sluice design - wyatt yeager msc.pdf

SLUICES

THEIR DESIGN, APPLICATION

AND OPERATION

SAVANA MINING EQUIPMENT LLC CALIFORNIA, USA

Placer System Design & Fabrication, Geological, Mining and Lands Consultants

Phone: +353 83 452 5859 Europe

+1 925 822 8852 USA

www.savanamining.com

Email: [email protected]

PREPARED BY:

Wyatt Yeager MSc – Savana Mining Equipment

http://www.savanamining.com/

mailto:[email protected]

OVERVIEW

The dramatic increase in gold price during the years 2008 to 2010 created renewed interest

in the development of alluvial gold mines and in particular has turned some previously

uneconomic properties into viable mining ventures.

The resulting interest has seen existing manufacturers of alluvial recovery equipment

increase production and has introduced many new constructors into the market place.

While conventional plant; screens, trommels, pumps, jigs and sluices remain popular there

are many new innovations, while as yet untried in the long term, also available.

The world economic downturn had resulted in equipment costs rising and many mine

owners are thus forced, because of budgetary constraints, to seek less expensive recovery

circuit equipment. The sluice remains one of the least expensive and “reasonably” efficient

means of recovering heavy minerals. However, the capital savings do come with certain

drawback’s as sluices certainly do not offer the consistently high recoveries of machines such

as jigs and centrifugal concentrators.

This report is intended as a broad background to sluicing, a guide to the types of sluices

available, their design, characteristics and application. It is by no means intended to be a

complete guide to sluicing and some comments and recommendations here may not be

relevant to certain mining properties and their development.

Sluices are one of the earliest means of recovering minerals. Agricola in his De Re Metallica

(1556) makes mention and describes in detail the use of strakes and sluices. The design of

the sluice has altered little from those times. Modern engineering practices have resulted in

more efficient riffle designs, the use of expanded mesh and 3M Nomad matting are two

modern variations of the older wooden riffle and coconut fiber matting used up to as

recently as the 1950’s.

Sluices are at their most efficient when treating “Long Range” type feeds and do not respond

well to “Short Range”, closely classified feeds. Having said that the combination of large

boulders with fine sands is to be avoided. Similarly sluices are well suited to free running

gravelly ground but not so well suited to fine sandy or clay rich feed.

While many operators claim high recoveries, +90%, when using sluices, this is not normally

the case. Such high claims are often made as a result of the operator not actually knowing

his average head grade or of his failure to account for losses of fine mineral across the sluice

to tailings. Many operators never even know they have fine mineral in their feed stock.

REPRODUCED FROM DE RE METALLICA, 1556

This report attempts to provide the reader with basic background information on the sluice

as a gravity recovery mechanism. Much of the data contained in this report has been derived

from old texts (See Bibliography), from the author’s personal observations and from

anecdotal information provided by miners and prospectors. For those readers who require

further information they are referred to the texts detailed later in this report and in

particular to the very thorough study by Clarkson, 1990.

TABLE OF CONTENTS

PAGE NO.

1.0 INTRODUCTION 1 – 2

2.0 COMPONENTS OF THE SLUICE 3 – 8

3.0 TYPES OF SLUICE 9 – 12

4.0 DESIGN CRITERIA 13 – 22

4.1 PRODUCTION RATE 14 - 15

4.2 SCREENING 15

4.3 FEED RATE / SLUICE CAPACITY AND DESIGN 15 – 22

4.4 WATER CONSUMPTION 22

5.0 GENERAL CONSIDERATIONS 23 – 26

5.1 SCREENING 23

5.2 MISCONCEPTIONS 24 – 26

6.0 SLUICE LOSSES 27 - 31

7.0 RECOMMENDATIONS 32 - 34

8.0 BIBLIOGRAPHY 35 - 36

LIST OF FIGURES

PAGE NO.

FIGURE 1 COMPONENTS OF A HUNGARIAN RIFFLED SLUICE 3

FIGURE 2 EXPANDED MESH RIFFLES / MAT COMBINATION 8

FIGURE 3 LONG TOM AND RIFFLED SLUICE 10

FIGURE 4 KEENE TYPE UNDERCURRENT SLUICE 10

FIGURE 5 OSCILLATING TYPE SLUICE 11

FIGURE 6 INFORMATION QUESTIONNAIRE 13

FIGURE 7 GOLD SIZE ANALYSIS QUESTIONNAIRE 14

FIGURE 8 CROSS SECTION OF RIFFLE RECOVERY MECHANISM

AFTER CLARKSON, 1990 18

FIGURE 9 TYPES OF HUNGARIAN RIFFLES 18

FIGURE 10 BENT FLAT BAR RIFFLES 19

FIGURE 11 INCORRECT RIFFLE PLACEMENT (AFTER CLARKSON, 1990) 20

FIGURE 12 SELECTIVE DATA REPRODUCED FROM CLARKSON (1990) 26

LIST OF PHOTOS

PAGE NO

PHOTO 1 PERUVIAN GROUND SLUICE, BOULDER RIFFLES 5

PHOTO 2 ARTISANAL SLUICE, MEXICO 5

PHOTO 3 ARTISANAL SLUICE, SULAWESI, INDONESIA 6

PHOTO 4 ARTISANAL SLUICE, CAMEROON WEST AFRICA 6

PHOTO 5 HUNGARIAN RIFFLED NUGGET TRAP SLUICE 7

PHOTO 6 EXPANDED MESH SLUICE RIFFLES 7

PHOTO 7 ARTISANAL SLUICE, SULAWESI, INDONESIA 27

PHOTO 8 INDONESIA, MINUTES AFTER PHOTO 7 28

PHOTO 9 CAMEROON, WEST AFRICA, NOTE EXTREME SLUICE ANGLE 29

PHOTO 10 CAMEROON, WEST AFRICA, NOTE LAMINAR FLOW ACROSS RIFFLES

INDICATIVE OF POTENTIAL GOLD LOSSES 30

PHOTO 11 CAMEROON, REPLACEMENT SLUICE SYSTEM, EXCESSIVE WATER

FLOW 30

1.0 INTRODUCTION:

Sluices are effectively a form of “stirred bed”. In other words the feed, normally “Long

