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(11) | EP 0 265 359 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Device for depositing metals on objects in suspension |
| (57) A device for depositing metals on objects, based on the fact that these are deposited
because the objects that are to be metallized remain in suspension owing to countercurrent
of a fluid that avoids the objects from falling by gravitation. The fluid provokes
deposition of the metal through a chemical or autocatalytic reaction, without any
external electric current being used, and the device forms a closed fluid flow circuit,
whose impulse suspends the objects that are to be metallized, and with a maximum impulse
of the fluid flow they may be extracted fully metallized, whereupon a new metallization
cycle can commence. All this with the necessary controls and apparatus for temperature,impulse,
emptying, filter, etc ... |
[0001] As introduction to the following technical report on a device for the deposition of metals on objects in suspension, it must be considered that whereas it is not admitted that certain industrial processes can be made in a routine manner, this leads to new situations and new layouts that prove valid, without this representing a great complication in the process studied, and however, they adapt perfectly to the evolutive process to which the industry is submitted.
[0002] There are quite a few cases that are known and which may be used as example of this reflection and one of them is the case of paint. For centuries it has been thought that a paint should have as its base an oil or a resin as filmogenous agent and to obtain a better application, the paint concept has always been associated with the concept of solvents or thinners. As the chemical industry began to develop and natural products were analysed, artificial resins appeared and lastly a large number of products were synthesized and proved better than the natural and artificial ones because they were studied with a specific purpose in mind. A great change started in the Sixties and the innovation, in this field, adopted two different directions: research on paint that would use water as thinner and paint without thinner.
[0003] All this technological development furthermore purported improvements in quality, and the possibility of automatically applying these new paints, for which automatic pistols had to be developed and which concluded by becoming true robots.
[0004] Safety standards also played an important role, both from a point of view of hygiene and safety, upon eliminating organic inflammable solvents.
[0005] The use of powder paints and varnishes could also be made by immersing the hot metal parts in them, however the fluidized bed technique had to appear in order that the part receive the same thickness.
[0006] In steel quenching, fused salts that are difficult to control in terms of their power of oxidation, have been replaced by furnaces with controlled atmosphere. Recently, at the ITEB, furnaces heated by propane have been used, because a sufficiently reducive atmosphere can be obtained that can be used for this purpose, although the furnace has been designed for ceramics. When a part is to be cooled, in a controlled way, during quenching, fluidized beds are used with solid particles of high calorific capacity.
[0007] In other words, there is an immense technical capacity that opens the way to new possibilities for using many products and furthermore, new products emerge to be used with new techniques.
[0008] This short introduction which could have been extended to many other well known cases, is simply a support and a justification, by introducing innovator ideas in the Imitation Jewelry Sector, within the specific activities of the Research Association, which is a Sector that is quite indifferent when it comes to studying improvements, but which accepts them fast once their profitability has been proved.
[0009] When presenting the different steps that are involved in applying a layer of gold to a metal object, we must not forget that in addition to attempting to rationalize the process, we must consider the properties that the gold film should have, in order to find a place on a market, as part of a finished product.
[0010] With this process, we have reviewed the possibilities that are offered by the world market of gilt objects, field of immitation jewelry, and the factors that could help obtain a good portion of it. The gilding process must be automatized better, and its quality must be first class in fine layer, less than 1µm.
[0011] Within this conviction and consequent with it we are going to undertake the study of these two targets, not in their entirety, but in specific acttions. We are going to study how to eliminate a manual process that consumes a considerable amount of manpower and at the same time, try to improve the quality of the deposited gold. For greater clarity we shall recall some of the steps that are followed in gold-plating pieces for imitation jewelry.
[0012] In the glavanic processes, each piece must be suspended in the bath, using a device that permits electrical connection between this piece and the cathode. This is achieved by placing the pieces in frames, called racks, with an insulant cover to eliminate deposition of metal on them, except at the ends of their arms, bare metal.
[0013] The enormous variety of shapes and sizes of the pieces and other objects that are to be protected, using metals deposited from their salts that form the galvanic baths, has made it necessary to use two different processes.
