Bernshtein L.G. «Drying of sand for production of dry pack mortars»

Abstract

Sand drying is one of the most important stages of
a process of manufacturing dry pack mortars, that is responsible
for quality and cost of a final product to a considerable
extent.

Special features of sand drying for production of dry pack mortars

When drying sand for production of dry pack mortars,
some special features should be taken into account:

1. Moisture content of dried sand should not exceed
0.5%, that is why it is necessary to control this parameter
carefully.

2. Temperature of sand used for production should
be low, not more than 50°С, therefore a drier should be
equipped with a sand cooler. One of possible varieties is
a drum cooler that provides recycling of the heat emissioned
by sand at a drying process.

3. Taking into account poor conditions for fuel combustion
inside a drum, a drier should be equipped with a
combustion chamber. In the first place, it is essential for
driers that are fired with fuel oil and other types of fuel
which are capable of forming ash at incomplete combustion,
and ash impurities in sand used for production of
dry pack mortars are inadmissible.

4. Taking into account, that requirements to fraction
composition of sand used for production of dry pack
mortars are very strict, it is reasonable to equip driers
with a pack of sieving drums for primary sand fractionation.

5. There should be a roofed warehouse for a stock
of wet sand at a dehydrating plant. Absence of a warehouse
is frequently a reason for abnormal operation of
a drier. The best results are shown by roofed centrefeed
warehouses, which enable to stabilize a moist
content of sand.

6. Waste gases are usually treated with cyclone collectors
and hose filters. One should pay special attention
onto thermal conditions of those machines, such as presence
of heat-insulation, absence of inleakage, installation
in roofed premises. As soon as these conditions are
violated, vapor condensation arises, which leads to sealing
cyclone collectors and hose filters.

Sand can be dried in boiling bed drying plants, vibrofluidized
bed driers, and in rotating drying drums.
Boiling bed driers, and especially vibrofluidized bed
driers have extremely high thermal effectiveness. They
guarantee a moisture removal of 150–200 kg/m3 of a
plant, and horizontal strain of about 1000 kg of moisture
from 1 sq. m of a floor per hour. Specific heat consumption
under such conditions is within the limits of
5000–6500 kJ per 1 kg of evaporated moisture, and
vibrofluidized bed drying plants enable to reach the specific heat consumption of 4000–5000 kJ per 1 kg of
evaporated moisture.

Pic. 1 shows a scheme of a boiling bed drier that
consists of a kiln (3), drying chamber (8) and external
equipment for treatment of waste gas (10, 11, 12). Disadvantage
of boiling bed plants is an excessive power consumption,
which is necessary for a boiling bed, as well as
difficulties connected with upkeeping a boiling regime
when sand has a poly-fractional composition.

Pic. 1 Boiling bed drying plant for dispersion materials: 1 — inlet ventilator; 2 — fuel feeding; 3 — kiln; 4 — air inlet hole; 5 — emergency tube; 6 — tanker with material feeding system; 7 — floor; 8 — drying chamber; 9 — discharge of product; 10 — cyclone collector; 11 — multicyclone collector; 12 — exhaust blower; 13 — conveyor for ready product

Pic. 2 Vibrofluidized bed drying plant for dispersion materials: 1 — air delivery for burning; 2 — fuel inlet supply; 3 — kiln; 4 — air inlet hole; 5 — аварийная труба; 6 – vibration motor of a floor; 7 — feeder; 8 — floor; 9 — drying chamber; 10 — cyclone collector; 11 — exhaust blower; 12 — carriage to discharge blown away material from a vibrofluidized bed; 13 — product discharging; 14 — conveyor

To overcome aforementioned difficulties, vibrofluidized
bed drying plant has been developed. In this plant,
a boiling bed is created on account of vibration of a floor
(7), initiated by a special vibration motor (Pic.2).

Disadvantages of such plants are complexity of vibration
motor and an excessive power consumption at a
production capacity of over 1–2 tons per hour. For drying
over 2 tons of sand per hour drum driers are preferred.
A scheme of a drum driers is shown at Pic.3.

Major subunits of a drier — kiln (1), rotating drum
(9), discharging device (11) and a system of waste gas
treatment (12, 13). Disadvantage of this plant is a high
cost, advantage — high reliability at continuous operation
and productive capacity of several dozens of dry
sand per day.

For increasing intensity of drying process, a drum is
equipped with inserted elements, examples of which are
shown at Pic. 4.

Pic. 3 Drum drying plant: 1 — kiln; 2 — mixing chamber; 3 — cool air inlet hole; 4 — emergency tube; 5 — materials feeding; 6, 11 — end-cutting chambers of a drier; 7 — ring seals; 8 — retaining rings; 9 — metal drum; 10 — ring gear; 12 — cyclone collector; 13 — exhaust blower; 14 — conveyor; 15 — jaw lock; 16 — rolling bearings; 17 — ventilator of air delivery for burning; 18 — fuel inlet supply

Pic. 4 Inserted elements for drying drums: a) — cellular; b) — transition type; c) — lifting and bladed

An important stage in designing driers is a choice
between a cocurrent and counter-current scheme of
heat exchange. Cocurrent scheme is applied for drying
materials, for which overheating is not permissible, for
example for flammable, explosive substances or those
that lose quality at overheating. At this heat exchange
scheme, we have increased temperature of exhaust gases
and, respectively, excessive heat losses.

