LIQUID INJECTION OF VCL ₄ INTO SUPERHEATED TiCL ₄ FOR THE PRODUCTION OF Ti-V ALLOY POWDER

FIELD OF THE INVENTION

This invention relates to the production of alloys.

BACKGROUND OF THE INVENTION

The present invention relates to the production of metals and alloys using the general method disclosed in U.S. patent nos. 6,409,797; 5,958,106; and 5,779,761 , all of which are incorporated herein, and preferably a method wherein titanium or an alloy thereof is made by the reduction of halides in a flowing liquid stream of reducing metal.

Although the process and system hereinafter described pertains to titanium base alloys, it is applicable to a wide variety of alloys, wherein a superheated halide is used to vaporize a liquid halide to form an alloy in which the constituents include the superheated halide and in the liquid halide.

The Armstrong Process is defined in the patents cited above and uses a flowing liquid metal stream into which is introduced a halide vapor. The liquid metal stream may be any one or more of the alkali metals or alkaline earth metals or mixtures thereof, however, the preferred metal is sodium because of its availability, low cost and melting point, permitting steady state operations of the process to be less than 600° C and approaching or below 400° C. Preferred alternates are potassium or Nak while Mg and Ca are preferred alkaline earth metals. One very important commercial aspect of the Armstrong Process as disclosed in the above-referenced and incorporated patents is the ability to make almost any alloy wherein the constituents can be introduced as vapor into the flowing liquid metal. For titanium and its alloys, the most common commercial alloy is what is known as 6-4 alloy, that is 6% percent by weight aluminum, 4% by weight vanadium with the balance titanium, the ASTM B265 classifications for Ti are set forth in Table 1 hereafter (Class 5 is alloy 6-4). The ASTM 265 classification for commercially pure (CP) titanium is Class 2.

TABLE 1

Chemical Requirements

Element Composition %

Grade

1 2 3 4 5 6 ; _ 9 10

Nitrogen max 0.03 0.03 0.05 0.05 0.05 0.05 ( 103 C .02 0.03 0.03

Carbon max 0.10 0.10 0.10 0.10 0.10 0.10 ( 3.10 C .10 0.10 0.08

Hydrogen ⁸ max 0.015 0.015 0.015 0.015 0.015 0.020 ( 1015 C 1.015 0.015 0.015 ron Max 0.20 0.30 0.30 0.50 0.40 0.50 0.30 0.25 0.20 0.30

Oxygen max 0.18 0.25 0.35 0.40 0.20 0.20 0.25 C .15 0.18 0.25

Aluminum 5.5 to 4.0 to . 2 .5 to

6.75 6.0 3.5

Vanadium 3.5 to 2O tO

4.5 3.0

Tin 2.0 to .

3.0 .

Palladium 0.12 to . 6 ^" i2 tθ

0.25 0.25

Molybdenum 0.2 to 0.4

Zirconium

Nickel ό ^' .β to o.9

Residuals ^{C D E} αi ό ^" i ό!i αi αi σi b.1 c A b ^" i 0.1

(each), max

Residuals ^{C D E} . 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4

[total) max

Titanium ^F remainder remainder remainder remainde remainder remainder r emainder remainder remainder remainder

^A Analysis shall be completed for all elements listed in this Table for each grade. The analysis results for the elements not quantified in the Table need not be reported unless the concentration level is greater than 0.1 % each or 0.4% total.

^B Lower hydrogen may be obtained by negotiation with the manufacturer. ^c Need not be reported.

^D A residual is an element present in a metal or an alloy in small quantities inherent to the manufacturing process but not added intentionally.

^E The purchaser may, in his written purchase order, request analysis for specific residual elements not listed in this specification. The maximum allowable concentration for residual elements shall be 0.1% each and 0.4% maximum total.

^F The percentage of titanium is determined by difference.

In making 6-4 alloy, one of the problems is the instability of VCI ₄ . VCI ₄ is commonly transported as liquid vanadium tetrachloride, but liquid vanadium tetrachloride is unstable and decomposes to vanadium trichloride, the rate of decomposition being temperature dependent. Vanadium trichloride is less desirable as a feedstock for the Armstrong Process because it has a much higher melting and boiling point than vanadium tetrachloride. Moreover, decomposition of liquid

tetrachloride to solid trichloride in a vanadium tetrachloride boiler adversely affects boiler performance due to the solids build up resulting in poor boiler pressure control, premature failure of boiler heaters, line plugging, loss of usable feedstock and excessive maintenance.