Range”, is stirred or agitated at a rate insufficient to keep all the mineral grains and rock

particles in suspension. This action, independent of the container in which the feed has been

placed, results in reverse classification and gravitational stratification, the result being that

the heaviest particles settle the most rapidly and form the base or basal layer in the

container.

In the case of a sluice it is the tumbling action imparted by the riffles coupled with the flow

of the water across the riffles that effects separation and thus settling.

Sluices have been used in various forms throughout the world for the recovery of gold, tin

and tantalite and can be used for any mineral having a “Concentration Criteria” of 3.5 or

greater. Other applications are washing pyrite or slate from coal, lead shot recovery from

rifle and gun ranges, etc.

So what is a sluice? Basically it is an inclined trough or launder, usually on a slightly angled

slope, into which the feed ore is placed and washed down the trough by a rapidly running

stream of water. The sluice may have no riffles (tin streaming box) or have riffles of which

there is a wide variety that have been and are being used.

Recoveries vary widely and depend to a large extent not only on the feed type, but the

ability of the operator to monitor and adjust feed rate, water flow rate, sluice angle and

sluice riffle load. No feed is consistently the same, particularly alluvial or placer feeds, and

sudden changes in feed size, feed rate, water flow and other such variables are the main

cause of high losses over the sluice.

Sluices were widely used in gold and tin mining by artisanal miners, in small scale

mechanized operations and, up until the 1940’s, on large dredges. During that decade many

of the larger dredge and even land based mining operations discarded the sluice in favor of

modern pulsating jigs. The jig had much to offer, a smaller engineering footprint, more

automated operation and much higher and more consistent recoveries.

Notwithstanding these changes, the sluice is still popular and forms the basic tool of many of

the world’s artisanal miners. The Incas used natural rock sluices, see Photo 1, and wooden

launders or sluices are seen in artisanal mining operations on every continent. Photo 2.

Recoveries are always problematic. Unless the operator has some quantitative estimation of

his head feed grade then it is impossible to determine sluice efficiency. Many claim

recoveries of +90% and Clarkson, 1990, has documented very high recovery rates in studies

he conducted in the Klondike and Yukon.

Clarkson does, however, also report losses of between 0 and 71% in most operations, two

operations he studied losing more gold than they recovered. Other mines with sophisticated

screening systems prior to the sluice recorded recoveries of as high as 99%.

Clarkson concluded that all the mines without screening equipment would pay back all their

capital outlaid on screens in one season of operation.

So do sluices have any application in modern mining?

If operated carefully with adequate pre-screening and feed conditioning then they are

capable of high recovery rates and the recovery to cost ratio in such cases is certainly better

than other gravity machines. However sluices do not offer satisfactory recoveries of very fine

gold particularly fine flattened particles.

No writer appears comfortable in giving a lowest size limit for recoverability, some believe

that sluices can recover down to 200 mesh, this is unlikely! A conservative figure is more

likely to be 50 mesh (300 µm). Clarkson’s data indicates that in the Yukon deposits there is

little gold below 100 mesh and thus in that region the sluice is a popular recovery tool.

The work by Clarkson, while restricted in its location is a valuable guide as to sluice efficiency

and clearly demonstrates that for sluicing to be successful pre-sluice screening is required.

2.0 COMPONENTS OF THE SLUICE:

A sluice is effectively an open launder (or channel) into which are fitted riffles of one form or

another. It can be a constructed box of metal, wood or plastic or a ditch excavated into

bedrock (Inca Sluice). Raw feed, usually screen underflow product, is introduced into the

head of the sluice and a flow of water applied so as to move the feed down the length of the

launder across the riffles. The tumbling action imparted onto the feed by the water and the

tumbling action of the coarse particles over the riffles results in separation of the mineral

constituents in the feed, the heavy particles moving downward and becoming trapped on

the floor of the sluice and behind the riffles.

The various components of the sluice as set out in Figure 1 and described in more detail

below.

FIGURE 1 – COMPONENTS OF A HUNGARIAN RIFFLED SLUICE

(I) THE LAUNDER:

3M NOMAD

MAT UNDER

RIFFLED

SECTION

HUNGARIAN

RIFFLES

CONSTRUCTED IN

A STEEL FRAME

PIVOT POINT

NORMALLY A

BOLT EITHER

SIDE OF THE

LAUNDER

FASTENING

CLIP EITHER

SIDE OF

LAUNDER

FEED

DISTRIBUTION

BOX

SLICK PLATE

600 MM TO

1,000 MM

LONG

May be made from any material, normally it is made of steel or aluminum and apart from

artisanal mining operations, rarely now of wood. The launder may be straight or tapered

(Pinched Sluice). Old wooden sluices were tapered so that each section of the sluice could

be slotted together. Some modern sluices made of steel are also tapered.

