Jan Svoboda. Magnetic Techniques for the Treatment of Materials

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470 CHAPTER 6. INDUSTRIAL APPLICATIONS

Figure 6.1: Schematic illustrations of the trommel magnet system [S86] (cour-

tesy of Eriez Magnetics, Inc.).

6.1 Treatment of minerals

6.1.1 Recovery of grinding ball fragments from SAG and

ball mills

Grinding ball fragments from SAG and ball mills cause serious wear to down-

stream processing equipment. These fragments, recirculating in a milling circuit,

cause wear to sumps, pumps, hydrocyclones and interconnecting piping. Mag-

netic separation systems designed to remove grinding ball fragments from the

mill discharge have been developed and successfully applied in milling circuits.

Trommel and trunnion magnet systems developed by Eriez Magnetics were

applied at the Escondida Copper Concentrator in Chile and subsequently at

other concentrators, including Los Pelambres [S86].

The trommel magnet shown in Fig. 6.1 consists of an arc of permanent mag-

nets mounted at the discharge end of an existing trommel screen. The magnet

removes the grinding ball fragments from the oversize material allowing the

oversize ore to be returned to the circuit for processing. As can be seen in Fig.

6.1. the system consists of a blind trommel, magnet sector, support structure

and a discharge hopper. The function of the blind trommel is to provide a ﬂange

to transport the trommel oversize material through the magnetic ﬁeld.

During operation, the trommel screen oversize material moves across the

blind trommel prior to discharging into a sump. Any strongly magnetic mater-

ial will be attracted to the magnetic arc and held to the rotating blind trommel.

This material is then released as it rotates past the end of the magnet sec-

tor. Figure 6.2 shows an installed trommel magnetic system mounted on the

discharge end of a trommel screen.

After the installation of the trommel magnet in the Escondida concentrator

6.1. TREATMENT OF MINERALS 471

Figure 6.2: Trommel magnet (foreground) mounted at the discharge end of a

trommel screen (courtesy of Eriez Magnetics, Inc.).

Feed

preparation

Dense

medium

separation

Drain Rinse

Medium

storage

Medium

recovery

Densifica-

tion

Medium

recovery

Floats

Products

Water

Feed

Medium recovery circuit

effluent

Feed preparation effluent

Figure 6.3: Block ﬂowsheet of a dense medium plant.

472 CHAPTER 6. INDUSTRIAL APPLICATIONS

126 tonnes of grinding ball chips were removed on the ﬁrst day. This amount was

progressively decreasing with each subsequent day and after the ﬁfth day the

recovered fragments stabilized at about 7 tonnes per day [S86]. When installed

at the Los Pelambres Copper Concentrator (Chile) on a SAG mill, on average

16.6 tonnes of ball chips per day were collected.

The trunnion magnet was subsequently designed to collect magnetically the

grinding ball fragments directly from the mill discharge stream without the use

or requirement of a trommel screen. The magnet installed at a ball mill at Los

Pelambres collected, on average, 34.8 tonnes of ball chips per day.

Installation of these magnetic systems for the removal of grinding ball frag-

ments resulted in extension of the lifetime of pumps by 300%, the milling

throughput increased by 5% and the energy consumption decreased by 10%.

6.1.2 Heavy medium recovery circuits

Dense medium separation has been practised for a long time in the coal, iron

ore and diamond industries and in spite of its long history and relative matu-

rity the process is being steadily improved. The DMS plant block diagram is

shown in Fig. 6.3, while Fig. 6.4 illustrates the medium recovery circuit with

water recycling. Since the major component of the total loss of heavy medium

is the magnetic loss, signiﬁcant emphasis has been placed on the development

of the magnetic circuits and their operation. Improved drum and tank designs

provide better recoveries and correct adjustment and optimization of variables

aecting operation of magnetic separators also provide more e!cient and eco-

nomical operation of the circuit. Implementation of rare-earth drum magnetic

separation, with important consequences for better recoveries of heavy media

and for simpliﬁcation of the entire circuit, will be the next step for improvement

of this mature process.

6.1.3 Beneﬁciation of iron ores

EVTAC Mining Co., Minnesota, USA

EVTAC Mining, the magnetite concentrate and pellet producer, has recently

faced a problem of the reduced feed grade and the need to move into mining areas

of lower-grade ore with a higher silica content and lower magnetic susceptibility

iron minerals. In order to reverse the losses of the magnetic minerals, low-

intensity magnetic separators developed during the 1960s were replaced by more

e!cient Metso Minerals machines.

