Durrant 566-70 salt harvester collecting salt for
the first time by the clients driver under the
supervision of the ROV Durrant commissioning engineer.
ROV Durrant Engineering
In 2005, the annual world production of salt exceeded 250
million tonnes for the first time. Since then, it has been
fluctuating at around 260 million tpa. Recent upturn in demand,
driven by chemical markets in Asia and de-icing markets in
Europe, resulted in shortage of high quality solar salt,
tightening supply and sharply increasing prices.
Forecast demand for salt in 2015 is
290-300 million tpa. Approximately 40% of all salt produced
worldwide is manufactured by solar evaporation of sea water and
The solar salt manufacturing method
consists of evaporation of brines in a series of shallow ponds
up to saturation, followed by further evaporation of saturated
brine in crystallisers. Depending on the climatic conditions,
the salt crystal layer that grows at the crystalliser bottom
can become several centimetres thick in one year but under
favourable climatic conditions the thickness can reach a few
decimetres before it is harvested.
There are many methods of
crystallising and harvesting solar salt:
- Salt crystals can grow on
salt floor or on mud floor;
- The crystallisers can
receive pre-concentrated brine in series or in parallel;
- The salt can grow as a
compact layer or the crystals can be kept apart by regular
- The brine level can be
maintained deep or shallow;
- Thick or thin salt layer can
finally be harvested.
Solar salt harvesting
There are many combinations of salt production and harvesting
methods. In many parts of the world, for example in
the most primitive salt harvesting method - by bare hand - is
still being practised today.
Elsewhere, the salt harvesting
methods invented long ago for example in ancient Rome or by the
Arabs, are being practised. In India or in China, regular
raking of salt crystals in shallow ponds allows manual
harvesting of thicker salt layers.
Salt grown on the salt floor as a
compact thick salt layer is usually of better quality. However,
such salt must be harvested with mechanical machinery. The
productivity of mechanised saltworks is higher, the production
cost lower and the salt product is more competitive. The trend
towards mechanisation of solar saltworks is prevailing
A team of men pick the salt from the mud floor
of a crystalliser by shovels to suspended belt
conveyors, which transport the salt to a slurring
vessel located on a tractor. Salt is mixed with
brine and pumped to a mobile washplant located
at the side of the crystallising pond.
Salt Partners Ltd
In Tuticorin, India, perhaps the simplest method
of salt harvesting with wooden tools from shallow
earthen ponds is being practised. A similar salt
harvesting method is still being used in small
saltworks all around the world.
Salt Partners Ltd
Mechanical salt harvesting
Mechanical salt harvesting has been employed by solar saltworks
in a variety of forms, including simple power-enhanced devices,
standard construction machinery - such as mechanical shovels
and front end loaders - and purpose-built salt harvesters.
Perhaps one of the simplest
mechanical devices used in solar saltworks is a wooden or steel
plate with handles connected to a wrench of a tractor, which
provides the pulling power. The workers drag the device away
from the tractor to the far end of the crystalliser, lower it
to the salt and then, when the tractor starts pulling, balance
the plate so that the salt gets dozed to the side of the
crystallising pond where the tractor is standing. Productivity
of such devices is low and the people using them must work day
and night to earn the cost of living and the diesel fuel they
In many smaller mechanised
saltworks, standard construction machinery is being employed
for harvesting. This may include, for example, front end
loaders, graders, mechanical shovels, and tractor-scrapers.
Usually, the salt layer is first
ripped (scarified) to separate the salt crystals from each
other. Various devices are used for this task, including simple
hooks or rotating cutters. The loosened salt layer can then be
collected with excavators fitted with wide shovels onto heaps.
The same excavators, but fitted with high volume buckets, then
transfer the salt from the heaps to trucks, trailers or
dumpers, which transport the salt from the crystallisers to the
Much depends on the hardness of the
salt layer and the bearing capacity of the crystalliser floor.
A soft salt layer on a hard floor can be collected with
tractor-scrapers and transported to the washplant in a single
working cycle. Salt grown on a hard crystalliser floor can be
cut with front-end loader buckets that have been equipped with
a special cutting edge. The same front end loaders then lift
the salt and fill the trucks.
For higher capacity, the salt is
windrowed with graders and the salt is collected with
harvesters that lift the salt to trucks. After the harvest, the
crystalliser floor must be prepared for the next
In Mexico, salt is grown on a salt floor. The salt
layer is first ripped, then windrowed. The windrow
is collected by a tractor-pulled harvester, which
lifts the salt to a truck-pulled hopper-trailer equipped
with bottom discharge gates.
Salt Partners Ltd
Collecting and lifting salt harvester on double
wheels. The wheels on the left hand side drive
on the salt floor of the crystalliser. The wheels
on the right hand side drive on the salt layer.
