Background

With the advanced technology of the 21st century and the

know-how accumulated over the last few centuries, the

extraction of gold and other precious metals from ores with

low concentrations has become economically viable. The gold

recovery process is simple, well known, and technologically

proven. The company exploring for precious metals needs to

decide, based on the amount of gold in the ore, which method

to use for recovery of the gold. The 1970s-1980s saw a rapid

expansion in gold production from these low-grade deposits

around the world, and Durco Filters was there to assist with

process and filtration solutions of gold recovery for these mines.

Ore extraction is straightforward and requires just brute force.

Gold extraction from the ore requires specific chemicals to dissolve the gold. The gold recovery process however is a

different animal altogether. The choice of gold recovery can be

broken down into two basic processes:

1. Heap Leaching (one of many extraction methods) followed

by Merrill-Crowe (recovery)

2. Leaching and Carbon Adsorption (extraction) followed by

Merrill-Crowe or Electrowinning (recovery)

Both processes begin with leaching of some variety. Leaching

is a process whereby the gold is dissolved (extracted) from the

ore using a solution of Sodium Cyanide (NaCN). Durco Filters

by Ascension Industries is a key component used in both

processes of precious metal extraction and recovery.

Durco Filters in Gold Extraction & Recovery

Heap Leaching for Gold Extraction

As illustrated in the first four steps of Figure 2 below, the

process of extracting the gold from the ore is fairly simple. In the

leaching process, mined ore is crushed into small chunks and

heaped on an impermeable plastic or clay lined leaching pad.

This “heap” is an engineered structure designed to maximize

the precious metal extraction. Sodium cyanide solution (NaCN)

is then dripped or sprayed through nozzles over this heap. The

cyanide solution then percolates through the heap, dissolving

the precious metal and leaching it out of the heap (cyanidation).

With the heap leaching technology, gold can be extracted from

ores with as little as 0.01 oz. gold (Au) per ton.

Once heap leaching is done and the ore heap has been fully

leached and exhausted of precious metals, the pregnant leach

solution is further processed to filter out the fine ore solids and recover the gold.

Figure 1: Durco Filters Pressure Leaf Filter

Figure 2: Typical Leaching and Merrill-Crowe Process

Mining

FiltrationTechnical Brief

Calcening / Smelting + Further Processing

Vacuum Deaeration / Zinc Powder Addition + Mixing

Leach Clarification

Precious Metal Precipitate Recovery

Non-Pregnant Liquor Filtration

Ore Heap / Crushing / Milling / Leaching /

Decanting / Washing

Barren Solution

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Phone: +1.716.693.9381

www.durcofilters.comPhone: +1.716.693.9381

www.durcofilters.com

Durco Filters, and Proven, Trusted Solutions are trademarks of Ascension Industries, Inc.

Pressure Leaf Filters for Pregnant Leach

Clarification

Durco Filters Pressure Leaf Filters (PLFs) are used to clarify the

pregnant leach solution containing the dissolved gold solids.

The Durco Filters PLF (see Figure 1) targets the removal

of all of the fine particles of debris and sediment from the pregnant solution. The PLF is a filter that uses a substrate of metal, typically a fine mesh, upon which a layer of filter-aide is deposited during a precoat cycle. The pregnant solution is

then filtered through this filter-aide to produce a high degree of filtrate clarity.

The clear pregnant solution of gold and cyanide is now ready for

the recovery stages of the process, and the captured leached

ore solids that are removed by the PLF are sent to waste.

The specific design criteria for these types of filters vary with

the turbidity of the leach solutions. To avoid major upsets in

the plant, a minimum of two filters should be kept online while a third PLF is being cleaned, pre-coated, and brought back online in preparation for the next filter to go offline. The rotation of the Durco Filters Pressure Leaf Filters through the cleaning cycle allows the plant to be a continuously operating facility.

Filter Press for Gold Solids Recovery

Once the pregnant cyanide leach solution is clarified by the PLFs, the process continues with the addition of zinc (Zn) powder to precipitate the gold from the solution. These zinc and

gold solids are then removed from the leach solution through

the use of a Durco Filters designed Filter Press (see Figure 3).

The spent leach solution is collected and sent back for reuse

in the leaching phase, or to cyanide destruction and then to

discharge or waste.

The “cake” that is retained in the filter press contains zinc/gold

solids, which are then further processed by smelting to produce

gold “dore bars” containing up to 90% gold. Dore bars are then

typically sent to an external processor to be refined to bars of 999.9 parts per thousand pure gold.

Figure 3: Durco Filter Press

Carbon in Pulp & Carbon in Leach Extraction

CIL and CIP are variations of the conventional heap leaching

process. Ore is crushed, finely ground, and cyanide leached to dissolve the gold. Instead of separating ore solids from

the pregnant solution, as described in Pressure Leaf Filter for Pregnant Leach Clarification above, granular activated carbon is added to the leach slurry (pulp) to adsorb the gold.

The carbon adsorbs the gold from the slurry solution and is

removed from the slurry by coarse screening and elution. The

gold-free leach solution is then recycled, while the carbon is

processed to remove the concentrated gold. It is sent either

to electrowinning or to the Merrill-Crowe process for recovery

of the gold and other precious metals. Durco Filters designed

Leaching and Carbon Adsorption

During the leaching process, all of the gold within the ore

solids is leached with a cyanide solution. This is typically done

in a heap leach process. The pregnant leach solution along

with the fine ore solids is then concentrated in the carbon adsorption process. The adsorption of gold from the cyanide

solution makes use of gold’s natural affinity to adsorb onto carbon (usually coconut grade carbon is used in this process).