The “Long Tom” was tapered outwards in length; this effectively allowed the feed to spread

out across the launder. This slowed down the flow rate, thus assisting the heavy minerals to

settle more quickly.

(ii) SLICK PLATE:

In most modern sluice boxes there is a plain flat section at the head or feed end of the sluice

over which the feed passes prior to it reaching the riffled section of the launder. This blank

section is normally 2 to 4 feet, 600 mm to 1.2 m in length, is designed so as to allow the

heavy mineral in the feed stream to commence hindered settling prior to encountering the

first riffle.

(iii) THE RIFFLES:

There have been a wide variety of types of riffles used in sluices. The Incas cut ground

channels and used rock fill as the riffle medium. Photo 1 taken in Peru indicates that this

practice successfully continues even today.

Other artisanal miners use more conventional wooden sluice arrangements, Photos 2, 3 and

4, taken in Mexico, Indonesia and Cameroon, West Africa, respectively. Losses across these

sluices are normally quite high.

Riffles can thus be any material that imparts disturbance to the feed and promotes settling

out of heavy minerals.

Rocks, stones, and boulders, steel bars, wooden bars, wooden blocks, rails and poles (usually

set longitudinally), angle iron, flat bar and expanded mesh are and have been used. Modern

sluices usually use a combination of “Hungarian Riffles” and “Expanded Mesh” riffles placed

over synthetic matting such as 3M Nomad mat. See Figure 1.

PHOTO 1 – PERUVIAN GROUND SLUICE, BOULDER RIFFLES

PHOTO 2 – ARTISANAL SLUICE, MEXICO

PHOTO 3 – ARTISANAL SLUICE, SULAWESI, INDONESIA

PHOTO 4 – ARTISANAL SLUICE, CAMEROON WEST AFRICA

PHOTO 5 – HUNGARIAN RIFFLED NUGGET TRAP SLUICE

Hungarian Riffles are normally made of wood, steel or aluminum and are placed across the

launder with an overhang on the downstream side. See Figure 1 and Photo 5. Alternately

they can be made of angle iron, having the angle sloped slightly downstream.

Expanded Mesh riffled sluices are conventionally used as scavenger sluices, they have a layer

of expanded mesh (walkway mesh) over Nomad mat. Figure 2 and Photo 5. In this instance

the slope of the mesh faces downstream.

PHOTO 6 – EXPANDED MESH SLUICE RIFFLES

FIGURE 2 – EXPANDED MESH RIFFLES / MAT COMBINATION

3.0 TYPES OF SLUICE:

There are many variations on the basic sluice, and one has only to access the internet to see

the large number of groups now manufacturing commercial and hobby sluice boxes. Most

modern sluices are constructed of steel or lightweight aluminum bodies and steel riffles.

There are a number of manufacturers now constructing using plastics or Polyurethane with

preformed or molded riffles.

The list below is a few of the types of sluice in use, these being listed in no particular order of

efficiency:

Streaming Box – effectively a non riffled sluice generally used for cleaning up tin or

tantalum concentrates particularly when they contain accessory minerals such as

zircon, rutile, ilmenite and monazite.

The concentrate is placed at the head of the box into which is passed a regulated and

evenly distributed flow of clean water. The operator stands in the box and using a

flat mouthed shovel keeps turning the concentrate back on itself allowing the lighter

impurities to move down the box to tails. While this does result in some looses to

tails along with the impurities the resultant product remaining at or near the head of

the box can reach grades of over 74% Sn.

Long Tom – consists of a tapered non riffled launder into which the feed and water

are introduced. The box tapers outward at the discharge end and onto a punched

plate screen. Large rocks are removed from this section of launder by hand as is the

screen oversize that accumulates. Fines are passed though to a wide riffled sluice.

See Figure 3.

Undercurrent Sluice – comes in many forms and is basically used to recover very fine

gold that is conventionally lost to tailings. Figure 4 is of the Keene type Undercurrent

where the -1/2” material, the sands and fine gold are separated from the coarser

material using a ½” punched plate screen.

FIGURE 3 – LONG TOM AND RIFFLED SLUICE

Expanded Mesh Sluice – is depicted in Photo 6 and may be straight or tapered. They

are normally a combination of expanded mesh over matting and used for recovery of

fine gold lost across the primary sluice (Hungarian Riffled type).

FIGURE 4 – KEENE TYPE UNDERCURRENT SLUICE

Triple Run Sluices - rely on a punched plate distribution system to separate the fine

sands from the coarser gravels much like the simple Undercurrent Sluice. In this case

the fines are distributed to two side sluices while the coarse material continues down

the central run sluice. They are notoriously inefficient, (Clarkson, 1990) with much of

the fine gold being trapped in the high and turbulent water volumes required to

move the coarse material down the central sluice.

LONG TOMSCREEN

KEENE TYPE 3-STAGE SLUICE BOX

NUGGET TRAP RIFFLES

½’ PUNCHED PLATE SCREEN

+½” SCREEN PRODUCT

-½’ SCREEN UNDERFLOW PRODUCT

Oscillating Sluice Boxes – are being manufactured in a number of countries, some

impart a side to side motion, others a circular motion similar to panning. The

efficiency of these units compared to simple straight run stationary units is not

known. The diagram below illustrates the circular motion type sluice.