Observe that the wheels on the right hand side
are somewhat higher up. Therefore the harvester
collects the salt on the left hand side deeper than
on the right hand side.
Salt Partners Ltd
Solar salt harvesters
There are many types of salt harvesters. They vary in capacity,
in ground pressure, whether they cut the salt layer or just
collect it, etc. The main types are:
- Tractor drawn or self-propelled;
- On wheels or on Caterpillar
- Driving on salt bed or on
- Lifting or cutting the salt
- Cutting the salt layer
upwards or downwards;
- Discharging onto belt
conveyors or into trailers.
Tractor-drawn harvesters are
usually made for smaller capacities. Their salt lifting gear
can be propelled from the tractor drive shaft or they can have
their own engines.
Wheeled salt harvesters are usually
suitable only for crystallisers with a permanent hard salt
floor. For higher capacities and for harvesting of salt grown
on mud floors, only harvesters on Caterpillar tracks can be
employed successfully. This is particularly the case when the
harvester should separate the salt layer from the floor,
because this task requires pushing. The pushing power can be
best transmitted to the crystalliser floor by the Caterpillar
Unless the salt harvesters are
excessively heavy, the ground pressure exercised by the
Caterpillar tracks to the ground can be as low as the pressure
of the human foot. This makes it possible for the harvester to
drive even on the mud floor and lift the salt layer in the
front, provided that the salt layer is not too hard and
disintegrates on the lifting blade.
The configuration of such
harvesters works successfully within a given set of parameters,
such as the mud floor bearing capacity, thickness and hardness
of the salt, harvester width, weight, power, etc. If some
parameters change with time, or if such harvesters should be
employed in some other saltworks, the harvester may fail.
It is safer to design the
harvesters so that they drive on the salt layer. Then they can
be employed even in saltworks built on very soft ground. Some
such machines harvest the salt on the side.
Usually, the elevation of the
lifting blade is controlled by a worker, either sitting in a
cabin with good view of the lifting blade or walking along the
harvester. Such configuration limits the width of the
harvesting path to approximately one metre. Placing the
harvesting element behind the harvester facilitates a much
wider harvesting path.
Typically, the harvesting element
is 2.5 metres wide so that the harvester does not exceed the
width of vehicles normally permitted for road traffic. Wider
harvesters would require special transport should they be moved
from one place to another on public roads.
Salt grown on salt has the
advantage of minimised contamination with insoluble material
originating from the crystalliser bottom. However, the new salt
layer is firmly attached to the permanent salt floor. Unless
the new salt layer is very soft, it cannot be lifted - it must
be cut. This can be done either separately (ripping,
scarifying), prior to windrowing, collecting and lifting the
salt to the trucks, or in one working operation. Time, fuel and
manpower can be saved if these operations are performed
together by a single machine.
Simple ripping of the salt
frequently results in large salt lumps, or blocks, being
formed. Hard salt lumps cause problems in the washplant.
Cutting of the salt layer is possible with rotary devices,
which prevent the formation of lumps.
The rotation can be done either
from top to the bottom or vice versa. If the cutting is done
from the top to the bottom, then the salt is thrown behind the
rotary device. Therefore, behind the cutting drum a blade must
be provided, followed by a cover that facilitates collection of
the cut salt to a lifting device. Salt slipping below the
blade, remaining on the floor, can represent significant
harvesting loss. Besides, cutting elements that enter the salt
layer from the top to the bottom cause a breakage of the salt
crystals and create undesired fines.
Cutting of the salt layer from the
bottom to the top, in the direction of harvester movement,
lifts the salt layer and separates the crystals, minimising
their breakage. The crystalliser floor is swept clean of salt.
When the cutting elevation is effectively controlled, the
crystalliser floor behind the harvester is ready to receive
brine for the next crystallisation cycle.
Laser control of earth-moving
machinery is an established practice, for example in road
construction. In salt harvesting, the laser control is less
common. In principle, a rotating laser beam marks a perfectly
horizontal plane, for example four metres above the salt
crystalliser. The harvester is equipped with a receiver
reaching out above the harvester roof. The harvester is
equipped with devices that measure and control the distance
between the receiving point of the laser signal and the lowest
point of the cutting drum. Should this device indicate that the
distance is increasing, the controls of the cutting drum lift
it to the desired level, and vice versa.
Thus, the crystalliser floor needs
to be levelled only once, whether by some other equipment or by
the harvester itself. The next harvest will separate the new
salt layer from the crystalliser floor precisely at the correct
elevation. When salt is grown on mud floor, the contamination
of salt with insolubles will be minimised. When salt is grown
on salt, the floor is maintained clean and ready for the next
Salt harvester on Caterpillar trucks drives on the
mud floor of the crystallising pond. It pushes a
blade between the salt layer and the mud, lifting
the salt layer, which disintegrates on the blade.
