In the quartz sand purification process, the magnetic separation equipment
(especially the electromagnetic dry powder magnetic separator) undertakes the
key task of removing magnetic impurities (such as iron, manganese, titanium,
etc.), which directly determines the purity of the finished quartz sand. If the
iron removal effect is poor, the iron content in the finished sand exceeds the
standard (such as the iron content of the PV glass sand is ≤ 0.01%, and the
ordinary glass sand is ≤ 0.05%), which will seriously affect the quality of
downstream products, and even cause the product scrap. Many quartz sand
manufacturers have encountered the problem of "incomplete iron removal" of
magnetic separation equipment, but it is difficult to accurately locate the
cause. As a professional manufacturer specialized in R&D and manufacturing
of quartz sand magnetic separation equipment, we have combined experience of
hundreds of purification projects to find out the core causes of poor iron
removal effect and systematic solutions to help enterprises troubleshoot
problems efficiently.
Determine the criterion of "iron separation difference": avoid
misjudgment
Before troubleshooting, it is necessary to determine whether the iron removal
effect is really unqualified through scientific test, so as to avoid wasting
time due to improper test methods or standard confusion:
Direct detection: detect the iron content of the finished sand after magnetic
separation with an atomic absorption spectrophotometer or an X-ray fluorescence
spectrometer, and compare it with industry standards or customer requirements
(for example, the iron content of the electronic quartz sand shall be ≤ 0.001%).
If it exceeds the standard value, it can be determined that the iron removal
effect is poor;
Visual observation: spread the finished sand after magnetic separation on the
white paper and slide it on the sand surface with a strong magnetic rod. If a
large amount of black magnetic impurities are absorbed on the surface of the
magnetic rod, it indicates that the iron removal is not thorough;
Comparative test: take quartz sand samples before and after magnetic
separation, respectively weigh and then conduct secondary magnetic separation
with magnetic separation tube to calculate the removal rate of magnetic
impurities (removal rate=(magnetic impurity mass before magnetic separation -
magnetic impurity mass after magnetic separation)/magnetic impurity mass before
magnetic separation × 100%). If the removal rate is lower than 90% (≥ 99% for
some high-purity demand scenarios), targeted optimization is required.
Step 1: Troubleshoot the core parameters of equipment - "magnetic field
setting" is the key
The iron removal effect of electromagnetic dry powder magnetic separator
first depends on whether the core parameters such as magnetic field intensity,
magnetic field gradient and roller speed match the material characteristics, and
improper parameter setting is the most common cause.
Insufficient or uneven magnetic field intensity
Problem performance: the iron content of the finished sand exceeds the
standard, the magnetic products (including iron impurities) of the magnetic
separator are small, and a large amount of qualified quartz sand is mixed in the
impurities. The magnetic field intensity in the separation area is detected with
a Gauss meter, which is far lower than the rated value of the equipment (for
example, the rated magnetic field intensity of an electromagnetic magnetic
separator is 2000 Gauss, and the actual detection is only 1200 Gauss).
Cause analysis: First, aging of electromagnetic coil and interturn short
circuit lead to decrease of exciting current and decrease of magnetic field
intensity; Second, the material loss of the magnetic yoke (such as rust and
crack of the magnetic yoke), reduced magnetic conductivity and blocked magnetic
field conduction; Third, the magnetic circuit is not reasonably designed, the
magnetic field in the separation area is not evenly distributed, and there is a
"magnetic blind area". The magnetic impurities are discharged with the finished
product before being absorbed.
Solution: disconnect the power supply, disassemble the magnetic enclosure of
the magnetic separator, check whether the electromagnetic coil is blacked and
burnt, and measure the coil resistance with a multimeter (compared with the
standard resistance marked in the equipment manual, if the deviation exceeds
10%, the coil shall be replaced); If the magnetic yoke is rusted, grind it with
sand paper and apply anti-rust paint. In case of crack, replace the magnetic
yoke (DT4 electric pure iron material is selected, with higher magnetic
conductivity); For the equipment with uneven magnetic field distribution, the
manufacturer can be contacted to install auxiliary magnetic poles or adjust the
magnetic medium arrangement to ensure the uniform coverage of the magnetic field
in the separation area. If necessary, the high-gradient magnetic system shall be
replaced (if multilayer magnetic medium is used, the magnetic field gradient
shall be increased by more than 30%).