The original 98 concurrent Stearns low-intensity magnetic drum separators

(900×2400 mm) in the rougher circuit were replaced in 2001 by 20 Metso Min-

erals counter rotation LIMS, shown in Figs. 6.5 and 6.6. In addition to consid-

erable reduction in ﬂoor space, the replacement separators oered advantages

such as a longer pick-up zone, increased retention time and self-adjusting level

control. The metallurgical performance of the new separators exceeded not only

6.1. TREATMENT OF MINERALS 473

Fresh water

Drain and rinse

screens

Dirty water recycle

Primary LIMS

Secondary LIMS

Classifying

cyclone

Effluent

Figure 6.4: Dense medium recovery circuit with water recycling (adapted from

Dardis [D13]).

Figure 6.5: Metso Minerals low-intensity counter-rotation permanent magnet

drum separator (courtesy of Metso Minerals).

474 CHAPTER 6. INDUSTRIAL APPLICATIONS

Figure 6.6: Metso Minerals low-intensity permanent magnet drum separator

(courtesy of Metso Minerals).

Table 6.1: Metallurgical results of rougher optimization at EVTAC Mining [E6].

Parameter Metso LIMS Stearns LIMS

Feed [% solids] 30 - 35 42.3

Davis Tube concentrate [%] 6.5 13.1

Recovery [% Fe] 99.6 96.9

Davis Tube total conc. [%Fe] 68.9 68.5

Davis Tube total tails [%Fe] 54.9 57.5

Silica in conc. [%Si] 19.3 18.0

Silica rejection [%] 26.8 27.3

that of the original separators but also the set target. The metallurgical results

from the optimized rougher circuit are summarized in Table. 6.1.

Following the success of the rougher circuit optimization, the Metso LIMS

machines were tested in the cobber position. The new cobbers improved the

recovery and grades in both the concentrate and tails streams and their in-

stallation resulted in a reduction of 75% in the original equipment [E6]. The

metallurgical improvements are recorded in Table 6.2, while Fig. 6.7 illustrates

the pattern of the iron recovery and the feed grade as a function of time.

Iron ore beneﬁciation by the Ferrous Wheel magnetic separator

Ferrous Wheel magnetic separators, described in Section 2.4 have been applied

to the beneﬁciation of weakly magnetic iron ores at several mining operations.

6.1. TREATMENT OF MINERALS 475

Table 6.2: Metallurgical results of cobber optimization at EVTAC Mininig [E6].

Parameter Metso LIMS Stearns LIMS

Feed [% solids] 35 - 40 43.7

Feed grade [% Fe] 21.6 20.8

Concentrate grade [% Fe] 33.1 32.4

Recovery [% Fe] 97.2 92.4

Mass recovery [%] 63.9 59.5

Davis Tube total conc. [% Fe] 68.8 69.1

Davis Tube total tails [% Fe] 53.8 61.9

Silica in conc. [% Si] 33.7 34.2

Silica rejection [%] 22.2 22.7

Figure 6.7: Magnetic iron recovery and the feed grade at EVTAC (adapted from

[E6]).

476 CHAPTER 6. INDUSTRIAL APPLICATIONS

Figure 6.8: Installation of sixteen Ferrous Wheel magnetic separators to recover

hematite from tailings ponds (courtesy of Eriez Magnetics, Inc.).

Process

Water

pumps

Magnetic Separation

Circuit for MUT Plant

Process water

Thickener

For Filtration

Circuit

Transfer

tank

pump

Slimes

For Classifying

Circuit

Tailings

process water

Concentrate

+0,4 mm

-0,4 mm

Dilution

tank

pump

FW (05 equipment)

Concentrate

pump

Tailings

pump

HC-50a67

pump

Concentrate

tank

Tailings

+ Slimes pumps

protection screen

Process water

By-pass

Option

Figure 6.9: Magnetic separation circuit at the Mutuca Plant, Minas Gerais,

Brazil (courtesy of S.C. Amarante).

6.1. TREATMENT OF MINERALS 477

Table 6.3: Production-scale results at the Hercules Mine [P15].

Product Mass [%] Grade [% Fe] Recovery [% Fe]

LIMS 0-8 55 - 61 0-9

FW Rougher magnetics 18 - 35 57 - 60 22 - 41

FW Scavenger magnetics 13 - 34 55 - 57 15 - 38

Combined concentrate 49 - 58 56 - 58 58 - 68

Hercules Iron Mines, Coahuila, Mexico In 1995 the Grupo Acerero del

Norte’s Hercules Mine expanded its concentrator to recover iron values from mill

tailings. The expansion incorporates low-intensity drum magnetic separators

and matrix magnetic separators as a pre-concentration stage. Flotation is then

used to produce the ﬁnal iron concentrate for pelletization.

Approximately 6 million tonnes of ﬂotation tailings, averaging 50% Fe, mostly

hematite, are contained in tailings ponds. The tailings are relatively ﬁne at 93%

-75 m and 72% - 25 m. The non-magnetic fraction from the low-intensity

drum magnetic separator is treated by 15-ring Ferrous Wheel (FW) separators,

with 2.44 m diameter rings, equipped with the 14 mesh screen cloth matrix.