FIGURE 5 – OSCILLATING TYPE SLUICE

Hydraulic Riffled Sluices – these have been tried at a number of locations with

varying degrees of success. The normal installation sees every third riffle being

replaced by a square tube in which are drilled small holes, water is introduced into

the tube and thus out into the area behind the riffle. This appears to rely on the

settling velocity of the gold rather than creating disturbed vortices behind the riffles.

The riffles play an important role in sluicing, Peele notes three very important functions of

riffles, specifically he states:

“Riffles have three chief functions:

(a) To retard material moving over them and give it a chance to settle;

(b) To form pockets to retain gold which settles into them;

(c) To form eddies which roughly classify the material in the riffle spaces.

Their exact operation is not understood. Strength and shape of eddies (the “boil” of the

riffle) is affected by shape and spacing of riffles, their position with respect to direction of

flow, and the velocity of current. The boil must be strong enough to prevent the riffles

from filling with heavy sand (packing), and not too strong to prevent lodgment of gold.”

4.0 DESIGN CRITERIA:

Before considering the use of a sluice there are a number of parameters that must be taken

into account. The first and most important is “Know the Ground”, specifically:

What is the nature of the deposit, what are the average grade and feed size analysis and

composition of the ground? The data set out in Figure 6 is considered as minimum required

for adequate treatment plant design.

FIGURE 6 – INFORMATION QUESTIONNAIRE

In addition it is also important to know the size analysis of the gold or mineral to be

recovered. If for example the mineral is all in the 100 to 200 mesh size range then selection

of a system other than sluicing would be required. It is thus advisable to try to determine

grain size as follows:

FIGURE 7 – GOLD SIZE ANALYSIS QUESTIONNAIRE

Size Analysis May Be Applied To Any Mineral Species

4.1 PRODUCTION RATE:

This is normally based on the economics of the venture, that is, how much feed is required

to be processed to keep the operation profitable. Having settled on a figure then the design

process can be commenced. Taken into consideration must be:

Feed – what is to be presented to the sluice, is it to be a “long range” feed, that is a

feed with a wide size range or a “short range” feed, that is a more closely sized

material;

What is the nature of the feed material, is it sandy, cobble, clayey, etc.;

These factors determine the width and to some extent the length and the slope of the sluice.

4.2 SCREENING:

The screening of feed is of major importance. Traditionally sluices were fed “long range”

feeds and thus there was a requirement for large volumes of water to move the coarse

gravels down and out of the sluice. This normally resulted in much, if not all, of the fine gold

being carried away by the turbulent water flow and lost to tailings, in some cases as much

gold was lost as retained.

The use of modern earthmoving machinery and the availability of mechanical screens;

trommels, vibrating screens, etc., meant that the sluice could be fed a ‘short range’ feed

relatively easily. This resulted in improved recoveries and lowered water requirements.

It should be noted however that an extremely “Short Range” feed is not desirable as this can

lead to packing of the sluice by clays and fine sands, screening should be a happy

compromise between a Long and Short range feed.

Screening of feed is critical to good sluicing.

4.3 FEED RATE / SLUICE CAPACITY AND DESIGN:

Older texts relate feed rate to water carrying capacity, probably because most of the water

was not pumped but flumed to the boxes. Modern pumps remove those concerns and

getting water to the sluice is now considerably easier.

Calculation of Width:

A simple rule of thumb used by some manufacturers is that you require 1” of sluice

width per loose cubic yard of feed per hour.

Note this is a loose cubic yard (lcy) or material after excavation. In metric terms this

equates to 0.30 lcm / hour per centimeter of width.

Thus if you are intending to treat 10 lcy / hour across the sluice the width required

would be 10”. This may well be an oversimplification and I have seen it misused on

several occasions.

The problem in applying a rather rigid rule of thumb is that the ground being treated

is normally variable with size fractions changing both laterally and longitudinally

within the deposit. Thus the feed rates to the sluice must change to reflect these

variations.

It is often wise to add in a factor to the calculation, say 20%. Thus to achieve 10 y3 /

hour would require a sluice width of 10” +2” = 12”.

Clarkson quotes feed rates of 0.65 to 1.30 y3 /hour per inch of width and makes a

very valid point that Hungarian Riffled sluices would be the upper end and expanded

mesh the lower end of those feed rates.

Calculation of Length:

In most well operated sluice systems the bulk of the heavy mineral is retained in the

upper 30% of the riffled area, however, this is not always the case and while shorter

sluices are certainly less expensive to purchase or manufacture additional length is

desirable.

The author has observed losses of well rounded fines up to 1 oz nuggets across a 10

m sluice operating in the highlands of Papua New Guinea.

“Length does not affect capacity but does affect recovery rates”.

Many modern sluices are manufactured to material sheet sizes, thus 8’, 10’ and 12’

steel and aluminum sheet lengths usually control sluice length.

On a small 30 lcy / hour plant where say 50% of the feed passes to the sluice after

screening a length of 12’ would normally be recommended.

It is a simple matter to add sluice modules to increase length however this should

only be done where losses are high. These extra modules take additional clean-out

time and add greatly to plant operating workload and cost.

Riffle Action, Selection and Placement:

The Riffle Action or Mechanism:

It is important prior to selecting the riffle type to understand how the riffle works and

why it works. Clarkson, 1990 summarized very clearly the physical action within the

riffled bed and some of his comments are repeated here.

Good riffle designs should ensure that there is a loose, active bed of sand in the inter-

riffle spaces. This ensures that the bed is maintained in place while the only material

being lost is from the upper layers in the sluice. In this way heavy mineral is

continually being added to the inter-riffle active bed.