[0014] Those pieces which have a difficult shape or size to be easily held by mechanical means, are metallized introducing them in rotary platic drums, which have mesh walls or perforated plate, so that once all the pieces are immersed in the bath, they are all in contact with it. Inside the drumsk there are electric contacts, in connection with the cathode, so that all the pieces that turn with the drums, have an electric contact with one another but are constantly changing the points where the electric current passes, obtaining a continuous and uniform metal deposition.
[0016] Certain geometric shapes are difficult to cover using this method because they maintain a prolonged electric contact in one and the same zone and stick together, causing irregularities and surface flaws. These pieces, along with other larger ones which would need large size drums, are secured individually in the above-mentioned racks. There are certain pieces that can be hung from the ends of the arms of the racks with sufficient guarantee that they will not become detached from here, owing to the stirring movement of the bath or simply their transfer movement, although the great majority of these require that each piece be fixed to the rack with a conduction wire, which is generally brass.
[0017] This process of fixing millions of pieces consumes a considerable amount of labour which we should try and suppress. This is the basic target of this new Project and invention which is based on the following facts:
a) The chemical baths and the autocatalytic ones do not require external current for
depositing a metal on another and they are consequently appropriate to be used in
our development.
By autocatalytic bath we mean the composition that the reductive agent has, in solution
and the metal on which we want the deposition to be made does not act as such. A gold
bath is autocatalytic when it deposits gold on a gold surface.
b) The study of the fall of solids in a static fluid and in a fluid in movement will help to design and built an installation where our objective can be carried out.
THEORETIC BASIS OF THE PROJECT
Vertical free fall of an object in a fluid.
[0020] In a tube full of water (ρ₀=1) let us deposit on its surface a ball having a radius r and density ρ=1. The ball, deposited at a velocity v=0 will remain in suspension but will go towards the bottom more quickly if its density ρ is greater.
[0021] Mechanics of Fluids study these phenomena within the specific chapter on "Movement of solids within a fluid" and in the analysis of the problem, solids that have regular shapes such as in a sphere are taken, for greater simplicity. However, in the majority of cases, solids that have irregular shapes are used, and consequently the theoretic equations should be corrected if we want them to represent true facts. These correction factors are diffic ult to find through mathematic calculus and largely depend on the shape of the solid.
[0022] The more complicated is the geometry of the solid, in vertical fall, the greater will be the resistance that the fluid presents, and consequently, the time of fall to cover the same distance will also be greater.
[0023] In the simple case that is presented, the fall of flat solids will be experimented where, generally, the surface/weight ratio is large and consequently it will be necessary to adapt the laws that govern the fall to suit this fact.
[0024] The study is included in the vertical movement of solids in the interior of liquids and we start by considering that there is no friction.
[0025] If a solid having density ρ is allowed to fall in a liquid having density ρ₀, such that ρ₀<ρ, the forces that act are:
Weight = mg = Vµ g Thrust = V ρ g
[0026] The fundamental equation of the dynamic requires that:
F = ma = Vµ a = Weight - thrust
that is to say
that will be the acceleration with which the solid falls. In this case, the problem is reduced to applying the equations of the uniformally accelerated movement.
[0027] In the case of vertical fall of a solid in a resistant medium, this is studied from various angles depending on the fall velocity.
I Proportional resistance to the velocity.
II Proportional resistance to the square of the velocity.
III Proportional resistance to powers of velocity superior to 2.
[0028] This third case refers specially to movements of solids at high velocity, such as projectiles in the air and which are ruled by a standard formula:
R = K v n
where n = 5 for velocities like that of sound.
[0030] In the case where the resistance is proportional to velocity, such as the case of small velocities, the equation of the movement is:
where a_ is a constant that may have the shape of a = mk
and the integration gives us:
where v o is the velocity at the initial instant t = o
[0031] For a large value of t, we may say that the limit velocity V would have value:
the space covered by the body at the end of time t will be:
[0032] Returning to the sphere of radius r. When it is moved with uniformal movement and with very small velocity v in the interior of a fluid having viscosity coefficient η, resistance Fthat this offers the movement may be calculated, quite approximately, through the formula of Stokes.