SUMMARY OF THE INVENTION

Accordingly, it is a principal object of the present invention to provide a method of and system for producing alloys using the Armstrong Process in which halides which are unstable can be injected as liquids into superheated vapor to form a mixture of gases for alloy production.

Another object of the invention is to provide a method of producing an alloy, comprising providing a flowing stream of superheated halide vapor, introducing one or more liquid halides into the flowing superheated halide vapor to vaporize the liquid halides forming a mixture of gases in predetermined and controllable ratios, introducing the mixture of gases into a flowing stream of liquid alkali or alkaline earth metal or mixtures thereof establishing a reaction zone wherein the mixture of gases is reduced to an alloy and a salt, the liquid metal being present in a sufficient amount in excess of stoichiometric to maintain substantially all the alloy and salt below the sintering temperatures thereof away from the reaction zone.

Another object of the present invention is to provide a method of producing a Ti base alloy, comprising providing a flowing stream of superheated titanium tetrahalide vapor, introducing one or more liquid halides into the flowing superheated titanium tetrahalide vapor to vaporize the liquid halides forming a mixture of gases in predetermined and controllable ratios, introducing the mixture of gases into a flowing stream of liquid alkali or alkaline earth metal or mixtures thereof establishing a reaction zone wherein the mixture of gases is reduced to a titanium base alloy and a salt, the liquid metal being present in a sufficient amount in excess of stoichiometric to maintain substantially all the titanium base alloy and salt below the sintering temperatures thereof away from the reaction zone.

A further object of the present invention is to provide a method of producing a Ti base alloy, comprising providing a flowing stream of superheated titanium tetrachloride

vapor, introducing one or more liquid chlorides into the flowing superheated titanium tetrachloride vapor to vaporize the liquid chlorides forming a mixture of gases in predetermined and controllable ratios, introducing the mixture of gases into a flowing stream of liquid sodium or alkaline earth metal or mixtures thereof establishing a reaction zone wherein the mixture of gases is reduced to a titanium base alloy and salt, the liquid metal being present in a sufficient amount in excess of stoichiometric to maintain substantially all the titanium base alloy and salt below the sintering temperatures thereof away from the reaction zone.

A still further object of the present invention is to provide a system for producing an alloy, comprising a storage container for a first liquid halide and heating mechanism in communication therewith for providing a flowing stream of superheated halide vapor, a first detection and/or control device in communication with the flowing stream of superheated halide for detecting and/or controlling the mass flow rate thereof, a second storage container for a second liquid halide and mechanism in communication therewith for introducing the second liquid halide into the flowing stream of superheated halide vapor to vaporize the second liquid halide forming a mixture of gases in predetermined and controllable ratios, a second detection and/or control device in communication with the second storage container for the second liquid halide to measure and/or control the amount of second liquid halide introduced into the flowing superheated stream of halide, a storage container for a liquid alkali or alkaline earth metal and mechanism for providing a flowing stream of liquid alkali or alkaline earth metal or mixtures thereof and mechanism for introducing the mixture of gases into the flowing stream of liquid alkali or alkaline earth metal or mixtures thereof establishing a reaction zone wherein the mixture of gases is reduced to an alloy and salts, the liquid metal being present in a sufficient amount in excess of stoichiometric to maintain substantially all the alloy and salts below the sintering temperatures thereof away from the reaction zone, and control mechanism in communication with the first and second detection and/or control devices to control the amount of second liquid halide introduced into the flowing superheated stream of halide as a function of the mass flow rate of the superheated halide vapor to produce an alloy with predetermined

constituent concentrations.

