold during the life of the mine.

Most of the gold deposits in the Dominican Republic are in areas of magnetic low anomalies like that at the Placer Dome Mine.  Other similar anomalies are being explored in the Loma Hueca, the Cumani and the Loma Lovetan concessions by Linear Gold.  The work performed at Cumani indicated anomalies of gold in stream sediments and soil anomalies in the Rio La Savita drainage basin.

Another gold concession is held by Impact minerals in the eastern part of the country included in these concessions are the El Brujo, La Bruja, Tedeche, Barita, La Culebra and the Athena concessions.  Much of the gold is found in gold, silver, zinc mineral belts that are often localized at the intersection of major faults in the area.  These areas displayed gold values as high as 5.59 g/ton gold with higher values of silver and zinc. There has been a limited amount of prospecting done in the country, but what has been done indicates there is additional potential for further gold, silver and base metals yet to be found.

A major fault runs alongside these islands that has been caused by the rubbing together of the islands by the Caribbean Plate that are further east has generated the Lesser Antilles that are mainly volcanic in nature having several major eruptions in recent history including one on the Island of Martinique that was one of the most deadly eruptions in history.

Wednesday, February 15, 2012

Nancy Twinkie goes Big Time

A DeHaviland Beaver like the ones we had except ours were on floats not skis.
Photo by Dave Bezaire & Susi Havens

Every summer after black fly season was over Kenny, Hans and I used to go prospecting in Northern Canada in the Arctic Archipelago.  By that time I had married Nancy Twinkie.  It turned out to be an excellent choice and Nancy had developed into a good prospector and part time gold miner.  When she got wind of us going up north again she let out a howl that almost took the roof off the house.  She wanted to go too!   She also made it quite clear there was no reason why she shouldn’t.  Besides she could even get in a TV Special about prospecting in the far north.  Well, you know how a woman is when they get their head made up?

Kenny, Hans and I had a powwow about this development where at first the other two didn’t want to hear anything about Nancy’s plans.  She only howled louder, I never realized how much noise such a little snippet of a girl could make when she didn’t get her way.  I soon found out, so we decided Nancy could go with us as the camp cook, besides that we were sick of our own cooking.  It left too much to be desired.

With that we started the long trek by car to Winnipeg where we left our planes during the off season.  When we weren’t using them we leased them to the charter service in Winnipeg so they kept earning us money when we weren’t using them.  We had a pair of DeHaviland Beavers, a big single engine plane with a pair of pontoons bigger then canoes.  They also came with Pratt & Whitney radial engines that if they weren’t dripping oil we wouldn’t use them.

While we were getting the planes ready and buying what prospectors supplies we needed, Nancy went shopping for food.  She bought the usual, but even some things to eat we would never even thought of.  One of the items she bought was a big bag of marshmallows; can you just imagine taking a bag of marshmallows on a prospecting expedition?  I thought I’d never live that one down!

We took off on the long trip north to Victoria making our first landing on the lake at Thicket Portage.  It felt pretty good to have Nancy along she flew with me; Kenny and Hans were in the other plane.  After getting gas we continued to fly north to Churchill were we laid over for the night.  Rather then spending money it was our habit to sleep right in the plane.  The next morning we took off and continued north to our next gas stop at Baker Lake.  From Baker Lake we continued on north to our final destination at Cambridge Bay where we finally set down on the bay.

Cambridge Bay is on Victoria Island at 69o north.  The Arctic was abloom with flowers, and although the black fly season was over there were swarms of mosquitoes that took their place.  This surprised Nancy because she hadn’t counted on mosquitoes, and can they make your life miserable.

This story is going to take several episodes so be sure you see the next one: Nancy Twinkie meets the Inuit

Tuesday, February 14, 2012

Recovering Gold with Cyanide

A heap leach pile for extracting gold with cyanide in Elko, Nevada
Photo by USGS

How do you like that one of my relatives invented the MacArthur-Forrest process for recovering gold in the process that is more commonly known as the cyanide process.  This is one of the many techniques used to recover gold from low grade ore that works by dissolving gold and other metals is a solution of water and cyanide that causes gold to become water soluble.  Chemically this is called a water soluble coordination complex that is the most commonly used technique to recover gold.

Fifteen percent of the cyanide produced is used for gold recovery the balance is used in other industrial processes for such things as adhesives, plastics and insecticides.  Although both sodium and potassium cyanide are used the most commonly used is calcium cyanide.  Because of its extreme toxicity cyanide has been outlawed in many countries, but efforts are being made to replace its use in mining by less toxic chemicals.  One of these is sodium ferricynide that has been used under the name “hypo” in photography to dissolve silver in photographic materials.  Hypo also dissolves gold, but not as rapidly as cyanide.