The roller speed is too fast or too slow
Problem performance: when the rotating speed is too fast, the retention time
of magnetic impurities in the separation area is too short (less than 0.5
seconds), and the magnetic impurities will be thrown out with the finished sand
after being fully absorbed in the future, resulting in incomplete iron removal;
When the rotating speed is too slow, the magnetic impurities adsorbed on the
surface of the roller are easy to be "washed again" by the subsequent materials
and re-mixed into the finished products, and the production capacity is greatly
reduced.
Troubleshooting method: measure the actual rotating speed of the roller with
a tachometer and compare it with the "adaptive rotating speed" recommended in
the equipment specification (for example, for quartz sand with a particle size
of 0.1-0.5mm, the rotating speed of the roller is suggested to be 10-15r/min) to
judge whether it deviates from the reasonable range.
Solution: adjust the motor frequency converter parameters or replace the
drive pulley (change the transmission ratio), and adjust the roller speed to the
appropriate range - when handling fine quartz sand (<0.1mm), the rotating
speed can be appropriately reduced (8-12r/min) to extend the adsorption time;
When handling coarse quartz sand (>0.5mm), the rotating speed can be slightly
increased (15-20r/min) to avoid material accumulation; The iron content of 3
batches of finished sand shall be detected continuously after adjustment to
ensure stable iron removal effect.
Step 2: analyze material characteristics - "material adaptability" is the
basis
The particle size, moisture content and magnetic impurity form of quartz sand
directly affect the iron removal efficiency of the magnetic separation
equipment, and the material characteristics are not matched with the equipment,
so no high-quality equipment can play a role.
Material particle size difference is too large or fine powder is too much
Problem performance: the iron content of coarse particle (>1mm) and fine
powder (<0.074mm) in finished sand is obviously different, the iron removal
effect of coarse particle is good, and the iron content of fine powder is
seriously out of limits; Or the fine powder agglomerates in the separation area
and envelops the magnetic impurities, so that the impurities cannot be absorbed
by the magnetic field.
Cause analysis: fine quartz sand is light in weight, with large specific
surface area, easy to float with airflow and difficult to fully contact with the
surface of the magnetic separator drum; In addition, fine powder is easy to
agglomerate due to electrostatic action to form "powder", magnetic impurities
are wrapped inside, and the magnetic field cannot penetrate the "powder" to
realize adsorption.
Solution: add grading process before magnetic separation, use cyclone or fine
screening machine to divide quartz sand into coarse particle size (>0.1mm)
and fine particle size (<0.1mm) according to particle size, and respectively
enter magnetic separation equipment with different parameters for treatment -
use high gradient magnetic separator (magnetic field gradient>10 ^ 5 T/m) for
fine particle size materials, and install dispersion device (such as high-speed
rotating dispersion impeller) before feeding to break fine powder agglomeration;
If the proportion of fine powder is too high (>30%), airflow auxiliary device
can be added at the feed inlet of the magnetic separator to guide the fine
powder evenly distributed on the surface of the roller and improve the contact
efficiency.
Material moisture content is too high or contains sticky impurities
Problem performance: A large amount of wet materials are adhered on the
surface of the magnetic separator drum to form a "material layer", and the
magnetic impurities are covered by the wet materials and cannot be effectively
absorbed by the magnetic field; Or the materials contain cohesive impurities
such as clay and organic matter. The impurities are bonded with quartz sand and
magnetic particles to form a cluster and discharged with the finished product,
resulting in poor iron removal effect.
Troubleshooting method: detect the moisture content of the material with a
fast moisture meter. If the moisture content exceeds 6% (generally ≤ 5% for
electromagnetic dry powder magnetic separator), or observe the appearance of the
material, if obvious caking and sticky hands are found, it can be determined as
the moisture content or sticky impurities.
Solution: add drying process before magnetic separation, select roller drier
to reduce the moisture content of materials to 3% - 5%, and control the
temperature at 120-150 ℃ during drying to avoid discoloration of quartz sand
caused by too high temperature; If the material contains cohesive impurities,
scrubbing process (such as spiral scrubber) shall be added before drying to
remove impurities such as clay through mechanical friction. After scrubbing,
sedimentation tank shall be used to separate impurities and quartz sand to
ensure that the cleanliness of the material entering the magnetic separator
meets the standard.
Special form of magnetic impurities (weakly magnetic or encapsulated)
Problem performance: The content of finished sand iron exceeds the standard,
but the impurities in the magnetic products of the magnetic separator are less,
and most of them are strong magnetic iron (such as scrap iron), and the weak
magnetic impurities (such as hematite and titanite) are still left in the
finished products; Or magnetic impurities are wrapped by quartz sand particles
(forming "inclusions"), and the magnetic field cannot be absorbed.