Many early writers were of the opinion that it was the settling velocity of the gold

that resulted in its capture by the riffles. Clarkson however likened the sluice to a

centrifugal concentrator with the settling velocity only having the effect of allowing

the heavy particle to migrate to the base of the slurry column. At that point the

particles enter a turbulent vortex created by and in between the riffles.

It is effectively this vortex, a low pressure system, which draws the slurry column

down into the riffle area. There, under ideal conditions, the slurry is overturned and

flows down the face of the forward riffle and in a circular motion to create and

sustain the vortex. The gold and other heavies are driven, by centrifugal force to the

outside of the vortex and it is at the base of the vortex that centrifugal and

gravitational forces drive the heavies out of the vortex and into the matting. See

Figure 8.

To achieve this riffles should be of a height that does not interfere with the coarse

upper layer to an extent that extreme turbulence results in the active inter-riffle bed

being disturbed and lost to tailings.

The most commonly used form of riffle is the so called “Hungarian Riffle”. It comes in

many forms but all have one common factor, a slight angling, against the flow in the

case of angle iron and with the flow in other cases. The riffles usually have an

overhanging cap or bend. See Figure 9.

FIGURE 8 – CROSS SECTION OF RIFFLE RECOVERY MECHANISM

AFTER CLARKSON, 1990

FIGURE 9 – TYPES OF HUNGARIAN RIFFLES

EYE OF

VORTEXAREA OF

PACKED

SOLIDSAREA OF

PACKED

SOLIDS

BASE OF SLUICE

LIVE C

RESCENT

LIVE

DEPOSITION

ZONE

MATTING

HEIGHT OF

CUT TO

VORTEX

STREAMLINES

FREE WATER SURFACE

LOW

PRESSURE

SEPARATION

ZONE

BACK OF

DOWNSTREAM

RIFFLE

SUPPORTING

THE VORTEX

FINER / LIGHTER

MATERIAL

COARSER /

DENSER

MATERIAL

EXPANDED

MESH RIFFLES

BENT FLAT BAR

RIFFLES

ANGLE IRON

RIFFLES WITH

MODIFIED AND

SHORTENED

CAP

FLAT BAR

WOODEN

BLOCK WITH

STEEL CAP

FLOW

Riffle Selection:

Conventionally angle iron or angled Hungarian Riffles are used when the bulk of the

heavy mineral is larger than 1 mm (14 mesh). Where angle iron riffles are to be used

they are normally made from 1” angle with the top leg modified or reduced in length

to ½”. These types of riffle are normally inclined upstream at approximately 15

degrees.

Where bent flat bar is used riffles are normally inclined downstream, riffle heights

should not exceed 1-1/2”. The riffles are inclined forward at 15O and have a top

section bent forward a further 35O. Figure 10.

FIGURE 10 – BENT FLAT BAR RIFFLES

Straight flat bar riffles are normally only used for heavy minerals of greater than 2.5

mm (8 mesh) as they create greater turbulence and tend to dislodge upward all but

the heaviest and coarsest material. They are ideally suited to the recovery of coarse

nugget gold, say +1/2”.

Expanded mesh riffles are normally used where particle size is -1 mm (14 mesh).

They tend because of their shape to have a small sorting area and are thus prone to

variations in feed or water volume.

105O

35O

0.7

5"

0.50

"

3.0"

Riffle Placement:

This is critically important as incorrect placement will result in the accumulation area

either becoming packed with solids in the case of a narrow riffle spacing or either

scoured or packed solid in the case of extreme spacing. See Figure 11 (After Clarkson

1990).

FIGURE 11 – INCORRECT RIFFLE PLACEMENT

(AFTER CLARKSON, 1990)

The ideal riffle placement where modified 1” angle iron is being used is 2” with a 15

degree angle upstream. Bent flat bar riffles are angled downstream at 15O.

Sluice Operating Angle:

There is widely divergent opinion on the ideal operating angle of sluice boxes. The

operating angle and water volume will determine how the gravel is transported

across the sluice, that is whether is slides, and rolls across the tops of the riffles in the

case of the coarser particles or reacts in a turbulent or leaping motion (saltation) in

the case of the finer material.

VORTEX

AREA OF

PACKED

SOLIDS

15.0°

AREA OF

PACKED

SOLIDS

1 INCHTOO NARROW

3 INCHESTOO WIDE

1" ANGLE IRON

MODIFIED TOP

LEG ½”

As a general rule the steeper the operating angle the more violent the motion of the

particles.

A common rule of thumb used in old texts is a drop of 6” to 6.5” per 12’ of sluice

length or an angle of 2.5O (Taggart). Other texts such as Peele tabulate the

specifications of various mines; Peele notes angles varying from 2% to 15% of slope

(1O to 8.5O). In Guinea slopes were quoted as 2.5O for loose sand, 5.75O for loose

sand and gravel and up to 8.5O for coarse gravel and boulders.

Clarkson (1990) quotes angles of 3” per foot (14O) when using angle iron riffles while

Savana prefer a 6O slope for the primary sluice and even steeper for nugget trap riffle

boxes.

As the application of sluice boxes is so variable, each mine will have its inherent

differences in alluvium type, size ranges, etc.

