Basics in Mineral Processing-operation and Environment

8/8/2019 Basics in Mineral Processing-operation and Environment

1/7

Product Handbook 10:1

Operation and environment

Operation

and

environment

BASICS IN MINERAL PROCESSING

Operation and Environment Introduction

From environmental, health and safety point of view most mineral processingoperations have some negative affects on the working environment.

The main problems are related to

Dust (dry plants)

Noise (wet and dry plants)

Pollution (emissions other than dust to air and water)

Regarding pollution of water and air by various emissions we refer to the processsections 5 and 6 above (enrichment and upgrading).

Dust

Dust - size

When energy is introduced to rock, ore or mineral crystals will generate a dustemission. With dust in mineral processing we practically understand particlesbelow 100 micron in size. Above this size dry particles are easy to control andare quite harmless.

Dust - Chemical composition

A parameter of interest is the chemical composition. Hard rock in many cases ishazardous due to the silica content.

Free quarts (SiO2) is extremely dangerous and so are the rocks containing quartslike granite, gneiss a.o, see figure below. Fine silica can cause silicosis, a deadlylung disease. Mg-silicate of asbestos type is also very dangerous when inhaled,causing lung cancer.

As many of the silicates are hard and abrasive these dust fractions also arecausing heavy wear when exposed to bearings, motors etc.

Dust fractions of non -silica type are normallynot too dangerous for theoperators and give morelika a housekeepingproblem.

Granite

Rhyolite

Diorite

Andesite

Gab

bro

Basalt

SiO2

Abrasion curve

K-Felds

par

Quartz

Na,C

aFe

ldsp

ar

Ferro

mag

nesia

nmine

rals

70% SiO2

by weight

60

50

40

100% Mineralby volume

80

60

40

20

Surface (fine)

Deep (coarse)

Biotite Olivine

Dust SiO2 levels

SiO2levels in magmatic rock


2/7

10:2 Product Handbook


Operation

and

environment


Dust Control Basic

1. Let the dust report to the material flowby using dust suppression or enclosure.

2. Suppression by water or foam is cheap and handy but can only take care of thecoarser dust. Fine dust will remain a problem. If too much water is used the dustwill turn to sticky clay, causing down time in operation and freezing in cold cli-mate.

3. Enclosures of machines are very effective provided that you only encapsulatethe dust emitting part of the machine, not drives or other moving parts. Enclosures

are also very effective against wind emission of fines from conveyors and forsealing off transfer points, see below.

4. Dust removal by ventilation is used when the dust is the product (dry grinding offiller fractions) or when dust is not allowed in the final product or in the processingsystem, see ventilation criteria below.

Cyclone

Fan

Product

Totally sealed screen or feeder

Chute coverSeal

Seal

Chute

Seal

Conveyor cover

Dust collector

Dust-sealed stockpile chute

Equipment enclosure Wind enclosure

Dust removal

Enclosure

Some guidelines:


3/7



Operation

and

environment


Ventilation criteria

Dust capture velocity in m/s (ft/min)

= Ventilation criteria (Vc) in m/s/m (ft/min/ft)

= Air volume needed per open area of enclosure

Calculation of ventilation systems for dust removal is a tricky thing. Some estima-tion figures below:

Application Vc Comments

Feeders, surge bin openings 1,02 (200) General valuefor low-energyoperations

Transfer points 2,33 (1500) per enclosurearea

Screens 0,26 (50) per screen area

Crushers and dry mills 1,5 (300) not for air sweptmills

Dust collection

The dust removal and dust collecting systems are very similar to a normal dryclassification circuit. Dry classification is in fact a dust removal system where themax size of the dust is controlled by a classifier (or ventilation criteria), Seebelow.

Primary recovery of dust is normally done in a cyclone taking the major part. The

final recollection is done in a wet scrubber or a fabric filter

Wet scrubber has an advantage over fabric filter when the dust is combustible. Inall other cases the dry fabric filtration is more effective as no sludge handling isrequired (being the case with wet scrubbers).

FEEDHOPPER

CLASSIFIER DUSTCOLLECTOR

CYCLONE

MILL

FAN

PRODUCT

OPTIONIN CIRCUITDRYING

AIR HEATER


4/7



Operation

and

environment


Noise

General

In mineral processing there are a number of machines considered to be very noisy(crushers, screens and grinding mills are typical).

By definition noise is an undesirable sound. As sound is air borne sound pres-sure variations, we have to find a sound pressure level, which can be tolerated bythe operators. Noise is not only harmful to the hearing but also affects the heartaction and the ability of concentration. It also restricts verbal communication andthe observation of warning signals or dangerous situations.