F = 6πηrv (5)
[0034] If a sphere is left to fall in a fluid, the velocity will gradually increase until it reaches a limit. Calculus of this velocity can be made considering that between the weight of the sphere, the thrust and resistance F there is the following relation:
where
[0035] The condition imposed on Reynolds' number for the field of validity of Stokes' formula requires that the sphere be very small. With larger bodies, the resistance is no longer proportional to the velocity, that is to say, Stokes' formula is no longer valid and in accordance with Newton's law, it is admitted that resistance is proportional to the square root of velocity, which corresponds to case II.
[0036] We can therefore draw the conclusion that we pass from a system of resistance proportional to velocity to another where the resistance becomes proportional to the square root of velocity, because it is considered that the radius of the sphere increases in va , although with the same reasoning we can say that this might not occur, when the density of the sphere decreases even though its diametre increases, when v is maintained constant or with values that are valid for fulfilling Stokes' formula.
[0037] If we change the simile of the sphere, and substitute the spheres for disks for the same diametre and material, they will fall in water at variable velocity, in accordance with the area present at the time of fall, and their course will be represented by a broken line.
[0038] Having established this premise, it is not essential to know which mathematical formula governs the fall of metallic disks in water, but only what apparent velocity they reach.
Vertical fall of solids in a fluid in movement.
[0039] Within the two likely possibilities, when a fluid is made to flow in the tube of our project, only the direction contrary to the fall direction will be taken into consideration, and we shall try and obtain an equilibrium velocity.
[0040] The immeidate objective of the system is based on the following facts:
1.- After a metallic disk has been deposited at the bottom of the tube, which it has
rached after a fall time t from height L, we can say that the average velocity has been
because the fall has not taken place according to a uniformally accelerated movement,
neither with rectilineal course.
If we produce a current of water in the tube at velocity below v, the disk will logically ascend and descend in a continuous movement and theoretically
for a specific velocity v, the disk will remain practically in suspension, without touching the bottom of the
tube, or emerging from the top.
2.- For a specific velocity V c, the upward current will make the disk emerge via the top of the tube.
[0041] It may be admitted that, in practice, length L is not sufficient to keep the disk in suspension throughout its length. This means that length L of the tube is not sufficient for the kind of test that is to be carried out, that the velocity that was calculated is less than the limit velocity of that disk in that fluid and that a value of more than L should be used.
[0042] Since the target is however to try and keep a disk in suspension within the smallest possible space L, there are other factors that should be considered which have not been taken into account until now, one of the most important of these being to use a tube that has a variable section in certain parts of it.
[0043] Knowing the laws that govern the fall of solids in fluids under very specific conditions, experiments must be used as means of information when those specific conditions are not fulfilled due to change in geometric shape, temperature, density and viscosity of the fluid.
[0044] If the classification of the metals by their electrochemical activity is considered, then a metal that is located at the top of this may be covered by another metal that is situated under it.
[0045] An evident example of this is the deposition of copper on iron in an acidulated solution of copper sulphate. The two reactions that take place, one being anodic and the other cathodic, simultaneously, on the surface of the iron are:
Fe→Fe⁺⁺ + 2e (anodic) Eº = -0.44V
Cu⁺⁺ + 2e→Cu (cathodic) Eº = 0.337V
[0046] Once the iron has been completely covered by a layer of copper, the proces is stopped and there is no later increase in thickness. The deposit is normally thin (<1.0µm) and its adherence to the iron is not good.
[0047] Take a mechanical piece (M) that is joined to another piece (M₁) of a less noble metal and both immersed in the deposition solution containing ions (M₂ n⁺) of a more noble metal M₂, dissolution of the less noble tal M₁ (anodic) is produced, passing to the solution and the metal M₂ is deposited on the mechanical piece.
M₁→M₁ n⁺ + n e
M₂ n⁺ + n e→M
[0048] In this case, netallization may continue whilst dissolution of the anode M₁ takes place. There are gold solutions on the market than can provide thicknesses of 2.5µm, on substrates of silver, copper, brass, nickel, and tin. The processes are, in principle, similar to gilding by contact. The loss and gain of electrons is a basic part of the above commented processes, as it is really in all water deposit processes.
[0049] To avoid confusions, it is essential to examine the term deposition without current. Since the loss of an electron is essential, during the deposition we should base our action on other methods that are neither chemical methods,
1. Displacement reaction.
2. Galvanic dissolution reaction.
nor electric methods, such as the well-used electrodeposition.[0050] The following should be fulfilled in any process of deposition without current:
a) The solution must have a chemical reductive agent that acts as donor of electrons.
b) The process will continue to be catalysed by the deposited metal.