The simplest arrangement is to have the sluice set up so that the angle can be easily

altered, even while in operation. This arrangement would see variable slope

adjustment as follows:

Nugget Trap Sluice (0O to 15O);

Primary Sluice (5O to 12O); and

Expanded mesh Sluice (2O to 10O)

The Matting:

All modern sluices use some form of matting under the riffles. There are many

varieties available from simple coconut fiber, hessian sacking, astro turf, synthetic

carpet and 3M Nomad mat.

Each has its own characteristics, specifically:

Coconut / Astro Turf – have very limited storage capacity and are extremely

difficult to clean;

Monsanto Matting - has a hard backing that makes collection of gold difficult

and the long fiber strands project up into the vortex area and disrupt the

action and collection ability in the vortex; and

3M Nomad Mat – is the most widely used sluice matting. It does not interfere

with the vortex, most of its volume is available for capture of mineral

particles, and it has no stiff backing and is very easy to clean.

4.4 WATER CONSUMPTION:

In considering what water volume is required to operate the sluice successfully

consideration must be given to what max. and min. water volumes are locally available and

what is the quality of that water.

Clearly the volume of water to be introduced into the sluice depends to a large extent on the

type of ground being treated. Nominally Savana quote a rate of 10 gpm / cubic yard per

hour however that rate may be low for very rocky ground. Clarkson (1990) quotes volumes

of 320 gpm per foot of width or 26.7 gpm / inch of width.

Trying to cross reference these rather confusing figures indicates that in the Savana case the

water requirement would only be 120 gpm for a 12” sluice and in the Clarkson case 320

gpm. Who is correct, probably neither as actual volume still remains dictated by the nature

of the ground.

In considering the volumes to be used it is important to determine the water source within

the flowsheet. Where screening is being used the screen underflow will contain all the wash

water, for a 30 yph plant water volume is normally a maximum of 500 to 550 gpm. Clearly

that would be far too great for a 12” sluice so there must be an effort to either dewater the

feed or reduce the screen plant wash water.

Every sluice should be equipped with its own wash water source, usually a 2” water line with

valve for additional make-up water or for sluice clean-out.

5.0 GENERAL CONSIDATIONS:

Given that there are some excellent publications and reports available about sluicing this

section deals mainly with things the potential sluice operator should consider.

5.1 SCREENING:

It is quite clear from the material in the Clarkson (1990) report that pre-screening of the feed

to a sluice results in far higher recovery rates. The advantages of pre-screening feed are:

Much less water is required;

Barren gravels are eliminated from the sluice box;

The removal of larger barren rock reduces the riffle and box wear and thus reduces

maintenance and replacement costs;

The necessity for triple run and undercurrent sluices is removed and the difficulty

encountered in splitting feeds in these types of sluices totally eliminated; and

Last but not least the feed enters the box in a well conditioned state, that it is pre-

broken and free running and does not rely on the action of the box to separate the

mineral grains from the feed.

Numerous types of screen are available, trommels, vibrating screens, derockers, and

hydraulic finger grizzlies. Each has its own application to a particular operating scenario.

The trommel or scrubber trommel is probably the most efficient unit for cemented or high

clay gravels but it has high capital cost and usually requires the construction of high feed

ramps. The introduction of hopper – feeders ahead of the trommel assist in reducing feed

height.

Multi deck vibrating screens are also an efficient screening mechanism; they require less

feed height but are not suited to very clayey gravels or ground containing large boulders.

The derocker is more a moving grizzly feeder that has limited throughput and cannot size

feed to -2.5”.

Screening is thus strongly recommended.

5.2 MISCONCEPTIONS:

Testing of alluvial ground using sluice boxes is fraught with difficulty and many plant designs

have failed because of errors in interpretation of test data. Specifically:

Fine Gold – the presence or absence of fine gold in a sluice test must be treated

carefully. Even the worst sluice will recover fine gold and even the best operator will

lose fine gold;

Coarse Gold (Nuggets) - operators should be very aware of the “Nugget Effect”, one

nugget per cubic yard does not necessarily mean that this result will be repeated for

every yard treated. Expanded mesh test sluices will normally discard most nugget

material and retain the finer gold fractions while nugget trap sluices will efficiently

retain nuggets at the expense of fine gold;

Concentration – a high concentration of gold in the first few riffles of a sluice is not

an indicator of high efficiency, nuggets can be lost over even the most carefully

operated sluice and for testing it is strongly recommended that the primary sluice be

followed by a fines sluice and that tails be regularly sampled.;

Error – efficiency of a sluice should not be based on the amount of gold recovered.

Alluvial ground is highly variable both in grade and in mineral grain size. Many

samples and larger volume samples are usually required to satisfy sampling

reliability;

The Gold Pan – is far from a quantitative testing mechanism, “Nugget Effect” and

operator error make small volume dish samples at best qualitative;

The Bulk Test Plant – sample reliability can be calculated as a %, (See separate report)

and it is a good rule to apply some statistical analysis before embarking on a lengthy

and costly testing program. As a rule larger samples reduce sampling error

particularly where there is a large % of fine gold, or a very strong “Nugget Effect”. It

would be nice to see a Normal Statistical Distribution of gold sizing but that is unusual

to extremely rare.

In operating situations the claim that the sluice is catching 100% of the gold is a major

misconception. Many operators claim they have no fine gold because they just never see it,

probably because their test work was such that it was never recovered. Clarkson (1990) has

tabulated production and recovery rates from a series of mining operations, those data are

somewhat of an eye opener.

Losses are in some cases extreme, more gold being lost than recovered, losses of 0 to 71%,

etc. The following Table, Figure 12, is taken from his work and is a snapshot of some of

those data.