Counter-current scheme of heat exchange is preferred
for sand drying, Pic. 5.

Pic. 5 Scheme of material temperature change for material and drying agent in a drum drying plant. a) — co-current; b) — counter-current

Drying drum is designed on basis of a rate of evaporation
per unit heating surface g, which is determined
with a help a nomographic chart, Pic. 6.

Pic. 6 Nomographic chart for determining a rate of evaporation per unit heating surface of a drum, g kg/m3/hour at different temperature of при различных температурах heat carrier: 1 — 1200°С, 2 — 900°С, 3 — 700°С and final water content of 0,5%

Quantity of evaporated water can be found by the
following equation: (1), where Q — productive capacity
of a drum, dried sand is accounted, W1 – initial moisture
content, W2 — end moisture content, %.

Dimensions of a drum are determined by devising
a quantity of evaporate water onto a rate of evaporation
(2), selecting a diameter D from a standard range,
and counting a length by formula , where a ratio of
a length L to diameter D should be within the limits
of 6–10.

The time of dwell of a material in a drier can be
estimated by formula (3), where f — loading factor of
a drum, f =0,10–0,25, p — volume weight of a material,
kg/m. Rotation frequency of a drum is counted by formula
(4), where m=0,5 for rotary-vane heat exchangers,
at that k = 0,2–0,7 at cocurrent and к=0,5–0,7 at counter
current scheme, and for cellular heat exchangers m=1,0
and k=0,7–1,2 at cocurrent and k=1,2–2,0 at counter
current, i — slope of drum in per cent. Then, a drive
power is estimated N = 0,01 D2 Ln/kWt.

A scheme of working plant for production of sand is
given on Pic. 8.

The plant consists of a rotating drum with dimensions
of d2.0x20 m, drum cooler d2.0x6.0 m, external kiln,
system of sieving drums for primary fractionating, and
after-kiln system of waste gas purification. Production
capacity of the plant is about 4 tons of dry sand per hour.
Annual capacity of the plant is estimated at 30,000 tons.

Pic.7 Heat consumption per 1 kg of evaporated moisture

Pic. 8 Scheme of production line for sand drying and fractionating with the use of rotating drums: 1 — drying drum; 2 — external kiln; 3 — drum cooler; 4 — ladleman elevator for dry sand; 5 — sieving drum; 6 — silo towers for dry sand; 7 — ladleman elevator for wet sand; 8 — service bunker for wet sand; 9 — receiver for wet sand; 10 — hose filter; 11 — cyclone- precipitator; 12 — flue for waste gases from drying drum; 13 — gas flue to smoke exhauster; 14 — smoke exhauster

There are two types of burner devices for drum driers.
It is necessary to use long-flame burners for drum
driers without a cooler, to avoid overheating of sand and
problems caused by overheating, such as overheating of
transportation devices after drier, disintegration of organic
additives applied in production of dry pack mortars.
For drum driers with coolers, it is reasonable to use short-flame burners. It enables to reduce a specific heat
consumption and to improve the capacity of a plant.

Pic. 9 Two-drum drier: 1 — gas and material are delivered and moving in an internal drum; 2 — gas and material are moving in an external drum. 3 — external drum; 4 — internal drum; 5 — retaining rings; 6 — ring gear pare; 7 – screw feeder and a burner placed nearby. 8 — central fixed hole, formed at a drum rotation on account of eccentric placement of an internal drum, used as inlet for a screw feeder and a burner

Multi-drum drying plants.

Multi-drum drying plants of pipe-in-pipe type enable
to double production of dried sand per 1 sq. m of a
drum on account of eccentric placing an internal drum
inside an external drum. At operation, material and fuel
are delivered into an internal drum.

Development of multi-drum driers idea took shape
in creation of a three-drum plant shown at Pic.10.

Pic. 10 Three-drum drier

Table 1

Peculiar properties of multi-drum driers are the
following:

1. They are horizontally placed and therefore do not
create load onto fixed thrust rollers.

2. They have one retaining ring and one butt rigid
point of support.

3. Installation of an in-built cooler in them is not.

To improve drying performance, a new production
unit ‘inergit’ has been developed. High intensity of drying
in this unit is reached on account of increasing rotation
speed of a drum to a level close to critical, which
gives the material a waterfall character of movement.
This unit enables to achieve moisture removal level of
300–500 kg. of water per 1 cubic meter of a drum. This
leads to considerable reduction of investments and increase
of production capacity level.

Bibliography

1. Тепловые процессы и установки в технологии
строительных изделий и деталей, В.В. Перегудов,
М.И. Роговой. — М.: Стройиздат, 1983 г.

2. Проектирование цементных заводов. — СПб.: изд.
«Синтез» СПб технологического института, 1995 г.


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