[0051] Thus, when the surface of the substrate becomes covered by the metallic deposit, continuation of the process depends on the capacity of catalysis of the deposited metal.
[0052] The term "autocatalytic" is applied to this type of process and has not undergone the same confusion as the term "electroless".
[0053] Before a process of deposition of gold may be described as autocatalytic, that process must necessarily be capable of depositing gold on a gold substrate.
[0054] Let us now discuss the specific case that we are concerned with, namely to learn the possibilities that are offered by an autocatalytic formulation for the deposition of gold. Unless a work piece is a non-conductive material or has a highly complex geometry, it is best to use a conventional method of gilding.
[0055] The reasons that advise using standard electro-deposition methods are based on the fact that autocataly tic baths present:
1 A smaller deposition velocity.
2 The conditions of electro-deposition must be carefully supervised.
3 Substrates must be very clean.
4 The baths have a more limited duration.
5 The distribution of thicknesses of the deposits depends largely on the stirring conditions.
[0056] Before discussing in detail some of the formulations that may be used in this process, it is best to understand the mechanism of the reactions involved. An introduction into the kinetics of reactions is necessary, to help to choose the best viability of a system, although we cannot eliminate the fact that experiments are necessary in order to provide the pertinent proof.
[0057] In the autocatalytic deposition, a substrate is immersed in the deposition solution, whereupon reactions commence immediately and the metal is deposited on the surface of the substrate; this metal catalyses the reaction and provokes a continuation of the deposition.
[0058] The two most essential components, in a bath of this type, are the ions M n⁺ of the metal that is to be deposited, and the reductive agent (Red.) The deposition reaction is:
[0059] The reduction-oxidation reaction takes place on the surface of the metallic or metallized substrate, whilst the reductive, donating its electrons, is converted in its oxide form (Ox). Thus, the reaction may be considered as two semi-reactions redcx:
M n⁺ + n e →Mº
and
Red - n e →Ox
[0060] The equilibrium potentials of the electrodes of the metal E m (M n⁺ /M ) and of the reductive E R (Red/ Ox) may be obtained used Nernst's equation and the potentials Eº of the tables (oxidation-reduction standard potential).
[0061] Both potentials depend on the temperature of the solution and on the ionic concentrations, and also on the nature of the compounds used. Thus, the value of E R is strongly influenced by the pH of the solution.
[0062] The difference between Eº m and Eº R and, more correctly between E m and E R may help in a preliminary selection to determine the effectiveness of certain reductive agents. If the difference is too small (or negative), the reduction of the metal may not take place, whereas if it is too large, there may be a spontaneous decomposition of the solution.
[0063] Since the oxidation-reduction reactions take place on the surface of the substrate, the potential at which they take place drops in value between E m and E Rand is considered as mixed potential E PM. The deposition velocity is obtained from change in potential (E m→E PM)
[0064] Two techniques may be used to determine the deposition velocity using the mixed theoretic potential. These are:
a) The technique of extrapolation of Tafel's curve (or intersection) or
b) The technique of polarization resistance (sometimes called lineal polarization).
[0065] The first technique uses data obtained in anodic and/or cathodic polarization measurements. Where the potential curve (anodic or cathodic) intercepts the mixed potential E PM, the corresponding intensity i dep is taken to determine the deposition velocity; the anodic and cathodic currents are equal to the potential E PM, in other words, the reduction velocity of the metallic ions to metal is equal to the velocity of oxidation of the reductive.
[0066] In the second method, based on the technique of polarization resistance, it is based on the fact, observed in the corrosion process, that the density of current applied is approximately a lineal function of the potential of the electrode, so long as the measurements are maintained at ± 10 mv of the mixed potential E PM. Other kinetic parameters should however be obtained by some independent method, especially the Tafels gradients, for anodic and cathodic reactions.
[0067] In order to understand all this better, and merely by way of example, which is by no means restrictive, a sheet is appended hereto showing drawing where:
Fig. 1 - Represents the diagram of the functional installation.