It is clear from those data that prescreening of the feed prior to sluicing dramatically

improved recovery rates; further the addition of screening devices to mines currently

without that process improves recovery and reduces monetary losses. Certainly you cannot

capture all the gold or heavy mineral but the operator has choices which enable him to

minimize losses and quite significantly improve his monetary returns.

Sluices remain one of the least expensive gravity recovery processes available and while

relatively easy and inexpensive to operate still remain one of the most misunderstood

machines and processes available to the mine operator. It should be remembered however

that they are not suited to the recovery of extremely fine gold, that is, gold below 100 mesh

Tyler. Many deposits that are now being developed that contain high percentages of very

fine gold; the day of the sluice may well be numbered. The deposits with coarse nugget gold

are rapidly disappearing.

FIGURE 12 – SELECTIVE DATA REPRODUCED FROM CLARKSON (1990)

SLUICE

TYLER DIAM DISTRIBUTION RECOVERY DISTRIBUTION RECOVERY DISTRIBUTION RECOVERY DISTRIBUTION RECOVERY DISTRIBUTION RECOVERY

MESH MM % % % % % % % % % %

LOCATION S M U B A

+4 4.76

+8 2.38

+14 1.19 1 52 33 84 8 44 0 100 7 100

+28 0.59 13 68 37 88 28 68 35 100 39 100

+48 0.29 66 48 24 84 44 84 38 61 24 96

+100 0.14 20 36 6 60 18 64 21 78 25 96

-100 0.9 0.5 3 5 5

100.9 100.5 101 99 100

Raw Gold gm / hr 41 14 37 5 0.6

$/Hour $418.00 $145.00 $381.00 $47.00 $6.00

$ / 1,200 Hours $502,000.00 $174,000.00 $457,000.00 $57,000.00 $8,000.00

Overall Recovery 48% 84% 72% 79% 98%

Raw Gold gm / hr 38 11 36 4 0.3

$/Hour $390.00 $113.00 $367.00 $44.00 $3.00

$ / 1,200 Hours $468,000.00 $135,000.00 $440,000.00 $53,000.00 $3,000.00

Capital Cost $50,000.00 $100,000.00 $100,000.00 $1,000.00 $1,000.00

Operating Cost $5,000.00 $10,000.00 $10,000.00 $O $0

Overall Recovery 96 96 99 99 99

SIZE DISTRIBUTION AND RECOVERY

MONETARY VALUE OF GOLD LOSSES (GOLD CA $400.00 / OUNCE)

RECOVERABLE GOLD LOSSES (GOLD CA $400.00 / OUNCE)

SINGLE RUN HOMEMADE TRIPLE PEARSON TRIPLE ROTATING TROMMEL VIBRATING SCREEN

6.0 SLUICE LOSSES:

Losses across sluice boxes vary widely but as a rule are reduced by pre-screening the feed

material prior to sluicing. Recorded losses (Clarkson, 1990) varied from 0 to 71% with at

least two operations losing more gold than they recovered.

Unlike jigs, which in many respects are more forgiving in their application, sluices respond

badly to variations in feed rate, water flow, slope and feed type.

Water and feed flow are related.

Periods of no, or low feed, usually see little change in the water flow, thus in this instance,

the low feed causes the water to flush the sluice and some gold, particularly that material

sitting exposed between the riffles, will be lost to tailings.

Similarly surges in the supply of water will increase turbulence in the case of more water and

cause losses of fines to tails or in the case of lower water flow cause general losses across

the whole sluice. See Photos 7 and 8.

PHOTO 7 – ARTISANAL SLUICE, SULAWESI, INDONESIA

In Photo 7, a sluice fed by gravel pumping, several potential loss causing factors can be seen,

specifically:

The sluice is not level;

The sluice has to few riffles, note the riffle free section in the centre of the sluice;

The sluice is being fed a “Long Range”, -4” feed with no real screening and the

operators are using highly turbid re-circulated water.

PHOTO 8 – INDONESIA, MINUTES AFTER PHOTO 7

Note in Photo 8, taken only minutes after Photo 7, which the flow rate across the sluice has

dropped dramatically but that a surge of water can be seen at the head of the sluice. This

surge will probably have resulted in gold being scoured from the riffles.

In one instance the author has observed over-screening causing severe sluice losses, Photos

9, 10 and 11.

In this instance in Cameroon, West Africa, the operators were trying to treat extremely clay

rich ground and in the process screening to a -5 mm sluice feed.

Apart from very poor trommel design that saw screens blind to about 20% open space

availability, and fail to break up all the clay, the sluice was set at an extreme angle greater

than 15O. This created the mini Niagara falls seen in Photo 9 and was made worse by use of

excessive water flow.

The gold had little time to settle and no apparent vortex was able to form behind the riffles.

The clays formed hard packed areas behind the riffles

The solution was to install a dual sluice system of a Nugget Trap ahead of Expanded Mesh

sluices.

These units operated at flatter angles and resulted in better sluice performance although

excessive water flow was a common problem.

PHOTO 9 – CAMEROON, WEST AFRICA, NOTE EXTREME SLUICE ANGLE

PHOTO 10 – CAMEROON, WEST AFRICA, NOTE LAMINAR FLOW ACROSS RIFFLES

INDICATIVE OF POTENTIAL GOLD LOSSES

PHOTO 11 – CAMEROON, REPLACEMENT SLUICE SYSTEM

EXCESSIVE WATER FLOW

It is also important to stress that sluices are not suited to recovery of very fine mineral

product; sizes below 100 mesh are better recovered using other means. This fine size

material is normally lost across the sluice in the turbulent flowing upper layers.