The ferrite permanent magnets generate a magnetic induction of 0.1 T in the

open air in the upper rougher stage, while the magnets in the lower scavenger

zone produce a background magnetic induction of 0.22 T.

The results of the production-scale tests at Hercules are summarized in Ta-

ble 6.3. The best combined results of LIMS and Ferrous Wheel separators were

obtained at a feed rate of 26 t/h at 37% to 38% solids. As a result of success-

ful pilot-scale and production-scale tests, additional three low-intensity drum

magnetic separators (1220 mm diameter and 3170 mm drum width) and sixteen

Ferrous Wheel separators, shown in Fig. 6.8, were installed [P15].

Mutuca Plant, Minas Gerais, Brazil Four 15-ring Ferrous Wheel sepa-

rators were installed in 2001 at the Mutuca plant of MBR, Minas Gerais, the

second largest Brazilian iron ore producer, to treat 240 t/h of cyclone under-

ﬂow at the iron ore beneﬁciation plant. A schematic diagram of the magnetic

separation circuit of the MUT plant is shown in Fig. 6.9. The plant expan-

sion followed extensive laboratory and pilot-scale tests [A36]. The results of the

tests conﬁrmed that signiﬁcant reduction in concentration of aluminium-bearing

minerals and of silica could be achieved at high iron oxide recovery. It was also

observed that loss on ignition and concentration of manganese and phospho-

rus also decreased signiﬁcantly. Figure 6.10 illustrates the performance of the

Ferrous Wheel separator at the Mutuca plant. The Ferrous Wheel separator

installed at the Mutuca Iron Ore Plant is shown in Fig. 6.11, while Fig. 6.12

illustrates the positioning of the magnetics/non-magnetics splitter within the

Ferrous Wheel machine.

478 CHAPTER 6. INDUSTRIAL APPLICATIONS

Example of results for 75% TAM + 25% MUT

Gerência de Desenvolvimento e Tecnologia Mineral

PROCESS WATER

66.8 1.00

3.5m³/h

2.40 100

4 tph/ring

Fe Al

SiO

61.1 6.67

67.8 0.76

8.18 15

1.38 85

non-magnetic

netic

FEED

key

Ferrous Wheel

model FW 8-15

8' in diameter

Example of results for 75% TAM + 25% MUT

Gerência de Desenvolvimento e Tecnologia MineralGerência de Desenvolvimento e Tecnologia Mineral

PROCESS WATER

66.8 1.00

3.5m³/h

2.40 100

4 tph/ring

Fe Al

SiO

61.1 6.67

67.8 0.76

8.18 15

1.38 85

non-magnetic

netic

FEED

key

Ferrous Wheel

model FW 8-15

8' in diameter

Figure 6.10: Performance of Ferrous Wheel magnetic separators at the Mutuca

Iron Ore Plant, Brazil (courtesy of S.C. Amarante).

Figure 6.11: Ferrous Wheel magnetic separator installed at Mutuca Plant,

Brazil (courtesy of S.C. Amarante).

6.1. TREATMENT OF MINERALS 479

Figure 6.12: The positioning of a magnetics/non-magnetic splitter in the Ferrous

Wheel separator at Mutuca Plant (courtesy of S. C. Amarante).

Fábrica Mine, Forteco Minerac˛ão SA, Brazil

The concentrator plant of Fábrica Mine, which started beneﬁciating itabirite ore

in 1977, was using nine Jones wet high-intensity magnetic separators DP317,

with 1.5 mm gap and a magnetic induction of 1.2 T, to produce a concentrate

for pelletization. These WHIMS units were operating in parallel at a feed rate

of 110 t/h, with a feed grade of 47% Fe, of which maximum 0.8% was magnetite.

The increasing production rate and decreasing grade of the ore in the nine-

teen eighties resulted in the increase of the magnetite content, up to 3%, in

the feed to magnetic separators. This, combined with the need to increase the

magnetic ﬁeld strength to treat ﬁnely disseminated hematite, resulted in partial

clogging of the separation boxes.

After extensive pilot-scale testwork the WHIMS plant was modiﬁed into

a two-stage process using the existing nine Jones separators. The width of

the gap of the ﬁrst stage, with four machines, was increased to 3.5 mm, which

corresponded to a magnetic induction of 0.7 T. This ﬁrst medium intensity stage

was followed by a higher intensity WHIMS stage, with ﬁve machines operating

at 1 T and 1.5 mm gap width. Feed rates of up to 320 t/h in the ﬁrst and 240

t/h in the second stages could be achieved [H24].

The new circuit, the ﬂowsheet of which is shown in Fig. 6.13, allowed an

increase in the overall production rate from 832 t/h to 1200 t/h, and an increase

in the recovery of iron from 65% to 70%.