Cause analysis: the magnetic field intensity (1500-2500 Gauss) of ordinary
electromagnetic dry powder magnetic separator can only effectively absorb strong
magnetic impurities, and the adsorption capacity for weak magnetic impurities is
insufficient; If the quartz sand is not fully separated during crushing and sand
making, the magnetic impurities will be wrapped by the quartz sand to form a
"physical barrier", and the magnetic field cannot act on the internal
impurities.
Solution: replace magnetic separation equipment with high magnetic field
intensity (such as superconducting magnetic separator, the magnetic field
intensity can reach 5000-8000 Gauss) for weak magnetic impurities, or add
"multi-stage magnetic separation" process (2-3 stages magnetic separation,
gradually improve the iron removal efficiency) based on the existing equipment;
Crushing and sand making process shall be optimized for inclusion impurities.
Impingement sand machine or ball mill shall be selected to realize full
separation of quartz sand and magnetic impurities through strong impact and
grinding. After separation, quartz sand shall enter magnetic separation
equipment and iron removal efficiency can be increased by more than 40%.
Step 3: Inspect equipment structure and maintenance - "component state" is
the guarantee
Wearing, blocking or installation deviation of the drum, magnetic medium,
unloading device and other parts of the magnetic separation equipment will
directly affect the iron removal effect, and improper daily maintenance is easy
to cause such problems.
The roller surface is worn or attached with impurities
Problem performance: Scratch marks, dents or a layer of hard "scaling layer"
(mixture of fine quartz sand powder and magnetic impurities) are found on the
roller surface, and the surface is not smooth when touched by hand, so the
magnetic impurities cannot be closely absorbed on the roller surface during
magnetic separation, which is easy to fall off and mix into the finished
product.
Troubleshooting method: after shutdown, use a flashlight to illuminate the
roller surface and observe whether there is any abrasion or scaling. Use a blade
to slightly scratch the scaling layer. If the thickness exceeds 1mm, it will
obviously affect the magnetic field conduction.
Solution: if the roller surface is slightly worn, polish it with fine sand
paper and apply wear-resistant coating (such as ceramic coating, the
wear-resistant property is improved by 50%); Replace the roller (304 stainless
steel, surface finish Ra ≤ 0.8 μ m) in case of serious wear or crack; Regularly
(every shift) wash the roller surface with high pressure water gun (water
pressure 0.6-0.8MPa) to remove the scaling and avoid long-term adhesion of
impurities.
Blockage or damage of magnetic medium
Problem performance: the gap of magnetic medium (such as magnetic bar and
magnetic grid) in the separation area of magnetic separator is blocked by fine
quartz sand powder, the magnetic field cannot be normally penetrated, and the
adsorption of magnetic impurities is reduced; Or the magnetic medium breaks, the
magnetism declines, the local magnetic field disappears, and the "iron removal
blind area" is formed.
Troubleshooting method: disassemble the cover plate of the separation area of
the magnetic separator, take out the magnetic medium, observe whether there is
any blockage in the gap, and use a strong magnetic rod to absorb the surface of
the magnetic medium. If the adsorption force is significantly weakened, it
indicates that the magnetism declines.
Solution: Purge the clearance of magnetic medium with compressed air
(pressure 0.5MPa) to remove the blocked fine powder; If the magnetic medium is
broken, replace the magnetic medium with the same specification (select the
NdFeB permanent magnetic material with low magnetic degradation rate);
Magnetization treatment shall be carried out for the magnetic medium with
degraded magnetism (contact the professional manufacturer to use the magnetizer
to recover the magnetic field strength). It is suggested to test the magnetism
of the magnetic medium every half a year to ensure that the adsorption capacity
meets the standard.
Unloading device failure (scraper worn or brush loose)
Problem performance: The magnetic impurities adsorbed on the surface of the
roller can not be completely removed, and some impurities return to the
separation area with the rotation of the roller, and even are brought into the
finished sand. Observe the magnetic products at the discharge port, and it is
found that a large amount of impurities have not been unloaded.
Cause analysis: the scraper (or brush) rubs with the roller for a long time,
resulting in serious wear (the thickness of the scraper is reduced by more than
1/2); Or the fixing bolt of the scraper is loose, and the clearance with the
roller surface is too large (more than 0.5mm), so it cannot be closely
fitted.