A final object of the invention is to provide a system for producing a Ti base alloy, comprising a storage container for liquid titanium tetrahalide and heating mechanism in communication therewith for providing a flowing stream of superheated titanium tetrahalide vapor, a first flow meter in communication with the flowing stream of superheated titanium tetrahalide for measuring the flow rate thereof, a second storage container for a second liquid halide and mechanism in communication therewith for introducing the second liquid halide into the flowing stream of superheated titanium tetrahalide vapor to vaporize the second liquid halide forming a mixture of gases in predetermined and controllable ratios, a second flow meter and/or a scale in communication with the second storage container for the second liquid halide to measure the amount of second liquid halide introduced into the flowing superheated stream of titanium tetrahalide, a storage container for a liquid alkali or alkaline earth metal and mechanism for providing a flowing stream of liquid alkali or alkaline earth metal or mixtures thereof and mechanism for introducing the mixture of gases into the flowing stream of liquid alkali or alkaline earth metal or mixtures thereof establishing a reaction zone wherein the mixture of gases is reduced to a titanium base alloy and salts, the liquid metal being present in a sufficient amount in excess of stoichiometric to maintain substantially all the titanium base alloy and salts below the sintering temperatures thereof away from the reaction zone, and control mechanism in communication with flow meters and/or the scale to control the amount of second liquid halide introduced into the flowing superheated stream of titanium tetrahalide to produce a titanium base alloy with predetermined constituent concentrations.

The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of facilitating an understanding of the invention, there is

illustrated in the accompanying drawings a preferred embodiment thereof, from an inspection of which, when considered in connection with the following description, the invention, its construction and operation, and many of its advantages should be readily understood and appreciated.

FIGURE 1 is a schematic representation of a system for producing alloys according to the Armstrong Process incorporating the subject invention;

FIG. 1 A is a schematic representation of a reactor useful in the practice of the invention;

FIGS. 2-4 are SEMs of alloys made in accordance with the present invention; and

FIG. 5 is a plot of intensity versus energy level, in keV, for one spot of the alloy illustrated in the SEMs showing a small peak of about 5.3 keV is the K _β emission for V. DETAILED DESCRIPTION OF THE INVENTION

Because VCI ₄ is a stable compound in the vapor form but decomposes when present as a liquid, the decomposition rate being both temperature and time dependent, the subject invention solves a difficult problem in making the most commercially useful titanium alloy. By introducing VCI ₄ as a liquid, stored at a relatively low ambient temperature, directly into a super heated vapor without having to raise the temperature of the liquid over a longer period of time, significant losses of the VCI ₄ feedstock are prevented. Moreover, as previously indicated, a host of other problems are also solved by the subject invention including equipment failure, poor control of the amount of vanadium introduced due to build up of solids in the vanadium boiler, increased maintenance and boiler failure.

All of the figures included in this application are non-limiting specific examples of the invention. Although the superheated vapor used in the specific example herein is TiCI ₄ with optional aluminum trichloride intermixed therewith, the superheated vapor may be any halide or mixtures thereof that is suitable for the Armstrong process. Fluorides and borides are commercially available and for some alloy constituents may be required. The preferred halide is a chloride due to cost and availability. In general, the super heated halide may be one or more of titanium, vanadium, boron, antimony,

beryllium, gallium, uranium, silicon and rhenium. In addition, one or more liquid halides of the following elements may be used as alloy constituents: Al, B, Be, Bi, C, Fe, Ga, Ge ₁ In, Mo, Nb, P, Pb, Re, Sb, Si, Sn, Ta, Ti, V, and W. Certain halides sublimate rather than boil, so these, such as AICI ₃ , PtF ₆ and ZrCI ₄ , are introduced as vapor. The resulting alloy produced by this method and the system designed to provide same will include one or more of the following: Al, B, Be, Bi, C, Fe, Ga, Ge, Hf, In, Mo, Nb, P, Pb, Re, S, Sb, Si, Sn, Ta, Ti, U, V, W, and Zr. It should be noted that the alloy may contain non-metals such as carbon or boron or sulfur and in various amounts. The examples hereinafter set forth relate to titanium base alloys and particularly to titanium base alloys containing one or more of vanadium and aluminum but other alloys have been and are able to be made with the Armstrong Process. The introduction of some alloy constituents directly from the liquid has an additional advantage of facilitating the control of constituent concentrations.

Referring now to a non-limiting specific example, VCI ₄ is a stable compound in vapor form but the decomposition of liquid VCI ₄ is a problem when the liquid is heated beyond ambient temperatures in order to vaporize the same. The invention involves introducing a liquid halide into a super heated vapor stream of halides in order to flash the liquid VCI ₄ to the vapor phase from ambient temperatures directly without heating the liquid to its boiling point over a long period of time resulting in the aforesaid decomposition.