The salt falls from the blade to a rotating device,
which lifts the salt to a belt conveyor.
Salt Partners Ltd
On request of the client, this type Durrant
harvester has the drivers cabin located at the
with the pick roll and the conveyor just behind it.
The harvester drives on Caterpillar trucks only
on the crystalliser floor. Also this harvester is
Salt Partners Ltd
ROV Durrant salt harvesters
Salt Partners avail of more than 40 years of experience in
solar salt production and processing. In search for equipment
most suitable for solar salt production, the solar salt
harvesters made by ROV Durrant have been determined to fulfil
the requirements for versatile deployment under varying
ROV Durrant Engineering, based in
Port Elizabeth, South Africa, made its first salt harvesters in
1964. The first machine was capable of harvesting 80 tph of
solar salt. Its designation was SH80 (Salt Harvester 80 tph).
Later, it was recognised that the same machine could perform
differently, depending on the properties of the salt.
Therefore, the type designation was changed. Today, this
machine bears the type designation Durrant 130-60.
Over the years, larger machines
were designed, the output increased and reached 1,000 tph with
the type Durrant 916-120. Presently, the most popular size of
the Durrant salt harvesters is in the middle of this range.
Recently, ROV Durrant supplied type Durrant 560-75, Durrant
590-95 and Durrant 790-100 salt harvesters to solar salt
producers in Turkey, the Middle East and India.
The type designation, for example
Durrant 590-95, signifies the following:
- 5 is the type of the
- 90 is the size of the
engine, 9.0 litres;
- 95 is the diameter of the
cutting drum (pick roll) in centimetres. 95 cm pick roll is
designed to cut a salt layer 30 cm deep and fling it to the
In the Durrant 590-95
configuration, the harvesters have an instantaneous harvesting
capacity of 200 m3/h of the salt layer,
corresponding to approximately 350 tph of bulk salt. Depending
on turn-around times, waiting for trailers and operator breaks,
the effective harvesting rate is somewhat lower. With mobile
belt conveyors and mobile lorry loading hoppers, about 80% of
the instantaneous harvesting capacity is achievable.
ROV Durrants range of salt
harvesters is based on the chassis types in Table 2.
Mounted on the chassis are the
engines, hydraulic pumps, Caterpillar tracks, the pick roll,
the elevator and the cabin for the driver.
Unlike other systems, the Durrant
salt harvesters perform all the required harvesting operations
- The pick roll cuts the salt
and flings it to the elevator;
- The elevator lifts the salt
to trucks, trailers or to mobile belt conveyors;
- The harvester maintains the
crystalliser floor perfectly levelled for the next
No separate ripping, windrowing,
lifting and crystalliser floor maintenance after the salt
harvest is required.
The harvesters travel on the salt
layer and cut the salt behind them. The ground pressure is
similar to the pressure exerted by a human foot; therefore, the
harvesters can be used in crystallisers growing salt on soft
mud as well as on a hard salt floor.
The pick roll arrangement prevents
collection of unbroken salt lumps and formation of fines. The
level of the cutting drum is laser controlled, which
facilitates maximum salt recovery and minimises salt
contamination with impurities. After the salt harvest, the
crystalliser floor is perfectly levelled and ready to receive
brine for the next crystallisation cycle.
The Durrant salt harvesters are
designed to significantly improve the productivity of solar
saltworks and profitability of the salt production and
1. Salt harvesters of Ghana. 2007.
A film by Marcia Rock on YouTube.
2. P. de Flers, B. de Saboulin, J.
Clain. 1973. Harvest of solar salt at Salin de Giraud
France, Fourth International Symposium on Salt.
3. Gavin McLellan. 2008. Modern
mechanised salt harvesting in southern Africa, Proceedings
of the National Salt Conference, Gandhidham, India.
4. Vladimir M. Sedivy. 2010.
Mechanical solar salt harvesting worldwide,
Proceedings of the International Salt Summit, Ahmedabad,
Contributor: Vladimir M. Sedivy, President, Salt
email@example.com. Vladimir Sedivy received
his MSc degree in chemical engineering at the University of
Prague. His interest in salt started in 1973 when he became a
manager of salt projects in Africa and South America. He
invented the HYDROSAL salt purification process with
hydroextraction of impurities in 1978. As director of Krebs
Swiss, he developed and implemented more than 30 salt and
chloralkali projects in Europe, Africa, Middle East and India.
In 2003, he established Salt Partners. Since then, Salt
Partners have built several salt plants and carried out
consultancy assignments for banks and government organisations
worldwide. Salt Partners represent ROV Durrant in Europe, India
and in the US.
Spotlight on Salt Partners
Salt Partners are independent consultants and engineers active
in the field of salt and chloralkali production, salt
processing and hypersaline biotechnology. Salt Partners
worldwide reputation is based on 40 years of experience gained
in projects implemented worldwide.