In practice, this is accomplished by a series of five or six agitated tanks where carbon and ore slurry are contacted in

a staged countercurrent manner as seen in Figure 4 below. The gold concentration process adsorption occurs after the

leaching cascade section of the plant or simultaneously during

the process leaching and adsorption steps. In all processes,

the activated carbon is moved from one tank to another and

the gold, in its cyanide complex form, becomes attached (or

adsorbed) to the carbon particles.

Typically, a hot caustic solution or alcohol at 80-140°C is used to elute the concentrated gold from the carbon. Sometimes a

stripping solution contains ~25% of ethylene/propylene glycol

and ~2-3% caustic. The gold is eluted from the carbon when

the solution is heated (just under 200ºF) and sent to the carbon stripping system at a flow rate of twice the volume of the tank bed per hour. The resultant liquid stream from the elution

step contains concentrated gold and other precious metals,

which are then recovered from the cyanide complex solution

by either the electrowinning or Merrill-Crowe process. In the

carbon stripping (elution) process, the concentrated carbon/

gold complex is typically continued for 24 hours and results in stripped carbon having extremely low gold values. The stripped

carbon is then regenerated in an oxygen deficient heated kiln before being recycled back to the gold adsorption process.

There are essentially three process variations where carbon is

used with the Sodium Cyanide leach solution to extract and

recover the gold: Carbon in Pulp (CIP), Carbon in Leach (CIL)

and Carbon in Column (CIC).

Proven Liquid-Solid Separation Equipment

Phone: +1.716.693.9381

www.durcofilters.comPhone: +1.716.693.9381

www.durcofilters.com

Durco Filters, and Proven, Trusted Solutions are trademarks of Ascension Industries, Inc.

The use of a Durco Filters designed filter press will significantly aid in the efficient recovery of the gold solids. Specifically chosen filter cloths and decades of engineering and designing systems for this process have given Durco Filters the experience to

provide this type of process with years of superior performance.

Figure 4: Carbon in Column/Leach/Pulp

filter presses are used to remove carbon fines generated during the many adsorption process steps, thus keeping all of the

internal screens and filters free from premature plugging with carbon fines.

Carbon in Column Extraction

CIC starts with the standard heap leaching extraction process

as shown in Figure 4. The pregnant leach solution is then pumped upwards through a series of fluidized bed towers (columns) containing activated carbon.

As with CIL & CIP, the carbon adsorbs and concentrates the

gold from the slurry solution and is then removed from the slurry

by elution with both NaOH and NaCN solutions. The gold-

free leach solution and the carbon are then recycled, and the

concentrated gold solution is sent either to electrowinning or

to the Merrill-Crowe process for recovery of the gold and other

precious metals. Durco Filters designed filter presses are used to remove carbon fines generated during the many absorption process steps, thus keeping all of the internal screens and

filters free from premature plugging with these carbon fines.

Filter Press for Carbon Regeneration

In each of the CIP, CIL, and CIC processes, the gold that has

been adsorbed and concentrated on the activated carbon and

then is recovered from the carbon by elution typically with a

hot aqueous caustic/cyanide solution. Once stripped of gold

by elution, the carbon solids are regenerated for reuse. As

part of this regeneration process, Durco Filters designed filter presses are used to remove carbon fines generated during the many adsorption process steps, thus keeping all of the internal

screens and filters free from premature plugging with carbon fines. They are also used to concentrate the carbon solids and remove additional spent cyanide solution. The cyanide solution

is sent back for more leaching of gold from the heap-leached ore

and the carbon is sent back, after a high heat regeneration step,

to the head of the carbon cycle. The concentrated gold that has

been eluded from the carbon into the caustic/cyanide solution

is recovered from this solution using either electrowinning or

zinc precipitation (Merrill-Crowe).

The use of a Durco Filters designed filter press will significantly aid in the efficient recovery and dewatering of the carbon before the regeneration step. Specifically chosen filter cloths and decades of engineering and designing systems for this

process have given Durco Filters the experience to provide

this process with years of superior performance.

Trusted Solutions for Every Filter Application

Barren Solution

Ore Heap

Carbon Strip with Chemical

Carbon Fines Filtration Process

Multiple Carbon Steps with Column or Leach or Pulp

Precipitate Recovery Process

Electrowinning Process

Merrill-Crowe Process

Leachate

Enriched Carbon and Precious Metal

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Phone: +1.716.693.9381

www.durcofilters.comPhone: +1.716.693.9381

www.durcofilters.com

Durco Filters, and Proven, Trusted Solutions are trademarks of Ascension Industries, Inc.

Publication: A.22.B.1

01/2013

Typical Process Conditions for Durco Filters

Pressure Leaf Filter

Flow Rates Achievable: Up to 12,000 US gallons per minute with

standard models; customized filters can be designed for larger flows.

Equipment: Durco Filters Model HC, precoat, sluice discharge

(wet cake) pressure leaf filter, complete automation, frontal piping and other customized options available.

Filter Press

Sizing: Up to 200 cubic feet cake capacity. Customized filters for greater volume of cake capacity can be designed. Flow rates achievable: Up to 12,000 US gallons per minute.

Equipment: Durco Filters Model EP1200, filter press, oversized

stainless steel inlets and outlets, complete automation, sludge/

cake handling systems and customized options.

Process Conditions

1254 Erie AvenueNorth Tonawanda, NY 14120

Phone: +1.716.693.9381

Fax: +1.716.564.9044info@durcofilters.comwww.durcofilters.com

Continuing the tradition of over 130 years of Durco Filters™ quality and

innovation, as well as custom turnkey systems, Ascension Industries is your

provider of Proven, Trusted Solutions.

ASME Section VIII, China Code (SQL-AQSIQ), European– PED (97/23/EC) | ISO9001:2008

Proven, Trusted Solutions™

Durco Filters, and Proven, Trusted Solutions are trademarks of Ascension Industries, Inc.