With respect to titanium base alloys, a superheated stream of TiCI ₄ can be used to flash vaporize liquids of vanadium chlorides and other halides facilitating improved control and reducing equipment problems in a vanadium tetrachloride boiler, as previously discussed. The amount of superheat needed is dependent among other things on the respective amount of superheated vapor and liquid halide being injected and can be determined by a person within the ordinary skill in the art when the constituents are known, based on the specific heat of the superheated vapor and the specific heat and heat of vaporization of the liquid. An example calculation specific to flash vaporizing VCI ₄ with a superheated stream of TiCI ₄ is set forth below.

Properties and Assumptions

TiCI ₄ Mass Flow Rate = 2.5 Kg/min

VCI ₄ Mass Flow Rate = .091 kg/min

Cp TiCI _4gas = 94.9 Joule/Mol-K @ 533K

Cp VCI _4liq = 138.63 Joule/Mol-K @ 403K

H _vap VCI ₄ = 33 kJoules/Mol-K @ 503K

VCI ₄ MoI Wt. = 192.9g

TiCI ₄ MoI Wt. = 189.9g

MoI Wt V = 50.9g

MoI Wt Ti = 47.9g

Assume thermodynamic property variations are negligible over the temperature range considered (Ref. Chemical Properties Handbook, Carl L. Yaws, McGraw-Hill Handbooks).

To calculate the energy needed to vaporize the liquid VCI ₄ @ 500Kpa and 230°C using the properties and assumption above, the following calculations are made: (This is the energy to heat the VCI ₄ from 30°C to 230°C and the energy to vaporize the VCI ₄ ) (mol VCI ₄ /0.1929Kg)[(0.091 kg/60sec)(138.6J/mol-k)(230-30) + (0.091 kg/60sec)(33 kj/mol)] = 477j/sec needed to heat and vaporize the VCI ₄ at 50OkPa and at the stated flow rate.

Calculate the necessary superheat on the TiCI ₄ to provide the energy necessary to vaporize the VCI ₄ at 500 Kpa:

(Mass Flow _{TiCI4 Vap} )(C _{pTiCI4 Vap} )(T _{TiCI4 Sup8r(ieated} - 503k) = 477j/Sec (2.500kg/60sec)(mol TiCI ₄ /0.1899kg)(94.9 J/mol-K)( T _{TiCI4 Superhθatθd} - 503k) = 477 j/sec

^T _T iC _I 4 Superhea _t e _d = 525.8K = 252.8°C.

Thus, the superheat temperature above saturation required to provide the necessary energy to heat and vaporize the VCI ₄ | _iq in this example is (252.8°C -230°C): = 22.8°C of superheat. Example 1

Figure 1 is a schematic representation of the equipment used in the following example.

Referring now to Figure 1 , there is VCI ₄ reservoir 9 connected by a valve 1 to a source of argon, the reservoir 9 being supported on a weigh scale 10. A conduit is below the liquid level of the VCI ₄ in the reservoir 9 and extends through a series of valves 2 and 3 through a filter 6 into a gas manifold line 7. A separate argon purge is connected to the conduit leaving the VCI ₄ reservoir by means of a valve 11 and a flow meter 8 to control the flow rate of argon purge gas after a run has been completed.

Titanium tetrachloride from a boiler (not shown) flows into a superheater 5 through a conduit past valves 4 into a manifold receiving liquid VCI ₄ from the reservoir 9.

Other chlorides for alloy constituents can be introduced into the manifold containing the gas as illustrated in Fig. 1 or at any point before the introduction of the liquid VCI ₄ . After the liquid VCI ₄ is flashed to a vapor, the mixture of gases is then fed to the Armstrong reactor as illustrated in Fig. 1. Fig. 1 A is a replication of the reactor as illustrated in Fig. 2 of U.S. patent no. 5,958,106, issued to Armstrong et al. September 28, 1999, the entire disclosure of which was incorporated herein by reference. A reactor 20 has a liquid metal inlet 13 and a pipe 21 having an outlet or nozzle 23 connected to a source halide gas 22 and source of halide liquid 24.