Sound basic

The human sound pressure range from lowest sound to be heard and highestsound to stand without pain isfrom 0,00002Pa (2Pa) to 20 Pa. (1 psi = 6,89kPa).

To be more practical the sound pressure range above is converted to a soundpressure level by the formula:

Lp = 20x log P/Po (Po = 2 Pa) converting the range above over to 0-120 dB(decibel)!

Experienced sound change of dBDouble sound level + 10dBDouble sound sources + 3 dBDouble the distance to sound source - 6 dB

Hearing range for a normal ear

The lower limit is called the threshold of hearing and has a maximum sensitivityaround 3500 Hz (resonance frequency of the ear).

The upper line is the 120 dB sound pressure line (the pain line)

Mechanical noise is measured in dB (A) indicating that an A-filter is used,damping lower frequencies (of less harm to the operators).Infra-sound is sound with a frequency below 22 Hz. (Can be harmful at longerexposures)Ultra-sound is sound with a frequency above 18 kHz. (Can be harmful at longerexposures)

Speach


5/7



Operation

and

environment


Noise Reduction

There are 4 main ways to reduce the noise levels for processing systems includ-ing crushers, mills and screens.

Optimum operation

Use of internal polymers (wear material and wear products)

Use of external polymers (dust enclosures)

Enclosure with noise reduction walls

Optimum operation.

Mass flow equipment like crushers and screens are normally lower in noise whenthey are operated under optimum conditions and the material flow is absorbingpart of the noise (e.g. choke fed cone crushers). Reduced circulating loads alsolead to reduced noise levels.

Noise exposure risks

For continuous sound with a wide frequency range, a sound level below 85 dB(A)is acceptable for an 8 hour exposure per day with respect to the risk of hearingdamage.

If the sound level is higher an octave band analysis is necessary. This curve iscompared to the standard risk curves, see below.

130

120

110

100

90

8032 63 125 250 500 1000 2000 4000 8000 14000

= middle frequency of octave Band (Hz) (Impact crusher on 1 m distance).

Maximum acceptable exposureper 8 hours:

5. Less than 5 min

4. Less than 20 min

3. 1 - 2 h

2. 2 - 5 h

1. 5 - 8 h1

23

4

5

Internal polymers

The use of polymers as mill liners,screening media and wear protec-tion in material handling systems(chutes and transfer points) have adramatic effect on noise reduction.

For grinding mills a rubber liningcan reduce the noise level up to10 dB(A) compared to a steel lining

dB

90

80

70

60

125 250 500 1000 2000 4000 8000 Hz

dB(A)

Steel lining

Rubber lining


6/7



Operation

and

environment


External polymers

Using polymers as dust sealing enclosures of crushers, screens, conveyors,chutes, transfer points etc. will give a noise reduction of approx. 5-10 dB (A).The difference for a screen with steel wire deck and rubber deck is shown below.

A simple rule: The more polymers used for various purposes in the mineralprocess systems the lower the noise levels!

Noise reduction walls

Enclosure is an effective way of reducing noise. Enclosure can be more or less

extensive (enclosure of drive or machine or both). With a total enclosure noiselevels can drop by 10-15 dB (A).

Depending on duty the design of the noise reduction walls can differ in design:

31,5 60 125 250 500 1000 2000 4000 8000 16000 Hz

dB

100

90

80

70

60

Steel wire clothRubber elements

Rubber elements with dust encapsulation

50 150 150

Light andmedium

duty walls


7/7



Operation

and

environment


Heavy duty crusher wall , cross section

Plate 1,2 mm

RHS vert.

80x40x3 mm

Support

Weld

Beam

200 75 8,5

Service platform, not moving

Plate 1 mm

120-rail vertical

Absorbent 100 mm

Wood spacer 19 mm

Heavy rubber 5,2 mm

120-rail horizontal

Superior crusher112 DbA

Ear ProtectionWhen working in environments with continuous and high noise levels it is importantto use ear protection all the time. Also at sound levels of 75-80 dB (A) it is to berecommended to use ear protection even if recommendation says something else.Reason is that long exposure also at these levels can cause impairment ofhearing.

Good rules about ear protection:

Take some noise breaks now and then

Go for regular hearing tests

Check your ear protection equipment at certain intervals

Basics in Mineral Processing-operation and Environment

Documents

Transcript of Basics in Mineral Processing-operation and Environment