Application of waterjet in stone qu

Application of waterjet in stone quarrying and processing
Raimondo Ciccu*, University of Cagliari, Italy
(*) Corresponding Author’s Address: University of Cagliari, DICAAR - Dept of Civil and Environmental
Engineering, Piazza d’Armi, 09123 Cagliari, Italy, Phone: +39 070 675 5557 , fax: +39 070 675 5523, e-mail:
ciccu@unica.it
ABSTRACT
The power of a jet issued at high velocity can be properly exploited for a number of applications in
stone engineering. In the quarry the interest is limited to particular situations like the underground
extraction of granite block or in sandstone operations, whereas for marble the dominant technologies
are those based on diamond tools. Abrasive waterjet is currently used for contour cutting of stone
slabs where flexibility and accuracy are of a major concern. Finally waterjet, either plain or abrasive
laden, lends itself to surface finishing for which it proved to be superior to other rustic methods like
bush hammering and flaming. The paper refers on the contribution to the scientific knowledge and to
the development of waterjet technology in the field of stone at the University of Cagliari
Keywords: Stone, Extraction, Cutting, Surface finishing,
1. INTRODUCTION
High Velocity Waterjet can be regarded as one of
the major advances in the field of material
disintegration. Thanks to its inherent flexibility
and efficiency, the application has been rapidly
extended to many branches of industry for
working almost any kind of materials, from soft to
very hard ones.
As regards rock engineering, hydraulic energy
was first used for excavating soft rocks using low
pressure monitors. With the advent of the high
pressure technology also harder rocks have been
made amenable to waterjet treatment, opening a
new era in mining, quarrying, civil engineering,
mineral processing, waste treatment and
reclamation.
At present, waterjet lends itself to a variety of
operations such as excavation, drilling, slotting,
cutting, crushing and cleaning, alone or in
suitable combination with mechanical tools. The
prospects have been recently enhanced by the
development of abrasive jet technology,
especially for precision cutting.
2. WATERJET IN THE QUARRY
The only instance of industrial application of
waterjet in the quarry concerns sandstone, owing
to the very favourable textural characteristics of
this kind of rock. In the Rothbach quarry blocks
are extracted with a series of orthogonal slots
made with a swinging jet lance at a cutting rate of
6.5 m2/h (Figure 1)
Figure 1. The sandstone quarry at Rothbach
(France)
Concerning marble there is no interest in waterjet
since diamond tools (diamond wire and rock
cutter), are by far superior to any competitor on
both technical and economic grounds.
As for granite, blocks are generally extracted in
surface quarries according to bench methods
using different cutting technologies (diamond
wire, explosive splitting and wedge shearing).
In the future, however, the possibility of
developing the excavation underground is
expected to gain increasing popularity for a
number of reasons such as:
· less impact on the environment;
· no need for preliminary overburden removal;
· better overall recovery;
· possibility of working all year round in a
protected ambient;
· economic use of left-out stopes.
The access underground would consist of a large
gateway tunnel, from which production activity
can be developed. The excavation of the
gateway is made difficult by the absence of a
lateral access, since the only free face available
is the front of the advancing tunnel.
All blind slots at the face can be opened with
waterjet according to a regular grid, whose
geometry is dictated by the size of the blocks to
be extracted.
The hidden back face parallel to the front can be
redeemed either by using flat hydraulic jacks
introduced into the waterjet slots or by cutting
with diamond wire with the machine placed at
one side of the drift as shown in Figure 2.
Figure 2. Combination of waterjet and Diamond wire for the tunnel extraction of granite blocks.
The association of waterjet with diamond wire
offers a very interesting solution for mechanised
quarrying, since both are able to work in a
completely automated fashion.
3. CONTOUR CUTTING WITH ABRASIVE
WATERJET
AWJ technology is being increasingly used for
contour cutting of slabs in stone processing
plants, whereas for straight linear cutting
diamond circular blade is faster and cheaper,
especially in the case of thick slabs. A typical
equipment for contour cutting with abrasive
waterjet is shown in figure 3.
3.1. Conventional separation cutting
In separation cutting the jet cuts through a given
slab thickness with only one pass.
For a given machine setting (pressure, nozzle
Figure 3. Waterjet cutting table at the DICAAR
laboratories, University of Cagliari
diameter), experimental evidence has shown that
maximum specific erosion, i.e. the surface cut by
the unit mass of abrasive (cm2/g) achievable at
optimum traverse speed of the nozzle is affected
by some relevant characteristics of the abrasive
used, such as micro-hardness, particle size and
shape, density, as well as by the relative
proportion by volume of solids in water. On the
other hand, specific erosion is also chiefly
influenced by the hardness of the rock,
depending on the operational conditions.