For instance in FIG. 1A, the sodium entering the reaction chamber is at 200°C. having a flow rate of 38.4 kilograms per minute. The titanium tetrachloride from the boiler is at 2 atmospheres and at a temperature of 164°C, the flow rate through the line was 1.1 kg/min. Higher pressures may be used, but it is important that back flow be prevented, so the minimum pressure should be equal to or above that determined by the critical pressure ratio for sonic conditions, or about two times the absolute pressure of the sodium stream (two atmospheres if the sodium is at atmospheric pressure) is preferred to ensure that flow through the reaction chamber nozzle is critical or choked.

The description of the reactor in Fig. 1 A is found in the previously incorporated Armstrong et al. patents. The difference between the reactor illustrated in Fig. 1A herein and that as described in the '106 and other patents incorporated herein is that the liquid that is flashed in this present invention is injected from the source (9) as a liquid into the titanium tetrachloride after it leaves the boiler 22 and superheater (5)

under superheat conditions calculated in the manner hereinbefore described.

Referring to Fig. 1 , a liquid reservoir of VCI ₄ (9) is pressurized with Argon (1 ) to above the TiCI ₄ vapor pressure so that liquid VCI ₄ is capable of flowing into a pressurized TiCI ₄ vapor stream at a constant rate. The rate can be varied by adjusting the reservoir pressure or the spray orifice diameter. When the reaction process is started, the TiCI ₄ valves (4) open allowing superheated TiCI vapor to flow towards the reactor. Simultaneously, valve (3) opens allowing room temperature liquid VCI ₄ to flow through filter (6) and spray nozzle (7) into the superheated TiCI ₄ stream. The weigh scale 10 monitors VCI ₄ mass flow rate into the process. The superheated TiCI ₄ mixes with the liquid VCI ₄ , rapidly vaporizes it, and carries it to the Armstrong Reactor 20 (Fig. 1A) along with other metal chlorides from additional alloy boilers (not shown) to produce the desired powder. At the end of the run, the argon purge through flow meter (8) is used to drive out residual VCI ₄ from the injection nozzle and tubing to prevent decomposition of residual VCI ₄ plugging the delivery system.

In this example, TiCI ₄ pressure was 500Kpa and VCI ₄ reservoir pressure was 2400Kpa. During the course of the reaction, 232g of liquid VCI ₄ and 10,800 g of TiCI ₄ with 80 to 100 ⁰ C superheat were injected. This corresponded to 61.3 g V and 2,728g of Ti or 0.22 wt% V. The average chemical analysis showed a 0.23 wt% V in the powder demonstrating that the VCI ₄ injected into the TiCI ₄ stream made it into the reacted product. Further, X-ray mapping showed typical uniform distribution of the vanadium within the powder particles as shown in Fig. 5.

Using the equipment as shown in Figure 1 with the addition of liquid VCI ₄ flow control (PID) capability and the elimination of the spray nozzle (7) into the TICI ₄ tube replaced by a VA" tube leading directly into the superheated TiCI ₄ vapor, a TiV alloy was produced. Based on actual TiCI ₄ and VCI ₄ weights reacted during a run, a 5.1 wt% vanadium content was expected in the titanium powder that was produced. The actual measured vanadium content produced during the test as measured by direct current plasma emission spectroscopy per ASTM E1097-03 varied from 4.95% to 5.27% over six different samples.

In this example, the control system was programmed to produce a Ti-4%V alloy

as a function of actual TiCI ₄ flow. The TiCI ₄ pressure was approximately 50OkPa, the VCI ₄ reservoir pressure was approximately 800 kPa, the TiCI ₄ was superheated to greater than 285°C, the TiCI ₄ flow indicated approximately 2200g/min and the VCI flow indicated approximately 90g/min. Based on actual weights of metal chloride reactants used during this run, the metal powder chemistry was expected to be between 4.1 % and 4.2% vanadium. The vanadium concentrations are shown in Table 2.

TABLE 2

Method: Direct current plasma emission spectroscope - ASTM E 1097 03.