[0068] In this figure, the following components are described:
1.- Suspension tube.
2.- Charging hole.
3.- Closing lid.
4.- Protecting screen.
5.- Drive equipment.
6.- Return pipe.
7.- PH fluid control.
8.- Rotameter with thermostat.
9.- Ion consumption control.
10.- Calefactor.
11.- Filter.
12.- Flow valve.
13.- Flow control with digital thermometer.
14.- Basket.
15.- Blowoff valve.
[0069] With reference to the above-mentioned illustrations that represent a diagrammtic form of its industrial execution and are only included for informative pur poses, and consequently are in no way restrictive in a practical study of solid elements moving in fluids, the installation is presented and developed, where those solid elements may become suspended, in terms of their shape, their weight and the velocity imparted on the fluid, in a contrary direction to the vertical fall direction by action of gravity.
[0070] Free fall of a piece inside a tube that is full of a chemical or autocatalytic bath will constitute the most simple metallizing device when the time of fall, with the bath at a suitable temperature, is sufficient for depositing the required amount of metal.
[0071] Very simple reasons like recovery of the gold pieces, maintaining uniformity of the bath composition and its temperature, as well as the length of the tube, mean that it is almost impossible to use a device of this kind; however, the free fall of solid elements may be used if this is combined with a fluid movement, that has the same direction and in the opposite direction.
[0072] In the installation, that is represented in figure 1, solid elements may be metallized by combining two principles that have previously been well studied.
[0073] Firstly, if the installation is filled with a suitable fluid which is made to circulate with a centrifugal pump (5) towards the tube(1), a current of fluid is formed, whose velocity will be in terms of the pump delivery and the section of the rest of the installation. This velocity may be varied by using a flow valve (12). There is also a possibility of adapting an electronic velocity variator to the pump motor, by means of a rotameter (8), that is electronically connected to that velocity variator.
[0074] Once the movement of the fluid is established, in an upward direction along the tube (1), the temper ature of the fluid may be modified by using a calefactor covered with titanium (10). Temperature is controlled with a digital thermometer, the well of which is located at (13).
[0075] Stabilization of the temperature of the fluid is obtained by combining (10) and (13) using a thermostat.
[0076] Certain properties of the fluid such as pH, consumption of cerain ions, are determined by electrodes installed on (7) and (9), on the return pipe (1) with the drive equipment (5).
[0077] As safety elements, a protecting screen (4) is used to avoid solid elements from dropping on (5), and a filter (11) to retain possible solid impurities circulating in the equipment. Tube (1) has a closing lid (3) to avoid possible projections of fluid and to prevent foreign objects from falling in the tube (1).
[0078] Up to now, we have got a fluid to circulate through the device of figure 1 at variable velocity, so that if this is established appropriately, solid elements can be introduced via hole (2) of tube (1), and which are maintained in suspension in the fluid at an adequate velocity. The solid elements may also be introduced in tube (1) through hole (2) with the fluid at a resting state, held up by the protecting screen (4).When an appropriate velocity is given to the fluid, suspension is produced.
[0079] When the solid elements are introduced loose in tube (1), they may be taken out of the tube increasing the velocity of the fluid above the suspension velocity and are collected in the basket (14) that has been prepared for this purpose.
[0080] When the solid elements are introduced in the tube (1) in a perforated basket which adapts well to its walls, suspension is produced inside the basket and this also helps for emptying the pieces.
[0081] As already mentioned, a chemical or an autocatalytic bath has the property of depositing a specific metal, on a surface that has been suitably prepared, in a quantity that depends on the temperature and composition of the bath. When the device in figure 1 is filled with a chemical or autocatalytic bath and the appropriate velocity, pH and temperature conditions are established, a solid element whose surface has been appropriately treated may be metallized at the same time as it enters in suspension in the fluid. Once the calculated time has elapsed, it is taken out in the manner described above. On an industrial scale, a large number of pieces may be introduced, and thanks to the constant movement obtained in the suspension, they are not permanently in contact with one another to cause defects and irregularities on the layer of metal deposited.
1.- DEVICE FOR DEPOSITING METALS ON OBJECTS IN SUSPENS
ION, characterized because a fluid flows inside a tube, which causes suspension by
counter-current, of the objects that are to be metallized.