Solution: replace the worn scraper (select wear-resistant polyurethane
material, which is 3 times longer than the service life of rubber scraper) or
brush (select nylon wear-resistant brush); The gap between the scraper and the
roller shall be adjusted, and the gap shall be controlled within 0.1-0.3mm by
adjusting the adjusting bolt of the scraper bracket, so as to ensure that the
materials can be discharged completely and the roller surface will not be
damaged due to excessive friction; Install clearance monitoring device and
regularly check the fitness of scraper to avoid unloading failure due to
looseness.
Step 4: Troubleshoot the operation and process cooperation - the "process
connection" shall be smooth
The iron removal effect of the magnetic separation equipment is closely
related to the operation specifications of the operators and the process
coordination of the front and rear processes. Improper operation or poor process
connection will indirectly reduce the iron removal effect.
Unstable feed or overload
Problem performance: the feeding of feeder varies from time to time, the
materials in the magnetic separator separation area are stacked too thick (more
than 50mm), and the magnetic impurities in the bottom materials cannot contact
the magnetic field and are discharged with the finished products; Or the feeding
capacity exceeds the rated capacity of the equipment for a long time (e.g. the
rated capacity of the equipment is 10t/h, and the actual feeding capacity is
15t/h), the equipment operates under overload, and the iron removal efficiency
drops sharply.
Troubleshooting method: measure the actual feeding capacity with an
electronic belt scale and compare it with the rated capacity of the equipment;
Observe the material thickness in the magnetic separator separation area. If it
exceeds 1/5 of the drum diameter, it indicates that the feed is excessive.
Solution: adjust the feeder parameters (such as amplitude and frequency of
the vibrating feeder), and form closed-loop control with the feeder by
installing the material level sensor to keep the feed stable (fluctuation ≤
10%); To increase the capacity, the number of magnetic separators can be
increased (in parallel or in series) instead of forced overload operation; The
operator shall record the feeding amount and finished iron content every hour,
establish the related account of "feeding - iron removal effect" and adjust the
parameters in time.
Disjoint of previous and subsequent processes (if not removed or graded)
Problem performance: pre-treatment is not carried out before magnetic
separation, and large impurities (such as stones and metal blocks) are mixed in
the materials, blocking the separation area of the magnetic separator and
affecting the material flow; Or direct magnetic separation without grading,
coarse and fine particles are mixed, resulting in uneven iron removal effect,
and the iron content of some granular materials exceeds the standard.
Solution: pre-treatment process shall be added before magnetic separation,
such as grid (to intercept large impurities) and metal detector (to remove metal
blocks), so as to ensure that the materials entering the magnetic separator are
free of impurities; Based on the process sequence of "grading first and then
magnetic separation", the materials are divided by particle size by screening or
grading equipment, and the materials with different particle size correspond to
different magnetic separation parameters to improve the overall iron removal
efficiency; Regularly check the operation status of the equipment before and
after the process to ensure the smooth connection of the process and avoid the
effect of magnetic separation due to the failure of a certain link.
Preventive measures: improve the iron removal stability of magnetic
separation equipment
Solidification and regular calibration of parameters: for specific quartz
sand materials, determine the optimal magnetic field strength, roller speed,
feed volume and other parameters through tests, input them into the equipment
control system and solidify them to avoid random adjustment; Calibrate the
equipment parameters with Gauss meter and tachometer quarterly to ensure they
are consistent with the set values.
Establish the system of "daily inspection - weekly protection - monthly
maintenance": the operator shall check the roller surface, unloading device and
feeding quantity every day; The maintenance personnel shall clean the magnetic
medium and check the coil and magnetic yoke every week; Disassemble and overhaul
the bearing every month and replace the vulnerable parts to reduce the risk of
failure from routine maintenance.
Training for operators: regular training shall be carried out to enable
operators to master equipment parameter adjustment methods and common fault
judgment skills, and timely adjust parameters according to material property
changes (such as moisture content and particle size fluctuation), so as to avoid
reduction of iron removal effect due to improper operation.
Regular inspection of iron content of finished products: 3-5 batches of
finished sand samples shall be taken every day, iron content shall be inspected
with professional instruments, and "iron content change curve" shall be drawn.
In case of rising trend, equipment and process shall be checked timely to avoid
problem expansion.
As a professional manufacturer of quartz sand magnetic separation equipment,
we can customize electromagnetic dry powder magnetic separator according to
customers' demands for iron removal (such as common iron removal and high-purity
purification), and provide full-cycle services from "process design - equipment
manufacturing - installation and commissioning - operation and maintenance
training".