It is worth underlining that the quality of the cut
depends on the traverse velocity of the nozzle
that must be kept slow enough in order to limit
the roughness and waviness due to spreading
and instability of the abrasive flow.
The results of the investigation carried out at the
University of Cagliari show that peak specific
erosion Es (cm2/g) can be expressed as a
function of a global parameter Pm accounting for
the different characteristics of the abrasive as
shown in figure 4 for different materials taken into
consideration, including some rocks.
The structure of Pm is of the general form:
Pm = (Hm)a × (S)b × (r)c × (D)d × (P)e
where:
- Hm is the Knoop hardness [GPa]
- S is the shape factor of the abrasive
particles (dimensionless)
- r is the volumic mass of the abrasive
[g/cm3]
- D is the average particle size [mm]
- P is the proportion of solids in water by
volume [%o]
Figure 4 . Correlation lines of specific erosion as
a function of parameter Pm .
Exponents in the mathematical expression of Pm
have been calculated through a statistical
processing of experimental data by maximising
the correlation coefficient of the linear
relationship:
Es = K× Pm
Where K is a constant typical of each material,
obtained through the same computer procedure.
It was found that:
- exponent “c” and “e” are constant,
irrespective of the material;
- exponent “d” is constant equal to 0.3 except
for ductile materials;
- exponents “a” and “b” and coefficient K are
variable for the different materials.
Attempts have been made to express exponents
“a”, “b” and “d” and coefficient K as a function of
hardness that is believed to be the most
important material parameter capable of
describing its behaviour under the action of
abrasive waterjet.
The following remarks deserve some
consideration:
· An increase in abrasive hardness is always
beneficial, the more for harder materials,
whereas the advantage becomes less
important or even insignificant for softer
ones;
· also the shape factor is favourable, except
for the very hard porphyry rock, but the best
advantages of irregular particle shapes are
now for softer materials while for rocks the
gain is very poor, if any;
· regarding particle size, coarser particles
produce a better performance, although with
a decreasing marginal benefit;
· as for density, heavier abrasives are
increasingly detrimental for all the materials;
· finally, the influence of increasing the
abrasive load into a given volume of water is
always negative concerning the abrasive
efficiency, since specific erosion is inversely
proportional to the square root of the
solids/water ratio in the jet.
Therefore, although the most used abrasive is
garnet, cheaper and more efficient abrasives
should be preferred in particular cases like for
instance copper slag for soft marble.
3.2. Multiple pass strategy
In flat-bed contour cutting with abrasive waterjet,
a separation cut through a thick slab can be
made with multiple passes of the nozzle along
the planned profile at relatively high traverse
velocity. Since the relationship between depth of
kerf and traverse velocity is not a straight linear
one due to a gradual loss of efficiency of the jet, it
turns out that the overall time needed per metre
of cut and thus the unit cost of cutting can be
minimised by resorting to multiple passes. Of
course the quality of the cut surface is also
affected, either favourably or adversely according
to the particular conditions (figure 5).
0,00 R2 = 0,7868
0,20
0,40
0,60
0,80
1,00
1,20
1,40
1,60
0 1 2 3 4 5 6 7 8 9 10
Parameter Pm
Specific Erosion [cm2/g]
Perspex
Glass
Stainless steel
Marble
Granite
Alluminium
Basalto
Porfido
Results of cutting tests with abrasive waterjet on
a variety of materials clearly demonstrate the
advantages of using multiple passes at high
traverse velocity, consisting in:
· Reduced waviness except for heterogeneous
materials;
· constant cut quality over the entire area at
suitable conditions;
· almost zero trail-back;
· no significant taper;
· good separation cuts on either sides in case
of curved or angled section of the contour
profile;
· slow decrease in the incremental depth per
pass irrespective of the thickness of the
workpiece.
Figure 5. Features of surfaces cut with multi-pass
strategy on Granite (left) and marble (right)
The benefits offered by the concept of multiple
passes are much more important in the case of
complex profiles consisting of curved sections
and including angles, for which a deceleration of
traverse velocity is required in order to avoid the
problems related to trail-back, whereas this
measure is not needed in the case of multiple
passes. Moreover b