2.- DEVICE FOR DEPOSITING METALS ON OBJECTS IN SUSPENSION, in accordance with claim
1, and furthermore characterized because the upper part of the tube has charging hole
with closing lid and a protecting screen at the bottom which communicates with the
fluid drive equipment so that the objects that are to be metallized which are introduced
in the charging hole, even though more dense than the fluid, are maintained in suspension
by virtue of the current of fluid which the drive equipment produces inside the tube
in an upward direction and contrary to the falling objects by gravitation.
3.- DEVICE FOR DEPOSITING METALS ON OBJECTS IN SUSPENSION, in accordance with the
above claims, and furthermore characterized because when the objects that are to be
metallized have less density than the fluid, the tube and direction of the current
of fluid is reversed in order to maintain the suspension by counter-current.
4.- DEVICE FOR DEPOSITING METALS ON OBJECTS IN SUSPENSION, in accordance with the
above claims, and furthermore characterized because the fluid used on each cycle provokes
deposition of the metal by chemical or autocatalitic reaction, without the use of
external electrical current being necessary to provoke an electro-chemical reaction.
5.- DEVICE FOR DEPOSITING METALS ON OBJECTS IN SUSPENSION, in accordance with the
above claims, and furthermore characterized because the drive equipment communicates
with a return conduit that shuts the flow circuit of the fluid because it communicates
at one end with the tube of claim one, and at the other end with the drive equipment
which, in turn, communicates at the top with that tube.
6.- DEVICE FOR DEPOSITING METALS ON OBJECTS IN SUSPENSION, in accordance with the
foregoing claims, and furthermore characterized because the objects that are to be
metallized are introduced via the charging hole of the tube in a perforated basket
which will also be used to take them out at the end of the metallization process.
If, however, through the drive equipment, the flow of fluid that must circulate through
the tube is increased to the maximum, the objects may be collected and separated from
the fluid in a recipient inserted in the return conduit, without having to use the
perforated basket.
7.- DEVICE FOR DEPOSITING METALS ON OBJECTS IN SUSPENSION, in accordance with the
foregoing claims, and furthermore characterized because the return conduit includes
control devices of fluid, Ph, selective ion, a flow valve which permits the flow of
the fluid in the circuit to be varied, and a heater unit to be able to work at suitable
temperature in each process and be able to vary the deposition velocity of the metals,
in addition to a filter to eliminate foreign elements in suspension in the circulating
fluid.
8.- DEVICE FOR DEPOSITING METALS ON OBJECTS IN SUSPENSION, in accordance with the
foregoing claims, and furthermore characterized because there is a temperature control
and a fluid flow control fitted between the drive equipment and pipe protecting screen.
9.- DEVICE FOR DEPOSITING METALS ON OBJECTS IN SUSPENSION, in accordance with the
foregoing claims, and furthermore characterized because at the bottom of the return
conduit and immediately before it enters the drive equipment, there is a pipe for
emptying fluid,fitted with a valve.
10.- DEVICE FOR DEPOSITING METALS ON OBJECTS IN SUSPENSION, in accordance with the
foregoing claims, and furthermore characterized because by making the fluid flow at
slow velocity, the metallization process is initiated, connection start-up of the
calefactor equipment to that of the drive equipment, setting the work temperature
which is automatically stabilized by the thermostat.
11.- DEVICE FOR DEPOSITING METALS ON OBJECTS IN SUSPENSION, in accordance with the
f oregoing claims, and furthermore characterized because by reducing the flow of the
fluid to a minimum velocity, the objects which are to be metallized are located inside
the tube by means of the perforated basket, or on the screen that is situated at the
bottom, gradually increasing the flow of fluid until the desired degree of suspension
is obtained, remaining in operation until a chronograph disconnects the current of
the drive element when the preset time has elapsed.
12.- DEVICE FOR DEPOSITING METALS ON OBJECTS IN SUSPENSION, in accordance with the
foregoing claims, and furthermore characterized because by increasing the velocity
of the fluid to the maximum, in accordance with claim six, the tube is emptied of
metallic objects, which will be collected from the recipient of the return conduit,
thus leaving the device in a condition to commence a new cycle of metallization or
deposition of a metal on new objects in suspension.