Gold mining uses cyanide for both the MacArthur-Forrest Process for which it was originally developed and for the later heap leaching process that was developed for recovering gold from disseminated ore like that found in the Carlin Trend in Nevada.  In the MacArthur process the gold ore is first ground into a powder in a ball mill, a large tumbling barrel where the grinding action is provided by large steel balls.  The ore is then transferred to a vat where it is mixed with cyanide solution and agitated with compressed air.

The other place where cyanide is used is the Heap Leach Process here the ore is ground to ¼ minus and heaped up in huge piles usually covering several acres.  A solution of cyanide is sprinkled onto the pile with sprinklers where it seeps down through the heap dissolving the gold in the process.  This process goes on for a considerable time with the cyanide solution being recirculated several times until all the gold is dissolved.

In both processes the gold is precipitated from the pregnant solution with powdered zinc although in some cases it is recovered using activated charcoal filters.  The powder is black because it contains more then gold that is cast into a Dore bar and sent on for further refining to remove the impurities.

Thursday, February 9, 2012

The Miller Process for Refining Gold

Gold Bars   Agnico-Eagle

Ever wonder how most gold is refined?  No for the most part they don’t use the Wolhwill Process rather they use the Miller Process.  Unlike the Wolhwill Process described earlier in this series the Miller Process doesn’t use electricity but rather uses a stream of chlorine gas that is passed through a crucible of molten gold that produces gold that is 99.95 Fine.  It works to remove all the impurities because gold is least affected by chlorine so all the other impurities are removed as chloride slag floating on the surface of the molten gold where it is skimmed off.

In this process the dore bar is melted in an induction furnace where the chlorine gas is introduced into the molten metal by a rotating lance.  The lance is controlled in a very precise manner in a way there is no break through of the chlorine gas to the surface of the molten metal.  This removes what are termed “By-metals” like silver, copper, nickel along with any other metals.  The process also removes amphoteric elements like sulfur, selenium, tellurium, arsenic and bismuth.  These elements are all removed from the molten metal as chloride slag that floats to the surface of the charge where they are skimmed off for further treatment.

The process takes from 2 to 4 hours to complete with the chloride fumes being removed and filtered by a two stage wet filter or a two stage dry scrubbing system that maximizes the recovery of the chloride as well as the cleaning of the exhaust air from the process.  In the process slag is poured off the top of the melt into a crucible and allowed to slowly cool so any gold that has been caught up with the chlorides will settle to the bottom where it can be recovered as a button of gold when the crucible has cooled completely; it is then added to the remaining molten gold.  In this process the particles of gold remaining in the crucible are carefully removed by hand as well as the separation molds when they have cooled.

Once the gold has cooled it is poured through a ceramic filter directly into the granulation equipment or anode molds if the Wolhwill process is used to further refine the gold.

The main advantages of this process are its quick turnaround time, size of the charge that can be from 20 kg to as large as you like.  The rotating lances are made from temperature resisting ceramics for long life and evenly introduce chlorine to the melt.  The use of modern induction furnaces keep the consumption low using far less kilowatts then coil type furnaces for the same size of melted product.

In this process the gold produced is 99.95 Fine that is good enough for most commercial purposes.  Most of the metals remaining in the gold are mostly silver and copper that if desired can be removed using the Wolhwill Process producing gold bullion that is 99.999 Fine.   

The Foundations of a Good Gold Mining Business

The type of mine where good business practices are used.
Photo by Greverod

For small operations, mining for this precious metal takes a lot of personal sacrifice and faith. Recently,
reality television shows like Discovery Channel’s “Gold Rush” and “Bering Sea Gold” have brought the
business of gold mining to the general public’s attention. Television shows like these do try to show all
sides of the business, but as with all television, the glamorous side seems to come out more than the
gritty side.

At the heart of every good gold mining operation are a few basic yet necessary requirements. These
requirements provide the foundation for a legal and well-planned gold mining business. Before you go
prospecting for gold, you must be sure to take care of the following…

Locate a mining claim: In accordance with the Mining Law of 1872, a mining claim can only
be staked on federally-administered lands that have not been previously claimed or closed to
mineral entry under some special act, regulation or public land order. There are 19 states where
mining claims can be located.

Stake your claim: Federal law requires that claim boundaries be clearly marked. Most states also
have additional staking requirements.

Record the mining claim: In accordance with the Federal Land Policy and Management Act of
1976, all claims and sites must be recorded with both the state and the proper Bureau of Land
Management (BLM) office.

Secure an investor or investors: The total amount of money needed to run a successful mining
operation will depend on the size and location of your claim, the number of employees hired
and the method of mining used. Typically, a company will bring in managers with prospecting
experience to assess the claim and decide on an operating budget. This is the amount proposed
to investors. Investors will require a return on their initial investment, and smart prospectors
will hire an attorney to ensure the mining company receives a fair deal.
Purchase the right equipment: Investing in the right prospecting tools is essential for success.
When you use the right equipment, you find more gold and make more money.