Losses certainly can be minimized but not completely avoided.

7.0 RECOMMENDATIONS:

The following are some recommendations for those considering using sluice boxes for the

recovery of any heavy mineral, specifically:

i. “Know Your Ground”

The mine owner should make every effort to know the ground he intends to work.

Simple procedures can be adopted such as:

Conduct feed size analyses at several locations throughout the area to be

worked;

Determine the nature of the alluvium, whether it is sandy, clayey, rocky, etc.;

and

Take several bulk samples and from the concentrate determine the size range

of the mineral being sought.

Other information on items such as water supply, etc., is also valuable when

considering pumping and pipeline requirements.

ii. Feeding and Screening

Screening of the feed gravels is essential and where possible should be into three

fractions; +1”, -1” to +1/2” and -1/2”. The former is discarded to tailings while the

other two are sluiced to recover the heavy minerals.

Feed rates should be closely controlled either using mechanical feeders such as

conveyors, apron feeders, vibrating grizzly feeders or even hand operated monitors.

Sluices like constant feed without surging.

iii. The Sluices

Where a twin sluice system is to be used, that is where there are two size fractions

the following should be adopted:

The Coarse Fraction:

-1” to +1/2” should be passed across a nugget trap sluice with a minimum

length of 12’, set at a steep angle, at least 9O;

Ahead of the riffles the sluice should have a 3’ slick plate to assist in settling of

gold or heavy mineral in the slurry column prior to hitting the first riffle;

The sluice should be fitted with either angle iron or bent flat bar riffles as

described in the text, riffle spacing of 2” for angle iron and 3” for flat bar;

Water may have to be introduced and should not be less than 10 gpm / cubic

yard of feed, but no less than 150 to 200 gpm per foot of sluice width. The

operator may find higher water rates are required to move the gravel down

the sluice and should avoid the phenomenon of “Rooster Tailing” by adjusting

slope and water volume until a good turbulent flow is achieved;

An over turbulent water motion (scoured bed) and / or a flat flowing water

surface (indicating a packed sluice bed) will result in gold losses; and

The riffles should be laid on Nomad Mat and clamped tightly down.

The -1/2” Fraction:

Should be processed using two sluice runs in series, specifically:

Nugget Trap Section:

A first nugget trap type sluice having a slick plate at its head and angle iron or

bent bar riffles set tightly over Nomad Mat;

Riffle layout as described in the text, normally a 2” spacing or 3” in the case of

flat bar riffles;

The sluice run should be no less than 10’ and ideally 12’, width is determined

by feed volume;

Slope should be steep 5O to 9O depending on the wash type;

The riffles clamped down on Nomad Mat; and

Water volume determined by feed rate;

Expanded Mesh Sluice :

A 12’ to 16’ section of sluice fitted with coarse expanded mesh tightly

clamped on Nomad Mat; and

The sluice should be wider than the Nugget Trap section, the difference

should be around 20%, and the sluice set to operate at a flatter angle.

Ideally the sluices should be constructed so as to have adjustable operating angles

and provision made to be able to introduce extra water at the head of the sluice.

iv. General:

High sluice recoveries can only be achieved by good design and care in their

operation.

Sluices should not be allowed to pack with concentrate and should be cleaned out

on a regular basis.

Where nugget ground is encountered they can be fitted with locked mesh security

screens.

8.0 BIBLIOGRAPHY:

AGRICOLA, Georgius. 1556

De Re Metallica.

Translation by Herbert Clark and Lou Henry Hoover, 1912

The Mining Magazine, London

BOWIE, Jr., Aug. J. 1895

A Practical Treatise On Hydraulic Mining In California,

D. Van Nostrand Company, New York.

CLARKSON, Randy. 1990

Placer Gold recovery research, Final Summary.

Klondike Placer Miners Association, New Era Engineering Corporation.

2010

The Use of Nuclear Tracers to Evaluate the Gold Recovery Efficiency of Sluice boxes

Gravity Gold, 2010, Proceedings AusIMM

GRIFFITH, S.V. 1938

Alluvial Prospecting and Mining.

Mining Publications, Ltd, London

HARRISON, H.l.H. 1946

Examination, Boring and Valuation of Alluvial and Kindred Ore Deposits.

Mining Publications Ltd., The Mining Magazine, London.

1962

Alluvial Mining for Tin and Gold.

Mining Publications Ltd., The Mining Magazine, London.

LONGRIDGE, C.C. 1902

Hydraulic Mining, Part III

The Mining Journal, London

1906

Gold Dredging, Annual Supplement, 1906


Gold and Tin Dredging and Mechanical Excavators


MACDONALD, Eion H. 1983

Alluvial Mining, The Geology, Technology and Economics of Placers.

Chapman And Hall, London

PEELE, Robert. 1945

Mining Engineers Handbook, Third Edition.

John Wiley & Sons, Inc. London

RICHARDS, Robert H., 1903

Ore Dressing, Vol. I & II

The Engineering & Mining Journal, London

TAGGART, Arthur F. 1945

Handbook of Mineral Dressing, Ores and Industrial Minerals

John Wiley & Sons, Inc. London

WELLS, John H. 1969

Placer Examination, Principles and Practice,

Bureau of Land Management, US Government Printing Office, Washington DC.

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