The Titanium (Ti) - Vanadium (V) alloy sample ©) was analyzed on a Zeiss Supra40VP Scanning Electron Microscope (SEM), a variable-pressure system with a PGT energy-dispersive X-ray detector. The secondary electron detector operating at 20 kV was used for the SEM micrographs shown in Figure 2. This micrograph reveals typical Armstrong powder morphology with feature size similar to commercially pure (CP) Ti. Eleven spots were selected from an image similar to Figure 2 for quantitative elemental analysis (spotlight). The individual results from this spotlight analysis are given in Figure 3. The x-ray information showed a fairly uniform distribution of vanadium in titanium with an average value for V of 4.38%, see Table 3.

TABLE 3

Spotlight Summary Report Concentrations by Weight %

Tag # C Ti V

1 97.83% 2.17%

2 98.18% 1.82%

3 98.15% 1.85%

4 89.73% 10.27%

5 92.09% 7.91%

6 96.52% 3.48%

7 98.47% 1.53%

8 95.89% 4.11%

9 92.56% 7.44%

10 97.68% 2.32%

11 94.90% 5.10%

Average V 4.38%

Summary of the elemental concentrations derived from emission data for 11 random spots from an SEM image similar to Fig. 2.

Composition elemental mapping of the V concentration distribution in the titanium was performed using the K orbital x-ray emission data measure by a detector in the SEM. One issue in analyzing the x-ray emission information for a Ti-V alloy is that the Ka peak of V is near the Ti K _β peak making it difficult to directly map elemental V based on the V K _α data. In order to get an elemental map of V, without the masking effect of the Ti K _β peak, its K _β peak was used. The K _α data for V is much weaker but is not confounded by other possible elements in this range.

In Figure 3 the secondary electron image is given along with the elemental mapping data for Ti and V based on K _α emission data. With the confounding of the Ti Kp data at the same energy level as the V K ₀ , the results may not give an accurate map

of the V concentrations. The V K _p peak was used to map the elemental concentration of V, as shown in Figure 4. Since there are no other peaks masking the V K _β peak, it is assumed that the V mapping results should be more accurate.

The intensity results of the x-ray energy emission for the Armstrong Ti-4V powder sample is given in Figure 5. The high intensity peak at 4.51 keV is the K _α peak for Ti while the V K _α peak should appear at 4.95 keV, it is in part hidden by the secondary Ti K _β peak at about 4.9 keV. The V K _β peak however can be seen unabated at about 5.3 keV. Sample C (Figs. 3 and 4) contains Ti-V powder with feature size similar to Armstrong CP Ti powder. X-ray analysis indicates minimal segregation of the V element in the Ti alloy.

Although the specific experiments or examples set forth above relate to titanium and vanadium, and more particularly, to the use of a titanium tetrachloride superheated vapor to flash vaporize ambient liquid vanadium tetrachloride, the invention extends beyond the specific examples and is not to be limited thereby. More specifically, a wide variety of superheated halides including mixtures thereof may be used in the subject invention including titanium, boron, antimony, beryllium gallium, uranium, silicon and rhenium to name a few. The liquid halide may include one or more of boron, beryllium, bismuth, carbon, iron, gallium, germanium, indium, molybdenum, niobium, phosphous lead rhenium, antimony, silicon, tin, tantalum, titanium vanadium and tungsten.

Moreover, more than one liquid halides may be introduced and more than one halide may be used as the superheated halide. In addition, the invention includes serial introduction of liquid halides and serial introduction of halide vapors. For instance, a titanium tetrachloride vapor may be superheated to flash vaporize a liquid such as but not limited to vanadium tetrachloride, and thereafter, additional halides such as those of bismuth, iron or any of the other previously named halides may be added as vapors or as liquids, as necessary.

The calculation for the amount of superheat needed is based on the examples hereinbefore set forth.

In order to make the most commercially useful alloy of titanium which is called 6-4 titanium, that is 6 percent by weight aluminum and 4 percent by weight vanadium

with the balance titanium, aluminum trichloride has to be introduced into the titanium tetrachloride either before or after the liquid vanadium tetrachloride is flashed from liquid to vapor. The amounts of alloy constituents can be closely controlled using either the liquid or the vapor form, depending on instrumentation and the like. Other alloys can be made using the present invention including 6-4 titanium with boron additions as well as many other alloys.

While the invention has been particularly shown and described with reference to a preferred embodiment hereof, it will be understood by those skilled in the art that several changes in form and detail may be made without departing from the spirit and scope of the invention.