Hire experience: For the most part, you want your employees to have previous experience in the
specific job they will be performing. This reduces the time and money used for training.
Meet all federal and state mining safety and labor law requirements: There are laws and policies
that must be followed in order to continue operation. Each state has its own safety training
policy, so check them out before opening the operation.

Maintain workers’ compensation insurance: Even with proper training, accidents can happen.
Protect your company by maintaining the right workers’ compensation insurance. In some
states, it is actually a law to maintain workers’ compensation insurance, so check your state’s
requirements before hiring.

Maintain the mining claim: Under the federal mining law, all claims must follow the annual
mining claim maintenance requirements and pay an annual maintenance fee.

In addition to these practices, all gold mining operations should hire an accountant and office manager
to maintain all financial, employee and other records. A good operation keeps great records and a close
eye on profits and losses.

For more information on federal mining laws, visit the U.S. Department of the Interior Bureau of Land
Management website and search “gold mining.”

About the Author: Allison Lane owns the website  and enjoys writing guest blog posts on various topics of interest.

Monday, February 6, 2012

Refining Gold using Electrochemical Processes

An electrolytic cell used in the commercial refining of gold using the electro-chemical process.

This is a continuation of the previous article about the recovery of gold from concentrated stream heavies.  In this article we will examine the WolhwillProcess that was originally invented in 1874 and is still in use today. This process is used to produce gold that is 99.999% pure.  As an industrial-scale technique the Wohlwill process that was invented by Emil Wohlwill for refining gold using methods developed for electrochemistry.

The process makes use of a bar cast from impure gold that has been separated from concentrate like that in the first part of the process we explained in an earlier article.  This is done by pouring the gold and other metals into a crucible, melting then and pouring them into a mold producing what is called a “dore” bar composed of gold, silver and PGM.  If the amount of gold present in the dore bar is less then 95% it will be necessary to add enough pure gold to bring its content up to that level otherwise the other metals will be carried over in the finished product making the gold less then 99.999 fine.  The Dore bar acts as an anode in the process.

The cathode in this reaction is made from thin sheets of 24k gold that are spaced on each side of the Dore bar that acts as the anode.  Providing the anode is composed of more then 95% gold the remaining metals in the cathode will be deposited in the bottom of the electrolytical cell as anode sludge that can be recovered and the values of silver and PGM recovered in a later process.  

The gold along with the other metals are dissolved at the anode when current is delivered to the cell with pure gold coming through the chloauric from ion transfer where it is plated as pure gold at the cathode.  After the anode is completely dissolved the cathode is removed and melted down or processed for making the end product that is usually gold bullion pure gold like this is too soft for most other uses.

At 99.999 fine this gold is too soft for most purposes other then bullion so it must be alloyed with other metals most commonly silver although in some cases copper is also used.  For making jewelry the most common karats used are 14 and 18 k.  This is done by remelting the bullion and forming it into the shapes desired.

There are plenty of gold refiners that will refine the concentrate into pure gold, but they often keep the platinum values as part of the refining fee. 

Wednesday, February 1, 2012

Cleaning up Concentrates containing Gold and Platinum

Hematite particles forming ferro-fluid with a rare earth magnet beneath a sheet of glass.

One of the problems in the recovery of gold,silver and platinum from concentrates of stream heavies is the removal of iron oxides that make up the majority of heavies. Most of the other heavies are composed of silicates and in rare cases oxides of various elements. In the case of iron oxide there are two types present in most heavies. The most easily removed is magnetite because it is attracted to an ordinary magnet with the best being an Alnico magnet. This will quickly remove the magnetite that is the easiest to remove. Hematite is the second iron oxide in heavies that is not attracted to an ordinary magnet, but is attracted to a rare earth magnet that can be ordered on the internet. The remaining heavies composed of mineral silicates and oxides being much less dense and having a far lower specific gravity then gold, silver or platinum that is left behind in the pan.

The iron oxides can be removed in two stages; the first is removing magnetite. There are commercial magnetic separators available, but for the small amount usually encountered an ordinary magnet will suffice. A small amount of concentrate that has been thoroughly dried is placed on a sheet of paper and the magnet is passed over the concentrate just above its surface, but high enough not to touch the concentrate. The magnetite present will stick to the magnet so you will have to periodically remove it from the magnet.

The hematite is removed in the second step by using a small rare earth magnet by repeating step 1 using the more powerful magnet the remaining concentrate containing heavies, gold, silver and platinum group metals. This concentrate is poured into a separate container for further work in the process.

Although geologists know ways of separating heavies using dense fluids, but they are all highly toxic and shouldn’t be used by an untrained layman. The best way to remove them is with a small ten inch gold pan immersed in water using conventional panning methods. Its tedious work, but the pan full of concentrate can be worked down until the only thing left in the pan is gold, silver and platinum group metals (PGM).

The remainder in the pan is gold, silver and PGMs that are saved for further processing; because of the length of the whole process we are going to divide it into several articles we will publish in the near future. Watch for them!